My Project On Artificial Satellites.

INTRODUCTION
Space Exploration, quest to use space travel to discover the nature of the universe beyond Earth. Since ancient times, people have dreamed of leaving their home planet and exploring other worlds. In the later half of the 20th century, that dream became reality. The space age began with the launch of the first artificial satellites in 1957. A human first went into space in 1961. Since then, astronauts and cosmonauts have ventured into space for ever greater lengths of time, even living aboard orbiting space stations for months on end. Two dozen people have circled the moon or walked on its surface. At the same time, robotic explorers have journeyed where humans could not go, visiting all but one of the solar system's major worlds. Unpiloted spacecraft have also visited a host of minor bodies such as moons, comets, and asteroids. These explorations have sparked the advance of new technologies, from rockets to communications equipment to computers. Spacecraft studies have yielded a bounty of scientific discoveries about the solar system, the Milky Way Galaxy, and the universe. And they have given humanity a new perspective on the earth and its neighbors in space.

The first challenge of space exploration was developing rockets powerful enough and reliable enough to boost a satellite into orbit. These boosters needed more than brute force, however; they also needed guidance systems to steer them on the proper flight paths to reach their desired orbits. The next challenge was building the satellites themselves. The satellites needed electronic components that were lightweight, yet durable enough to withstand the acceleration and vibration of launch. Creating these components required the world's aerospace engineering facilities to adopt new standards of reliability in manufacturing and testing. On Earth, engineers also had to build tracking stations to maintain radio communications with these artificial "moons" as they circled the planet.

Beginning in the early 1960s, humans launched probes to explore other planets. The distances traveled by these robotic space travelers required travel times measured in months or years. These spacecraft had to be especially reliable to continue functioning for a decade or more. They also had to withstand such hazards as the radiation belts surrounding Jupiter, particles orbiting in the rings of Saturn, and greater extremes in temperature than are faced by spacecraft in the vicinity of Earth. Despite their great scientific returns, these missions often came with high price tags. Today the world’s space agencies, such as the United States National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), strive to conduct robotic missions more cheaply and efficiently.

It was inevitable that humans would follow their unpiloted creations into space. Piloted space flight introduced a whole new set of difficulties, many of them concerned with keeping people alive in the hostile environment of space. In addition to the vacuum of space, which requires any piloted spacecraft to carry its own atmosphere, there are other deadly hazards: solar and cosmic radiation, micrometorites (small bits of rock and dust) that might puncture a spacecraft hull or an astronaut's pressure suit, and extremes of temperature ranging from frigid darkness to broiling sunlight. It was not enough simply to keep people alive in space—astronauts needed to have a means of accomplishing useful work while they were there. It was necessary to develop tools and techniques for space navigation, and for conducting scientific observations and experiments. Astronauts would have to be protected when they ventured outside the safety of their pressurized spacecraft to work in the vacuum. Missions and hardware would have to be carefully designed to help insure the safety of space crews in any foreseeable emergency, from liftoff to landing.

The challenges of conducting piloted space flights were great enough for missions that orbited Earth. They became even more daunting for the Apollo missions, which sent astronauts to the moon. The achievement of sending astronauts to the lunar surface and back represents a summit of human space flight.

After the Apollo program, the emphasis in piloted missions shifted to long-duration spaceflight, as pioneered aboard Soviet and U.S. space stations. The development of reusable space craft became another goal, giving rise to the U.S. space shuttle fleet. Today efforts focus on keeping people healthy during space missions lasting a year or more—the duration needed to reach nearby planets—and in lowering the cost of sending satellites into orbit.

HISTORY OF SPACE EXPLORATION
The desire to explore the heavens is probably as old as humankind, but in the strictest sense, the history of space exploration begins very recently, with the launch of the first artificial satellite, Sputnik 1, which the Soviets sent into orbit in 1957. Soviet cosmonaut Yury Gagarin became the first human in space just a few years later, in 1961. The decades from the 1950s to the 1990s have been full of new "firsts," new records, and advances in technology.

First Forays into Space

Although artificial satellites and piloted spacecraft are achievements of the later 20th century, the technology and principles of space travel stretch back hundreds of years, to the invention of rockets in the 11th century and the formulation of the laws of motion in the 17th century. The power of rockets to lift objects into space is described by a law of motion that was formulated by English scientist Sir Isaac Newton in the 1680s. Newton’s third law of motion states that every action causes an equal and opposite reaction. As predicted by Newton’s law, the rearward rush of gases expelled by the rocket’s engine causes the rocket to be propelled forward. It took nine centuries from the invention of rockets and almost three centuries from the formulation of Newton’s third law for humans to send an object into space. In space, the motions of satellites and interplanetary spacecraft are described by the laws of motion formulated by German astronomer Johannes Kepler, also in the 17th century. For example, one of Kepler’s laws states that the closer a satellite is to Earth, the faster it orbits.

Rockets and Rocket Builders
Rockets made their first recorded appearance as weapons in 12th century China, but they probably originated in the 11th century. Fueled by gunpowder, they were launched against enemy troops. In the centuries that followed, these solid-fueled rockets became part of the arsenals of Europe. In 1814, during an attack on New Orleans, the British fired rockets—with little effect—at American troops.

In Russia, nearly a century later, a lone schoolteacher named Konstantin Tsiolkovsky envisioned how to use rockets to voyage into space. In a series of detailed treatises, including "The Exploration of Cosmic Space With Reactive Devices" (1903), Tsiolkovsky explained how a multi-stage, liquid fueled rocket could propel humans to the moon.

Tsiolkovsky did not have the means to build real liquid-fuel rockets. Robert Goddard, a physics professor in Worcester, Massachusetts, took up that effort. In 1926 he succeeded in building and launching the world's first liquid-fuel rocket, which soared briefly above a field near his home. Beginning in 1940, after moving to Roswell, New Mexico, Goddard built a series of larger liquid-fuel rockets that flew as high as 90 m (300 ft). Meanwhile, beginning in 1936 at the California Institute of Technology, other experimenters made advances in solid-fuel rockets. During World War II (1939-1945), engineers developed solid-fuel rockets that could be attached to an airplane to provide a boost during takeoff.

The greatest strides in rocketry during the first half of the 20th century occurred in Germany. There, mathematician and physicist Hermann Oberth and architect Walter Hohmann theorized about rocketry and interplanetary travel in the 1920s. During World War II, Nazi Germany undertook the first large-scale rocket development program, headed by a young engineer named Wernher Von Braun. Von Braun's team created the V-2, a rocket that burned an alcohol-water mixture with liquid oxygen to produce 250,000 newtons (56,000 lb) of thrust. The Germans launched thousands of V-2s carrying explosives against targets in Britain and the Netherlands. While they did not prove to be an effective weapon, V-2s did become the first human-made objects to reach altitudes above 80 km (50 mi)—the height at which outer space is considered to begin—before falling back to Earth. The V-2 inaugurated the era of modern rocketry.

A2 Early Artificial Satellites
During the years following World War II, the United States and the Union of Soviet Socialist Republics (USSR) engaged in efforts to construct intercontinental ballistic missiles (ICBMs) capable of traveling thousands of miles armed with a nuclear warhead. In August 1957 Soviet engineers, led by rocket pioneer Sergei Korolyev, were the first to succeed with the launch of their R-7 rocket, which stood almost 30 m (100 ft) tall and produced 3.8 million newtons (880,000 lb) of thrust at liftoff. Although its primary purpose was for use as a weapon, Korolyev and his team adapted the R-7 into a satellite launcher. On October 4, 1957, they launched the world's first artificial satellite, called Sputnik ("fellow traveler"). Although it was only a simple 58-cm (23-in) aluminum sphere containing a pair of radio transmitters, Sputnik’s successful orbits around Earth marked a huge step in technology and ushered in the space age. On November 3, 1957, the Soviets launched Sputnik 2, which weighed 508 kg (1121 lb) and contained the first space traveler—a dog named Laika, which survived for several days aboard Sputnik 2. Due to rising temperatures within the satellite, Laika died from heat exhaustion before her air supply ran out.

News of the first Sputnik intensified efforts to launch a satellite in the United States. The initial U.S. satellite launch attempt on December 6, 1957, failed disastrously when the Vanguard launch rocket exploded moments after liftoff. Success came on January 31, 1958, with the launch of the satellite Explorer 1. Instruments aboard Explorer 1 made the first detection of the Van Allen belts, which are bands of trapped radiation surrounding the earth (see Radiation Belts). This launch also represented a success for Wernher von Braun, who had been brought to the United States with many of his engineers after World War II. Von Braun’s team had created the Jupiter C (an upgraded version of their Redstone missile), which launched Explorer 1.

The satellites that followed Sputnik and Explorer into earth orbit provided scientists and engineers with a variety of new knowledge. For example, scientists who tracked radio signals from the U.S. satellite Vanguard 1, launched in March 1958, determined that Earth is slightly flattened at the poles. In August 1959 Explorer 6 sent back the first photo of Earth from orbit. Even as these satellites revealed new details about our own planet, efforts were underway to reach our nearest neighbor in space, the moon.

Unpiloted Lunar Missions
Early in 1958 the United States and the USSR were both working hard to be the first to send a satellite to the moon. Initial attempts by both sides failed. On October 11, 1958, the United States launched Pioneer 1 (see Pioneer (spacecraft)) on a mission to orbit the moon. It did not reach a high enough speed to reach the moon, but reached a height above Earth of more than 110,000 km (more than 70,000 mi). In early December 1958 Pioneer 3 also failed to leave high Earth orbit. It did, however, discover a second Van Allen belt of radiation surrounding Earth.

On January 2, 1959, after two earlier failed missions, the USSR launched Luna 1 (see Luna (space program)), which was intended to hit the moon. Although it missed its target, Luna 1 did become the first artificial object to escape Earth orbit. On September 14, 1959, Luna 2 became the first artificial object to strike the moon, impacting east of the Mare Serentitatis (Sea of Serenity). In October 1959, Luna 3 flew around the moon and radioed the first pictures of the far side of the moon, which is not visible from the earth.

In the United States, efforts to reach the moon did not resume until 1962, with a series of probes called Ranger. The early Rangers were designed to eject an instrument capsule onto the moon’s surface just before the main spacecraft crashed into the moon. These missions were plagued by failures—only Ranger 4 struck the moon, and the spacecraft had already ceased functioning by that time. Rangers 6 through 9 were similar to the early Rangers, but did not have instrument packages. They carried television cameras designed to send back pictures of the moon before the spacecraft crashed. On July 31, 1964, Ranger 7 succeeded in sending back the first high-resolution images of the moon before crashing, as planned, into the surface. Rangers 8 and 9 repeated the feat in 1965.

By then, the United States had embarked on the Apollo program to land humans on the moon (see the Piloted Spaceflight section of this article for a discussion of the Apollo program). With an Apollo landing in mind, the next series of U.S. lunar probes, named Surveyor, was designed to "soft-land" (that is, land without crashing) on the lunar surface and send back pictures and other data to aid Apollo planners. As it turned out, the Soviets made their own soft landing first, with Luna 9, on February 4, 1966. Luna 9 radioed the first pictures of a dusty moonscape from the lunar surface. Surveyor 1 successfully reached the surface on June 2, 1966. Six more Surveyor missions followed; all but two were successful. The Surveyors sent back thousands of pictures of the lunar surface. Two of the probes were equipped with a mechanical claw, remotely operated from Earth, that enabled scientists to investigate the consistency of the lunar soil.

At the same time, the United States launched the Lunar Orbiter probes, which began circling the moon to map its surface in unprecedented detail. Lunar Orbiter 1 began taking pictures on August 18, 1966. Four more Lunar Orbiters continued the mapping program, which gave scientists thousands of high-resolution photographs covering nearly all of the moon.

Beginning in 1968 the USSR sent a series of unpiloted Zond probes—actually a lunar version of their piloted Soyuz spacecraft—around the moon. These flights, initially designed as preparation for planned piloted missions that would orbit the moon, returned high-quality photographs of the moon and Earth. Two of the Zonds carried biological payloads with turtles, plants, and other living things.

Although both the United States and the USSR were achieving successes with their unpiloted lunar missions, the Americans were pulling steadily ahead in their piloted program. As their piloted lunar program began to lag, the Soviets made plans for robotic landers that would gather a sample of lunar soil and carry it to Earth. Although this did not occur in time to upstage the Apollo landings as the Soviets had hoped, Luna 16 did carry out a sample return in September 1970, returning to Earth with 100 g (4 oz) of rock and soil from the moon's Mare Fecunditatis (Sea of Fertility). In November, 1970, Luna 17 landed with a remote-controlled rover called Lunakhod 1. The first wheeled vehicle on the moon, Lunakhod 1 traveled 10.5 km (6.4 mi) across the Sinus Iridium (Bay of Rainbows) during ten months of operations, sending back pictures and other data. Only three more lunar probes followed. Luna 20 returned samples in February 1972. Lunakhod 2, carried aboard the Luna 21 lander, reached the moon in January 1973. Then, in August 1976 Luna 24 ended the first era of lunar exploration.

Exploration of the moon resumed in February 1994 with the U.S. probe called Clementine, which circled the moon for three months. In addition to surveying the moon with high-resolution cameras, Clementine gathered the first comprehensive data on lunar topography using a laser altimeter. Clementine’s laser altimeter bounced laser beams off of the moon’s surface, measuring the time they took to come back to determine the height of features on the moon.

In January 1998 NASA's Lunar Prospector probe began circling the moon in an orbit over the moon’s north and south poles. Its sensors conducted a survey of the moon's composition. In March 1998 the spacecraft found evidence of a significant amount of water in the form of ice mixed with lunar soil at the moon’s poles. Lunar Prospector also investigated the moon's gravitational and magnetic fields.

Scientific Satellites
Years before the launch of the first artificial satellites, scientists anticipated the value of putting telescopes and other scientific instruments in orbit around Earth. Orbiting satellites can view large areas of Earth or can provide views of space unobstructed by Earth’s atmosphere.

Earth-Observing Satellites
One main advantage of putting scientific instruments into space is the ability to look down at Earth. Viewing large areas of the planet allows meteorologists, scientists who research Earth’s weather and climate, to study large-scale weather patterns (see Meteorology). More detailed views aid cartographers, or mapmakers, in mapping regions that would otherwise be inaccessible to people. Researchers who study Earth’s land masses and oceans also benefit from having an orbital vantage point.

Beginning in 1960 with the launch of U.S. Tiros I, weather satellites have sent back television images of parts of the planet. The first satellite that could observe most of Earth, NASA’s Earth Resources Technology Satellite 1 (ERTS 1, later renamed Landsat 1), was launched in 1972. Landsat 1 had a polar orbit, circling Earth by passing over the north and south poles. Because the planet rotated beneath Landsat’s orbit, the satellite could view almost any location on the Earth once every 18 hours. Landsat 1 was equipped with cameras that recorded images not just of visible light but of other wavelengths in the electromagnetic spectrum (see Electromagnetic Radiation). These cameras provided a wealth of useful data. For example, images made in infrared light let researchers discriminate between healthy crops and diseased ones. Six additional Landsats were launched between 1975 and 1998.

The success of the Landsat satellites encouraged other nations to place earth-monitoring satellites in orbit. France launched a series of satellites called SPOT beginning in 1986, and Japan launched the MOS-IA (Marine Observation System) in 1987. The Indian Remote Sensing satellite, IRS-IA, began operating in 1988.

Astronomical Satellites
Astronomical objects such as stars emit radiation, or radiating energy, in the form of visible light and many other types of electromagnetic radiation. Different wavelengths of radiation provide astronomers with different kinds of information about the universe. Infrared radiation, with longer wavelengths than visible light, can reveal the presence of interstellar dust clouds or other objects that are not hot enough to emit visible light. X rays, a high-energy form of radiation with shorter wavelengths than visible light, can indicate extremely high temperatures caused by violent collisions or other events. Earth orbit, above the atmosphere, has proved to be an excellent vantage point for astronomers. This is because Earth's atmosphere absorbs high-energy radiation, such as ultraviolet rays, X rays, and gamma rays. While such absorption shields the surface of Earth and allows life to exist on the planet, it also hides many celestial objects from ground-based telescopes. In the early 1960s, rockets equipped with scientific instruments (called sounding rockets) provided brief observations of space beyond our atmosphere, but orbiting satellites have offered far more extensive coverage.

Britain launched the first astronomical satellite, Ariel 1, in 1962 to study cosmic rays and ultraviolet and X-ray radiation from the sun. In 1968 NASA launched the first Orbiting Astronomical Observatory, OAO 1, equipped with an ultraviolet telescope. Uhuru, a U.S. satellite designed for X-ray observations, was launched in 1970. Copernicus, officially designated OAO 3, was launched in 1972 to detect cosmic X-ray and ultraviolet radiation. In 1978 NASA's Einstein Observatory, officially designated High-Energy Astrophysical Observatory 2 (HEAO 2) reached orbit, becoming the first X-ray telescope that could provide images comparable in detail to those provided by visible-light telescopes. The Infrared Astronomical Satellite (IRAS), launched in 1983, was a cooperative effort by the United States, the Netherlands, and Britain. IRAS provided the first map of the universe in infrared wavelengths and was one of the most successful astronomical satellites. The Cosmic Ray Background Explorer (COBE) was launched in 1989 by NASA and discovered further evidence for the big bang, the theoretical explosion at the beginning of the universe. The Hubble Space Telescope was launched in orbit from the U.S. space shuttle in 1990, equipped with a 100-in (250-cm) telescope and a variety of high-resolution sensors produced by the United States and European countries. Flaws in Hubble’s mirror were corrected by shuttle astronauts in 1993, enabling Hubble to provide astronomers with spectacularly detailed images of the heavens.

Other Satellites
In addition to observing Earth and the heavens from space, satellites have had a variety of other uses. A satellite called Corona was the first U.S. spy satellite effort. The program began in 1958. The first Corona satellite reached orbit in 1960 and provided photographs of Soviet missile bases. In the decades that followed, spy satellites, such as the U.S. Keyhole series, became more sophisticated. Details of these systems remain classified, but it is has been reported that they have attained enough resolution to detect an object the size of a car license plate from an altitude of 160 km (100 mi) or more.

Other U.S. military satellites have included the Defense Support Program (DSP) for the detection of ballistic missile launches and nuclear weapons tests. The Defense Meteorological Support Program (DMSP) satellites have provided weather data. And the Defense Satellite Communications System (DSCS) has provided secure transmission of voice and data. White Cloud is the name of a U.S. Navy surveillance satellite designed to intercept enemy communications.

Satellites are becoming increasingly valuable for navigation. The Global Positioning System (GPS) was originally developed for military use. A constellation of GPS satellites, called Navstar, has been launched since 1978; each Navstar satellite orbits the earth every 12 hours and continuously emits navigation signals. Military pilots and navigators use GPS signals to calculate their precise location, altitude, and velocity, as well as the current time. The GPS signals are remarkably accurate: Time can be figured to within a millionth of a second, velocity within a fraction of a kilometer or mile per hour, and location to within a few meters or feet. In addition to their military uses, slightly lower resolution versions of GPS receivers have been developed for civilian use in aircraft, ships, and land vehicles. Hikers, campers, and explorers carry hand-held GPS receivers, and some private passenger automobiles now come equipped with a GPS system.

Planetary Studies
Even as the United States and the USSR raced to explore the moon, both countries were also readying missions to travel further afield. Earth’s closest neighbors, Venus and Mars, became the first planets to be visited by spacecraft in the mid-1960s. By the close of the century, spacecraft had visited every planet in the solar system, except for the outermost planet—tiny, frigid Pluto.

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Mercury Only one spacecraft has visited the solar system's innermost planet, Mercury. The U.S. probe Mariner 10 (see Mariner) flew past Mercury on March 29, 1974, and sent back close-up pictures of a heavily cratered world resembling Earth’s moon. Mariner 10’s flyby also helped scientists refine measurements of the planet's size and density. It revealed that Mercury has a weak magnetic field but lacks an atmosphere. After the first flyby, Mariner 10's orbit brought it past Mercury for two more encounters, in September 1974 and March 1975, which added to the craft's harvest of data. In its three flybys, Mariner 10 photographed 57 percent of the planet's surface.

Venus
The U.S. Mariner 2 probe became the first successful interplanetary spacecraft when it flew past Venus on December 14, 1962. Mariner 2 carried no cameras, but it did send back valuable data regarding conditions beneath Venus's thick, cloudy atmosphere. From measurements by Mariner 2's sensors, scientists estimated the surface temperature to be 400° C (800° F—hot enough to melt lead), dispelling any notions that Venus might be very similar to Earth.

In 1974 NASA launched Mariner 10 toward a double encounter with Venus and Mercury. As it flew past Venus on February 5, 1974, Mariner 10's cameras took the first close-up images of Venus's clouds, including views in ultraviolet light that recorded distinct patterns in the circulation of Venus’s atmosphere.

The USSR explored Venus with their Venera series of probes. Venera 7 made the first successful planetary landing on December 15, 1970, and radioed 23 minutes of data from the Venusian surface, indicating a temperature of nearly 480° C (900° F) and an atmospheric pressure 90 times that on Earth. More Venera successes followed, and on October 22, 1975, Venera 9 landed and sent back black and white images of a rock-strewn plain—the first pictures of a planetary surface beyond Earth. Venera 10 sent back its own surface pictures three days later.

Beginning in 1978, a series of spacecraft examined Venus from orbit around the planet. These probes were equipped with radar that pierced the dense, cloudy atmosphere that hides Venus's surface, giving scientists a comprehensive, detailed look at the terrain beneath. The first of this series, the U.S. Pioneer Venus Orbiter (see Pioneer (spacecraft)), arrived in December 1978 and operated for almost 14 years. The spacecraft’s radar data were compiled into images that showed 93 percent of the planet's large-scale topographic features.

The Soviet Venera 15 and 16 orbiters reached Venus in October 1983, each equipped with radar systems that produced high-resolution images. In eight months of mapping operations, two spacecraft mapped much of Venus's northern hemisphere, sending back images of mountains, plains, craters, and what appeared to be volcanoes.

After being released from the space shuttle Atlantis, NASA's radar-equipped Magellan orbiter traveled through space and reached Venus in August 1990. During the next four years Magellan mapped Venus at very high resolution, providing detailed images of volcanoes and lava flows, craters, fractures, mountains, and other features. Magellan showed scientists that the surface of Venus is extremely well preserved and relatively young. It also revealed a history of planetwide volcanic activity that may be continuing today.

Mars
On July 14, 1965, the U.S. Mariner 4 flew past Mars and took pictures of a small portion of its surface, giving scientists their first close-up look at the Red Planet. To the disappointment of some who expected a more Earthlike world, Mariner's pictures showed cratered terrain resembling the moon's surface. In August 1969 Mariner 6 and 7 sent back more detailed views of craters and the planet's icy polar caps. On the whole, these pictures seemed to confirm the impression of a moonlike Mars.

NASA's Mariner 9 went into orbit around Mars in November 1971, providing scientists with the first close-up views of the entire planet. Mariner 9's pictures revealed giant volcanoes up to five times as high as Mount Everest, a system of canyons that would stretch the length of the continental United States, and—most intriguing of all—winding channels that resemble dry river valleys of Earth. Scientists realized that Mars's evolution had been more complex and fascinating than they had suspected and that the planet was moonlike in some ways, but surprisingly Earthlike in others.

The USSR's Mars probes were stymied by technical malfunctions. In November 1971 the Mars 2 spacecraft (see Mars (space program)) went into orbit around the planet and released a landing capsule that crashed without returning any data. Mars 2 became the first artificial object to reach the Martian surface. In December 1971 a lander released by the Mars 3 orbiter reached the surface successfully. However, it sent back only 20 seconds of video signals that included no data. In 1973 two more landing missions also failed. In 1988 the USSR made two unsuccessful attempts to explore the Martian moon Phobos. Contact with the spacecraft Phobos 1 (see Phobos (space program)) was lost due to an error by mission controllers when the spacecraft was on its way to Mars. Phobos 2 reached Martian orbit in January 1989 and sent back images of the planet, but failed before its planned rendezvous with Phobos.

The U.S. Viking probes made the first successful Mars landings in 1976. Two Viking spacecraft, each consisting of an orbiter and lander, left Earth in August and September 1975. Viking 1 went into orbit around Mars in June 1976, and after a lengthy search for a relatively smooth landing site, the Viking 1 lander touched down safely on Mars's Chryse Planitia (Plain of Gold) on July 20, 1976. The Viking 2 lander reached Mars's Utopia Planitia (Utopia Plain) on September 3, 1976. Each lander sent back close-up pictures of a dusty surface littered with rocks, under a surprisingly bright sky (due to sunlight reflecting off of airborne dust). The landers also recorded changes in atmospheric conditions at the surface. They searched, without success, for conclusive evidence of microbial life. The landers continued to send back data for several years, while the orbiters took thousands of high-resolution photographs of the planet.

On July 4, 1996, 21 years after Viking 1 arrived, NASA's Mars Pathfinder spacecraft landed in Mars's Ares Vallis (Mars Valley). Pathfinder used a new landing system featuring pressurized airbags to cushion its impact. The next day, Pathfinder released a 10-kg (22-lb) rover called Sojourner, which became the first wheeled vehicle to operate on another planetary surface. While Pathfinder sent back images, atmospheric measurements, and other data, Sojourner examined rocks and soil with a camera and an Alpha Proton X-ray Spectrometer, which provided data on chemical compositions by measuring how radiation bounced back from rocks and dust. The mission ended when the spacecraft ceased responding to commands from Earth in October 1997.

NASA's Mars Global Surveyor went into orbit around Mars in September 1997. Designed as a replacement for NASA's Mars Observer probe, which failed before reaching Mars in 1993, Mars Global Surveyor is equipped with a high-resolution camera, instruments to study the planet's atmosphere, topography and gravity, surface composition, and magnetic field. Global Surveyor reached orbit around Mars in the fall of 1997, but a problem with an unstable solar panel delayed the start of mapping for about a year. (In the meantime, Mars Global Surveyor began relaying high-resolution images of select areas in early 1998.) Its mapping operation was slated to begin in the spring of 1999 and to last for one Martian year (about two Earth years). Unlike previous Mars probes, Mars Global Surveyor adjusts its orbit using a technique called aerobraking, which relies on friction with the planet's upper atmosphere—rather than rocket engines—to slow the spacecraft to bring it into a proper mapping orbit.

Mars Pathfinder and Mars Global Surveyor were part of a series of spacecraft that NASA plans to send to Mars about every 18 months. Mars Surveyor 98, the next mission in the series, includes the Mars Climate Orbiter and the Mars Polar Lander, which are scheduled for launch in December 1998 and January 1999, respectively.

The Outer Planets
The giant gaseous world Jupiter, the solar system's largest planet, had its first visit from a spacecraft—Pioneer 10—on December 1, 1973. Pioneer 10 flew past Jupiter 21 months after launch and sent back images of the planet’s turbulent, multi-colored atmosphere. Pioneer 10 also investigated Jupiter's intense magnetic field, and the associated belts of trapped radiation. Acting like a sling-shot, Jupiter's powerful gravitational pull accelerated the spacecraft onto a new path that sent it out of the solar system. Pioneer 10 traveled beyond the orbit of Pluto in 1983.

Pioneer 11 made its own inspection of Jupiter, passing the planet on December 1, 1974. Like its predecessor, Pioneer 11 got a gravitational assist from Jupiter. In this case, the spacecraft was sent toward Saturn. Pioneer 11 reached this ringed giant on September 1, 1979, before heading out of the solar system. NASA maintained periodic contact with Pioneer 11 until November 1995, when the probe's power supply was almost exhausted.

In 1977 the twin Voyager 1 and 2 probes (see Voyager) were launched on the most ambitious space exploration missions yet attempted: a grand tour of the outer solar system. Voyager 1 reached Jupiter in March 1979 and sent back thousands of detailed images of the planet's cloud-swirled atmosphere and its family of moons. Other sensors probed the planet's atmosphere and its magnetic field. Voyager discovered that Jupiter is encircled by a tenuous ring of dust, and found three previously unknown moons. The most surprising discovery of the Voyager probes was that the Jovian moon Io is covered with active volcanoes spewing ice and sulfur compounds into space. Io was the first world other than Earth found to be geologically active.

Voyager 1 continued on to a rendezvous with Saturn in November 1980. Its images detailed a variety of complex and sometimes bizarre phenomena within the planet's rings. It also photographed the Saturnian moons, including planet-sized Titan. Voyager 1 found Titan’s surface obscured by a thick, opaque atmosphere of hydrocarbon smog.

Voyager 2 made its own flybys of Jupiter in July 1979 and of Saturn in August 1981. It continued outward to make the first spacecraft visits to Uranus in January 1986 and Neptune in August 1989. Like Pioneer 10 and 11, the Voyagers are now headed for interstellar space. On February 17, 1998, Voyager 1 became the most distant human-made object, reaching a distance of 10.5 billion km (6.5 billion mi) from Earth. Scientists hope it will continue sending back data well into the 21st century.

NASA's Galileo orbiter reached Jupiter in December 1995. The spacecraft deployed a probe that entered Jupiter's atmosphere on December 7, 1995, radioing data for 57 minutes before succumbing to intense pressures. The probe sent back the first measurements on the composition and structure of Jupiter's atmosphere from within the atmosphere. The Galileo spacecraft then began a long-term mission to study Jupiter's atmosphere, magnetosphere, and moons from an orbit around the planet. NASA expected the spacecraft to keep returning data until the end of 1999.

NASA’s Cassini spacecraft set out toward Saturn and Saturn’s moon Titan in October 1997. Cassini is scheduled to reach Saturn in 2004 and to release a probe into Titan’s atmosphere.

Other Solar System Missions
Aside from the planets and their moons, space missions have focused on a variety of other solar system objects. The sun, whose energy affects all other bodies in the solar system, has been the focus of many missions. Between and beyond the orbits of the planets, innumerable smaller bodies—asteroids and comets—also orbit the sun. All of these celestial objects hold mysteries, and spacecraft have been launched to unlock their secrets.

A number of the earliest satellites were launched to study the sun. Most of these were earth-orbiting satellites. The Soviet satellite Sputnik 2 (see Sputnik), launched in 1957 to become the second satellite in space, carried instruments to detect ultraviolet and X-ray radiation from the sun. Several of the satellites in the U.S. Pioneer series of the late 1950s through the 1970s gathered data on the sun and its effects on the interplanetary environment. A series of Earth-orbiting U.S. satellites, known as the Orbiting Solar Observatories (OSO), studied the sun's ultraviolet, X-ray, and gamma-ray radiation through an entire cycle of rising and falling solar activity from 1962 to 1978. Helios 2, a solar probe created by the United States and West Germany, was launched into a solar orbit in 1976 and ventured within 43 million km (27 million mi) of the sun. The U.S. Solar Maximum Mission spacecraft was designed to monitor solar flares and other solar activity during the period when sunspots were especially frequent. After suffering mechanical problems, in 1984 it became the first satellite to be repaired by astronauts aboard the space shuttle. The satellite Yohkoh, a joint effort of Japan, the United States, and Britain, was launched in 1991 to study high-energy radiation from solar flares. The Ulysses mission was created by NASA and the European Space Agency. Launched in 1990, the spacecraft used a gravitational assist from the planet Jupiter to fly over the poles of the sun. The European Space Agency launched the Solar and Heliospheric Observatory (SOHO) in 1995 to study the sun's internal structure, as well as its outer atmosphere (the corona), and the solar wind, the stream of subatomic particles emitted by the sun.

Asteroids are chunks of rock that vary in size from dust grains to tiny worlds, the largest of which is more than a third the size of Earth’s moon. These rocky bodies, composed of debris left over from the formation of the solar system, are among the latest solar system objects to be visited by spacecraft. The first such encounter was made by the Galileo spacecraft, which passed through the solar system's main asteroid belt on its way to Jupiter. Galileo flew within 1600 km (1000 mi) of the asteroid Gaspra on October 29, 1991. Galileo's images clearly showed Gaspra's irregular shape and a surface covered with impact craters. On August 28, 1993, Galileo passed close by the asteroid 243 Ida and discovered that it is orbited by another, smaller asteroid, subsequently named Dactyl. Ida is the first asteroid known to possess its own moon. On June 27, 1997, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft flew past asteroid 253 Mathilde. NEAR was scheduled to rendezvous with the asteroid Eros in February 1999 for a detailed study.

Comets are icy wanderers that populate the solar system's outermost reaches. These "dirty snowballs" are chunks of ice and dust. When a comet ventures into the inner solar system, some of its ices evaporate. The comet forms tails of dust and ionized gas, and many have been spectacular sights. Because they may contain the raw materials that formed the solar system, comets hold special fascination for astronomers. Although several comets have been observed by a variety of space-born instruments, only one has been visited by spacecraft. The most famous comet of all, Halley’s Comet, made its most recent passage through the inner solar system in 1986. In March 1986 five separate spacecraft flew past Halley, including the USSR’s Vega 1 and Vega 2 probes, the Giotto spacecraft of the European Space Agency, and Japan's Sakigake and Suisei probes. These encounters produced valuable data on the composition of the comet's gas and dust tails and its solid nucleus. Vega 1 and 2 returned the first close-up views ever taken of a comet’s nucleus, followed by more detailed images from Giotto. Giotto went on to make a close passage to Comet P/Grigg-Skjellerup on July 10, 1992.

Piloted Spaceflight
Piloted spaceflight presents even greater challenges than unpiloted missions. Nonetheless, the United States and the USSR made piloted flights the focus of their Cold War space race, knowing that astronauts and cosmonauts put a face on space exploration, enhancing its impact on the general public. The history of piloted spaceflight started with relatively simple missions, based in part on the technology developed for early unpiloted spacecraft. Longer and more complicated missions followed, crowned by the ambitious and successful U.S. Apollo missions to the moon. Since the Apollo program, piloted spaceflight has focused on extended missions aboard spacecraft in Earth orbit. These missions have placed an emphasis on scientific experimentation and work in space.

Vostok and Mercury
At the beginning of the 1960s, the United States and the USSR were competing to put the first human in space. The Soviets achieved that milestone on April 12, 1961, when a 27-year-old pilot named Yury Gagarin made a single orbit of Earth in a spacecraft called Vostok (East). Gagarin’s Vostok was launched by an R-7 booster, the same kind of rocket they had used to launch Sputnik. Although the Soviets portrayed Gagarin's 108-minute flight as flawless, historians have since learned that Vostok experienced a malfunction that caused it to tumble during the minutes before its reentry into the atmosphere. However, Gagarin parachuted to the ground unharmed after ejecting from the descending Vostok.

On May 5, 1961, the United States entered the era of piloted spaceflight with the mission of Alan Shepard. Shepard was launched by a Redstone booster on a 15-minute "hop" in a Mercury spacecraft named Freedom 7. Shepard’s flight purposely did not attain the necessary velocity to go into orbit. In February 1962, John Glenn became the first American to orbit Earth, logging five hours in space. His Mercury spacecraft, called Friendship 7, had been borne aloft by a powerful Atlas booster rocket. After his historic mission, the charismatic Glenn was celebrated as a national hero.

The Soviets followed Gagarin's flight with five more Vostok missions, including a flight of almost five days by Valery Bykovsky and the first spaceflight by a woman, Valentina Tereshkova, both in June 1963. By contrast, the longest of the six piloted Mercury flights was Gordon Cooper's 34-hour mission in May 1963.

By today's standards, Vostok and Mercury were simple spacecraft, though they were considered advanced at the time. Both were designed for the basic mission of keeping a single pilot alive in the vacuum of space and providing a safe means of return to the earth. Both were equipped with small thrusters that allowed the pilot to change the craft's orientation in space. There was no provision, however, for altering the craft's orbit—that capability would have to wait for the next generation of spacecraft. Compared to Mercury, Vostok was both roomier and more massive, weighing 2500 kg (5500 lb)—a reflection of the greater lifting power of the R-7 compared with the U.S. Redstone and Atlas rockets.

Voskhod and Gemini
In early 1961—just weeks after Shepard had become the first American in space—President John F. Kennedy challenged the nation with this ambitious goal: to land a man on the moon and return him safely to Earth by the end of the decade. With a total cost estimated at $25 billion in 1960s dollars, the Apollo program became a massive effort utilizing the combined energies of 400,000 people at NASA, other government and academic facilities, and aerospace contractors.

NASA realized, however, that it would not be possible to jump directly from the simple Mercury flights in Earth orbit to a lunar voyage. The agency needed an interim program to solve the unknowns of lunar flights. This became the Gemini program, a series of two-astronaut missions that took place in 1965 and 1966.

The Gemini missions were intended to develop and test the building blocks of a lunar flight. For instance, Gemini astronauts had to maneuver and dock two orbiting spacecraft, since astronauts would need to execute such a maneuver before and after landing on the moon. Gemini included long-duration spaceflights of a week or more—the amount of time necessary for a lunar landing flight—as well as space walks that demonstrated the ability of an astronaut to perform useful work in the vacuum of space, and controlled reentry into Earth’s atmosphere. The Gemini spacecraft had less than twice the crew space of Mercury, but it was far more capable. Gemini crews could change their orbits, and even use a rudimentary onboard computer to help control their craft. Gemini was also the first spacecraft to utilize fuel cells, devices that generated electrical power by combining hydrogen and oxygen.

At the same time, the USSR was preparing a new-generation of spacecraft for its own moon program. The Soviets staged a series of intermediate flights in a craft designated Voskhod (Sunrise). Described as a new spacecraft, Voskhod was actually a converted Vostok. In October 1964 Voskhod 1 carried three cosmonauts—the first multi-person space crew—into orbit for a day-long mission. By replacing the Vostok ejection seat with a set of crew couches, designers had made room for three cosmonauts to fly, without space suits, in a craft originally designed for one.

In March 1965, just weeks before Gemini's first piloted mission, Voskhod 2 carried two space-suited cosmonauts aloft. One of them, Alexei Leonov, became the first human to walk in space, remaining outside the craft for about 10 minutes. In the vacuum of space Leonov's suit ballooned dangerously, making it difficult for him to reenter the spacecraft. Voskhod 2 proved to be the last of the series. Further Voskhod flights had been planned, but they were canceled so that Soviet planners and engineers could concentrate on getting to the moon.

Ten piloted Gemini missions took place in 1965 and 1966, accomplishing all of the program's objectives. In March 1965 Gus Grissom and John Young made Gemini's piloted debut and became the first astronauts to alter their spacecraft's orbit. In June, Gemini 4's Ed White became the first American to walk in space. Gemini 5's Gordon Cooper and Pete Conrad captured the space endurance record with an eight-day mission. Gemini 7's Frank Borman and Jim Lovell stretched the record to 14 days in December 1965. During their flight they were visited by Gemini 6's Wally Schirra and Tom Stafford in the world's first space rendezvous. Neil Armstrong and Dave Scott succeeded in making the first space docking by mating Gemini 8 to an unpiloted Agena rocket in March 1966, but their flight was cut short by a nearly disastrous episode with a malfunctioning thruster. On Gemini 11 in September 1966, Pete Conrad and Dick Gordon reached a record altitude of 1370 km (850 mi). The final mission of the series, Gemini 12 in November 1966, saw Buzz Aldrin make a record five hours of space walks. At the conclusion of the Gemini program, the United States held a clear lead in the race to the moon.

Soyuz and Early Apollo
By 1967 the United States and the USSR were each preparing to test the spacecraft they planned to use for lunar missions. The Soviets had created Soyuz (Union), an Earth-orbiting version of the craft they hoped would fly cosmonauts to and from the moon. They were also at work on a Soyuz derivative for flights into lunar orbit, and a lunar lander that would ferry a single cosmonaut from lunar orbit to the moon's surface and back. Two parallel Soviet moon programs were proceeding—one to send cosmonauts around the moon in a loop that would form a figure-8, the other to make the lunar landing.

Meanwhile, the United States continued work on its Apollo spacecraft. Apollo featured a cone-shaped command module designed to transport a three-man crew to the moon and back. The command module was attached to a cylindrical service module that provided propulsion, electrical power, and other essentials. Attached to the other end of the service module was a spidery lunar module. The lunar module contained its own rocket engines to allow two astronauts to descend from lunar orbit to the moon's surface and then lift off back into lunar orbit. The lunar module consisted of two separate sections: a descent stage and an ascent stage. The descent stage housed a rocket engine for the trip down to the moon. The descent stage fit underneath the ascent stage, which included the crew cabin and a rocket for returning to lunar orbit. The astronauts rode to the surface of the moon in the ascent stage with the descent stage attached. The descent stage remained on the lunar surface when the astronauts fired the ascent rocket to return to orbit around the moon.

The year 1967 brought tragedy to both U.S. and Soviet moon programs. In January, the crew of the first piloted Apollo mission, Gus Grissom, Ed White, and Roger Chaffee, were killed when a flash fire swept through the cabin of their sealed Apollo command module during a pre-flight practice countdown. Subsequent investigation determined that frayed wiring probably provided a spark, and the high-pressure, all-oxygen atmosphere and flammable materials in the spacecraft created the devastating inferno. In April, the Soviets launched their new generation spacecraft, Soyuz 1, with Vladimir Komarov aboard. Consisting of three modules, only one of which was designed to return to the earth, Soyuz could carry a maximum of three cosmonauts. After a day in space Komarov was forced to end the flight because of problems orienting the craft. After reentering the atmosphere the Soyuz's parachute failed to deploy properly, and Komarov was killed when the spacecraft struck the ground.

By the end of 1967 NASA achieved a welcome success for Apollo with the first test launch of the giant Saturn V moon rocket, designed by a team headed by von Braun. Measuring 111 m (363 ft) in length (including the Apollo spacecraft), the three-stage Saturn V was the most powerful rocket ever successfully flown. Its five first-stage engines produced a combined thrust of 33 million newtons (7.5 million lb). The first Saturn V test flight, designated Apollo 4, took place in November 1967, and propelled an unpiloted Apollo command and service module to an altitude of 18,000 km (11,000 mi) before the spacecraft returned to the earth.

In October 1968 a redesigned, fireproof command module made its piloted debut as Wally Schirra, Donn Eisele, and Walt Cunningham reached Earth orbit in Apollo 7. During the 11-day test flight, the command and service modules checked out perfectly. Apollo 7's success paved the way for NASA to send the crew of Apollo 8, Frank Borman, Jim Lovell and Bill Anders, on the first voyage to the moon. Borman's crew became the first men to ride the Saturn V booster on December 21, 1968. About two hours after launch, the Saturn's third stage engine re-ignited to send Apollo 8 speeding moonward at 40,000 km/h (25,000 mph). Some 66 hours later, on December 24, 1968, they reached the moon and fired Apollo 8's main rocket engine to go into lunar orbit. They spent the next 20 hours circling the moon ten times, taking photographs, making navigation sightings on lunar landmarks, and beaming live television pictures back to Earth. Just after midnight on December 25, the astronauts fired the service module's main rocket engine to blast out of lunar orbit and onto a course for Earth. After a fiery reentry, the heat-shielded command module splashed down in the Pacific Ocean on December 27.

The Soviets, meanwhile, flew a successful piloted Soyuz mission in October 1968. Soyuz 3 carried cosmonaut Georgi Bergovoi in orbit around Earth for four days. The USSR also sent two Zond craft, specially designed for missions around the moon, on unpiloted flights around the moon and back to Earth. Zond spacecraft were modified Soyuz craft. A pair of cosmonauts prepared for their own mission around the moon in early December 1968, just ahead of Apollo 8. But concern over problems on the unpiloted Zond flights caused Soviet mission planners to postpone the attempt, and the flight never took place. Apollo 8 was not only a triumph for NASA—it also proved to be the decisive event in the moon race.

Humans on the Moon
Having sent astronauts into lunar orbit and back to Earth, NASA faced even more daunting hurdles to achieve Kennedy's challenge for a moon landing before the end of the 1960s. Apollo 9 in March 1969 tested the entire Apollo spacecraft, including the lunar module, in Earth orbit. In May 1969, Apollo 10 carried out a dress rehearsal of the landing mission, with the command and service modules and lunar module in lunar orbit. With these crucial milestones accomplished, the way was clear to attempt the lunar landing itself. On July 16, 1969, the crew of Apollo 11—Neil Armstrong, Mike Collins, and Buzz Aldrin—headed for the moon to attempt the lunar landing.

On July 20, while in lunar orbit, Armstrong and Aldrin passed through a connecting tunnel from the command module, Columbia, to the attached lunar module, named Eagle. They then undocked, leaving Collins in orbit, alone in Columbia, 111 km (69 mi) above the moon. After shifting the low point of their orbit to 15,000 m (50,000 ft), Armstrong and Aldrin fired Eagle's descent rocket to slow the craft into its final descent to the moon's Mare Tranquilatis (Sea of Tranquillity). An overloaded onboard computer threatened to abort the landing, but swift action by experts in mission control allowed the men to continue. Armstrong was forced to take over manual control when he realized that Eagle was heading for a football-field-size crater ringed with boulders. He brought Eagle to a safe touchdown with less than a minute’s worth of fuel remaining before a mandatory abort. "Houston," Armstrong radioed, "Tranquillity Base here. The Eagle has landed."

Hours later, Armstrong and Aldrin were sealed inside their space suits, ready to begin history's first moonwalk. At 10:56 PM Eastern Daylight Time, Armstrong stood on Eagle's footpad and placed his left boot on the powdery lunar surface—the first human footstep on another world. Armstrong’s famous first words on the moon were, "That's one small step for man, one giant leap for mankind." (He had intended to say "That’s one small step for a man, one giant leap for mankind," and that is how the quote is worded in many accounts of the event.) Aldrin followed Armstrong to the surface 40 minutes later. During the moonwalk, which lasted about two and a half hours, the men collected rocks, took photographs, planted the American flag, and deployed a pair of scientific experiments. Their landing site, a cratered plain strewn with rocks, proved to have "a stark beauty all its own," in Armstrong's words. Aldrin called the appearance of the lunar surface "magnificent desolation."

Inside Eagle once more, Armstrong and Aldrin tried unsuccessfully to get a good night's sleep. On July 21, after a total of 21½ hours on the moon, they fired Eagle's ascent engine and rejoined Collins in lunar orbit. On July 24, after a flawless mission, Armstrong, Aldrin, and Collins returned to the earth, carrying 22 kg (48 lb) of lunar rock and soil. Kennedy's challenge had been met with months to spare, and NASA had shown that humans were capable of leaving their home world and traveling to another.

Six more lunar landing attempts followed Apollo 11. All but one of these missions were successful. In November 1969 Pete Conrad and Alan Bean made history's first pinpoint landing on the moon, touching down less than 200 m (less than 600 ft) from the robotic Surveyor 3 probe, which had been on the moon since April 1967 (see Surveyor (spacecraft)). In their 31½ hours on the moon, Conrad and Bean made two moonwalks and collected 34 kg (76 lb) of samples.

In April 1970 Apollo 13 almost ended tragically when an oxygen tank inside the service module exploded. The spacecraft was 300,000 km (200,000 mi) from Earth. The accident left the command and service modules without propulsion or electrical power. Astronauts Jim Lovell, Jack Swigert, and Fred Haise struggled to return to Earth using their attached lunar module as a lifeboat, while experts in mission control worked out emergency procedures to bring the men home. Although the mission failed in its objective to land in the moon's Fra Mauro highlands, Apollo 13 was an extraordinary demonstration of the Apollo team's ability to solve problems during a spaceflight. The mission's goals were achieved in February 1971 by Apollo 14 astronauts Alan Shepard, Stu Roosa and Ed Mitchell.

Lunar exploration entered a more ambitious phase with Apollo 15 in July 1971, when Dave Scott and Jim Irwin landed at the base of the moon's Apennine mountains. Their lunar module had been upgraded to allow a stay of nearly three days on the lunar surface. Improved space suits allowed the men to take three moonwalks, the longest of which lasted more than seven hours. They also brought along a battery-powered car called the Lunar Rover. With the rover, the astronauts ranged for miles across the landscape, even driving partway up the side of a lunar mountain. They picked up some of the oldest rocks ever found on the moon, including one fragment that proved to be 4.5 billion years old, almost the calculated age of the moon itself.

Two more lunar landings followed before budget cuts ended the Apollo program. The final team of lunar explorers were Apollo 17's Gene Cernan, a former Navy fighter pilot, and Harrison "Jack" Schmitt, a geologist-astronaut who became the first scientist to reach the moon. They explored the moon's Taurus-Littrow valley while crewmate Ron Evans orbited overhead. During three days on the moon, Cernan and Schmitt collected 110 kg (243 lb) of samples, including an orange soil that gave new clues to the moon's ancient volcanic activity.

While the Apollo program racked up successes, the Soviet lunar program was plagued by setbacks. The Soviets built a moon rocket of their own, the giant N-1 booster, which was designed to produce 44 million newtons (10 million lb) of thrust at liftoff. In four separate test launches between 1969 and 1972, the N-1 exploded within seconds or minutes after liftoff. Combined with the U.S. Apollo successes, the N-1 failures ended hopes of a Soviet piloted lunar landing.

Salyut Space Stations Even before the first human space flights, planners in the United States and the USSR envisioned space stations in orbit around the earth. The Soviets stepped up their efforts toward this goal when it became clear they would not win the moon race. In April 1971 they succeeded in launching the first space station, Salyut 1 (see Salyut). The name Salyut, which means "salute," was meant as a tribute to cosmonaut Yury Gagarin, the first person in space. Gagarin had been killed in the crash of a jet fighter during a routine training flight in 1968. Salyut consisted of a single module weighing 19 metric tons that offered 100 cu m (3500 cu ft) of living space. Cosmonauts traveled between the earth and the Salyut stations in Soyuz spacecraft. In June 1971 cosmonauts Georgi Dobrovolski, Vladislav Volkov, and Viktor Patsayev occupied Salyut for 23 days, setting a new record for the longest human space flight. Tragically, the three men died when their Soyuz ferry craft developed a leak before they reentered the atmosphere. The leak allowed the oxygen in the cabin to escape, suffocating the cosmonauts. The Soyuz returned to Earth under automatic control.

Six more Salyut stations reached orbit between 1974 and 1982. Two of these, Salyuts 3 and 5, were military stations equipped with high-resolution cameras to gather military information from orbit. Salyuts 6 and 7 served as orbital homes to cosmonauts during record-breaking space marathons. In 1980 Salyut 6 cosmonauts Leonid Popov and Valerie Ryumin logged a record 185 days in space. (Remarkably, Ryumin had spent 175 days aboard Salyut 6 during the previous year.) The longest mission to Salyut 7 was also a record-breaker, lasting 237 days—nearly eight months—in space. In 1985 Salyut 7's electrical system failed, forcing a team of cosmonauts to stage a repair mission to bring the stricken station back to life. In mid-1986, after two more crews had visited the station, Salyut 7 was abandoned for good.

The Salyut cosmonauts pushed frontiers of long-duration space flight, often with considerable difficulty. In addition to the medical effects of long-term exposure to weightlessness—including muscle atrophy, loss of bone minerals, and cardiovascular weakness—long-duration spaceflight can cause the psychological stresses of boredom and isolation, occasionally relieved by visits by new teams of cosmonauts. Supplies and gifts brought up by unpiloted versions of Soyuz spacecraft called Progress freighters also provided novelty and relief. The Salyut marathons paved the way for even longer stays aboard the space station Mir.

Skylab Space Station
Skylab, the first U.S. space station, utilized hardware originally created for the Apollo program. The main component, called the orbital workshop, was constructed inside the third stage of a Saturn V booster. It contained living and working space for three astronauts. Attached to the orbital workshop were the Apollo telescope mount (ATM), a collection of instruments to study the sun from space; an airlock module to enable two of the astronauts to make space walks while the third remained inside; and a multiple docking adaptor (MDA) for use by the Apollo spacecraft that would ferry the crew to and from orbit. Altogether, Skylab weighed 91 metric tons and offered 210 cu m (7400 cu ft) of habitable space.

Skylab's mission almost ended with its launch in May 1973. A design flaw caused the station's meteoroid shield to be torn off during launch, severing one of two winglike solar panels that were to convert sunlight to electricity for the space station. Mission controllers quickly went to work on a rescue plan that could be carried out by the first team of Skylab astronauts—Pete Conrad, Joe Kerwin, and Paul Weitz. After reaching the station in late May aboard an Apollo spacecraft, Conrad's crew installed a sunshield to cool the soaring temperatures inside the station. In a space walk repair effort, Conrad and Kerwin restored the necessary electric power by freeing the remaining solar wing, which had failed to deploy properly. The astronauts also conducted medical tests, made observations of the sun and earth, and performed a variety of experiments. Their 28-day mission broke the endurance record that had been set by the Salyut 1 crew two years before. Two more teams of astronauts reached Skylab in 1973, logging 56 and 84 days in space, respectively. The three Skylab missions gave U.S. researchers valuable information on human response to long-duration space flight.

Skylab was not designed to be resupplied, and by the late 1970s its orbit had decayed badly. Friction with gas molecules in the outer atmosphere had caused the spacecraft to lose altitude and speed, and controllers calculated that it would fall out of orbit by the end of the decade. Tentative plans to use the space shuttle to boost the station into a stable orbit did not come to pass—the shuttle was still in development when Skylab met its fiery end, breaking up during reentry in July 1979. Debris from Skylab landed in the Indian Ocean and in remote areas of Australia.

Mir Space Station
In 1986 the USSR launched the core of the first space station to be composed of distinct units, or modules. This modular space station was named Mir (Peace). Over the next ten years additional modules were launched and added to the station. The first of these, called Kvant, contained telescopes for astronomical observations and reached the station in April, 1987. Another module, called Krystal, was devoted to experiments in processing materials in zero gravity. In 1996 Prioda, the last module, was added, bringing Mir's total habitable volume to about 380 cu m (about 13,600 cu ft).

Cosmonauts have lived aboard Mir even longer than their Salyut predecessors did. In 1987 and 1988 Mir cosmonauts Vladimir Titov and Musa Manarov achieved the first year-long mission. In 1995 physician-cosmonaut Valeriy Polyakov completed a record 14 months aboard the station. Such long-duration missions have helped researchers understand the problems posed by lengthy stays in space—information vital to planning for piloted interplanetary voyages.

Beginning in 1995 Mir was the scene of joint U.S.-Russian missions. (Russia had taken over the Soviet space program after the collapse of the USSR in 1991.) The joint missions were to pave the way for the planned International Space Station (ISS; discussed below). U.S. space shuttles docked with Mir nine times, and seven U.S. astronauts lived aboard Mir for extended periods. One of them, Shannon Lucid, set the U.S. space flight endurance record of 188 days in 1996.

By 1997 the 11-year-old Mir was experiencing a series of calamities that included computer failures, an onboard fire, and a collision with an unpiloted Progress spacecraft during a rendezvous exercise. Subsequent repair missions returned the station to a relatively normal level of functioning. The Russian Space Agency plans to abandon Mir and cause it to reenter the earth’s atmosphere in 1999.

International Space Station One of NASA's most cherished goals was to build a permanent, Earth-orbiting space station. Although it received approval from President Ronald Reagan in 1984, the space station project (designated Space Station Freedom) faced huge political and budgetary hurdles. In 1993, after several redesign efforts by NASA, the station was reshaped into an international venture and redesignated the International Space Station. In addition to the United States, many other nations have joined the project. Russia, Japan, Canada, and the European Space Agency are producing hardware for the station. Launch of the first element, a Russian built Functional Cargo Block (known by its Russian acronym, FGB), was scheduled to occur before the end of 1998. The FGB will provide the power and propulsion needed during the ISS’s assembly. Once the ISS is complete, the FGB will be used mostly for storage. The first habitable part of the ISS—the Russian-made service module—was scheduled for launch sometime in 1999. Planned for completion in 2003, the ISS is designed to be continuously occupied by up to seven crewmembers. It is envisioned as a world-class research facility, where scientists can study Earth and the heavens, as well as exploring the medical effects of long-duration space flight, the behavior of materials in a weightless environment, and the practicality of space manufacturing techniques.

Space Shuttles
Even before the Apollo moon landings, NASA's long-term plans included a reusable space shuttle to ferry astronauts and cargo to and from an Earth-orbiting space station. Agency planners had hoped to pursue both the station and the shuttle during the 1970s, but in 1972, Congress approved funding only for the shuttle. With the orbiting space station on hold, NASA had to reevaluate the role of the shuttle. The agency came to envision the shuttle both as a "space truck" that could deploy and retrieve satellites and as a platform for scientific observations and experiments in space.

The space shuttle consists of three main components: an orbiter, an external fuel tank, and two solid rocket boosters. The winged orbiter contains the crew cabin, three liquid-fuel rocket engines for use during launch, and a 20-m (60-ft) long cargo bay. Overall, the orbiter is the size of a medium-sized passenger jet airplane. It is controlled by five onboard computers and is covered with thousands of heat-resistant silica tiles to protect it during the fiery reentry into Earth’s atmosphere. Following reentry the orbiter becomes an unpowered glider, and the shuttle's commander steers it to a landing on a runway. Five shuttle orbiters were built. The first one, named Enterprise, never flew in space, but was used for a series of approach and landing tests in 1977.

The shuttle’s other two components help the shuttle reach orbit. The external tank, which is the size of a grain silo, is attached to the orbiter during launch and provides fuel for its engines. The tank is discarded once the shuttle reaches orbit. The paired giant solid rocket boosters, attached to the external tank, provide additional thrust during the first two minutes of launch. After that, they fall away and are recovered in the ocean to be refurbished and reused.

On April 12, 1981—exactly 20 years after Gagarin's pioneering flight as the first human in space—the orbiter Columbia flew a near-perfect maiden voyage. Veteran astronaut John Young and first-time astronaut Robert Crippen piloted Columbia on the two-day mission, ending with a flawless landing on a dry-lakebed runway at California's Edwards Air Force Base. Three more qualifying flights followed, and in July 1984 the shuttle was declared operational. Over the next 17 months, 20 more shuttle missions, with crews of up to eight astronauts, racked up a string of accomplishments. Shuttle astronauts deployed and retrieved satellites using the orbiter's remote manipulator arm. In space walks, astronauts repaired ailing satellites; they also tested the Manned Maneuvering Unit, a self-contained flying machine with thrusters that use compressed nitrogen. They conducted a variety of scientific and medical research missions in a module called Spacelab, which was stored in the orbiter's cargo bay.

NASA had hoped that the reusability of the shuttle would make getting into space less expensive. The space agency expected that private companies would pay to have their satellites launched from the shuttle, which would provide a cost-effective alternative to launching by a conventional, "throw-away" rocket. However, the costs of developing and operating the shuttle proved enormous, and NASA found it was still a long way from reducing the cost of reaching Earth orbit. To offset these costs, the agency pushed for more frequent launches—in 1986 they hoped to launch 24 missions per year.

Then, on January 28, 1986, disaster struck. The shuttle Challenger exploded 73 seconds after liftoff, killing its seven-member crew, which included schoolteacher Christa McAuliffe (see Challenger Disaster). The tragedy shocked the nation and brought the shuttle program to a halt while a presidential commission tried to determine what had gone wrong. The Challenger disaster was traced to a faulty seal in one of the solid rocket boosters, and to faulty decision making by NASA and some of the contractors who manufacture shuttle components. After making several safety modifications, shuttle flights resumed in 1988.

Soviet officials viewed the U.S. program with some trepidation, fearing that the shuttle would be used for military offensives against the USSR. Partly in response, they built a heavy-lift booster called Energia, and a space shuttle called Buran ("snowstorm"). The Buran/Energia combination made only a single unpiloted, orbital test flight in November 1988. Unlike its U.S. counterpart, ground controllers could operate the Soviet shuttle remotely. Buran was far from ready to support piloted flight, and economic problems caused by the collapse of the USSR in 1991 ended the Buran program prematurely.

Beginning in 1995, the shuttle flew a series of missions to the Russian space station Mir. In 1998 the shuttle was scheduled to begin assembly of the International Space Station. The shuttle program's 100th mission is slated to take place before the end of 1999, and shuttle orbiters are expected to keep flying during the first decades of the 21st century.

SCIENCE OF SPACE EXPLORATION
Space is a harsh environment for humans and human-made machines. Radiation from the sun and other cosmic sources can weaken material and harm the human body. In the vacuum of space, objects become boiling hot when exposed to the sun and freezing cold when in the shadow of Earth or some other body. Scientists, engineers, and designers must make spacecraft that can withstand these extreme conditions and more.

General Principles of Spacecraft Design The challenges that spacecraft designers face are daunting. Each component of a spacecraft must be durable enough to withstand the vibrations of launch, and reliable enough to function in space on time spans ranging from days to years. At the same time, the spacecraft must also be as lightweight as possible to reduce the amount of fuel required to boost it into space. Materials such as Mylar (a metal-coated plastic) and graphite epoxy (a construction material that is strong but lightweight) have helped designers and manufactures meet the requirements of durability, reliability, and lightness. Spacecraft designers also conserve space and weight by using miniaturized electronic components; in fact, the space program has fueled many advances in the field of miniaturization.

Since the early 1990s, budgetary restrictions have motivated NASA to plan projects that are better, faster, and cheaper. In this approach, space missions requiring single large, complex, and expensive spacecraft are replaced with more limited missions using smaller, less expensive craft. Although this new approach has been successful with such spacecraft as the Mars Pathfinder lander, it is more difficult to apply to piloted spacecraft, in which the overriding concern is crew safety. However, engineers are always looking for new technologies to make spacecraft lighter and less expensive.

Getting into Space
One of the most difficult parts of any space voyage is the launch. During launch, the craft must attain sufficient speed and altitude to reach Earth orbit or to leave Earth’s gravity entirely and embark on a path between planets. Scientists sometimes find it helpful to think of Earth's gravitational field as a deep well, with sides that are steepest near the planet's surface. The task of the launch vehicle or booster rocket is to climb out of this well.

Although some launch vehicles consist of just a single rocket, many are composed of a series of individual rockets, or stages, stacked atop one another. Such multistage launch vehicles are used especially for heavier payloads. With a multistage rocket, each stage fires for a period of time and then falls away when its fuel supply is used up. This lightens the load carried by the remaining stages. In some liquid-fuel boosters, strap-on solid-fuel rockets are used to provide extra thrust during the initial portion of ascent. For example, the Titan III booster has two liquid-fuel core stages and two strap-on solid-fuel motors. The largest example of a successful multistage booster was the Saturn V moon rocket, which had three liquid-fuel stages and measured 111 m (363 ft), including the Apollo spacecraft, in length.

Despite their utility, most multistage boosters are not reusable, which makes them expensive. Cost-conscious engineers have focused on creating a single-stage-to-orbit (SSTO) vehicle. In an SSTO, the entire spacecraft and booster would be integrated into one fully reusable unit. If successful, this approach would reduce the costs of reaching Earth orbit. However, the technical challenge is enormous: A full 89 percent of an SSTO's total weight must be reserved for fuel, a much higher proportion than any previous launch vehicle. The payload, the crew, and the weight of the vehicle itself must make up only 11 percent of the SSTO’s total weight.

Navigation in Space Spaceflight requires very detailed planning and measurement to get a spacecraft into place or to send it on its proper path. Some of the Apollo spacecraft were able to travel from Earth to the moon (a distance of almost 390,000 km, or almost 240,000 mi) and land on the lunar surface within a few dozen meters (several dozen feet) of their target. Careful planning allowed the Mars Pathfinder spacecraft to fly from Earth to Mars, travelling more than 500 million km (more than 300 million mi), and land just 19 km (12 mi) from the center of its target area.

Flight Paths

To launch a spacecraft into orbit around Earth, a booster rocket must do two things. First it must raise the spacecraft above the atmosphere—roughly 160 km (100 miles) or more. Second it must accelerate the spacecraft until its forward speed—that is, its speed parallel to Earth’s surface—is at least 28,200 km/h (17,500 mph). This is the speed, called orbital velocity, at which the momentum of the spacecraft is strong enough to counteract the force of gravity. Gravity and the spacecraft’s momentum balance so that the spacecraft does not fall straight down or move straight ahead—instead it follows a curved path that mimics the curve of the planet itself. The spacecraft is still falling, as any object does when it is released in a gravitational field. But instead of falling toward Earth, it falls around it. See Orbit.

Using its own thrusters, a spacecraft can raise or lower its orbit by adding or removing energy, respectively. To add energy, the spacecraft orients itself and fires its thrusters so that its accelerates in its direction of flight. To subtract energy, the craft fires its engines against the direction of flight. Any change in the height of a spacecraft's orbit also produces a change in its speed and vice versa. The craft moves more swiftly in a higher orbit than it does in a lower one. By firing its rockets perpendicular to the plane of its orbit, the craft can change the orientation of its orbit in space.

To travel from one planet to another, a spacecraft must follow a precise path, or trajectory, through space. The amount of energy that a spacecraft’s launch rocket and onboard thrusters must provide varies with the type of trajectory. The trajectory that requires the least amount of energy is called a Hohmann transfer. A Hohmann transfer follows the shape of an ellipse, or a flattened circle, whose sides just touch the orbits of the two planets.

The trajectory must also take into account the motion of the planets around the sun. For example, a probe travelling from Earth to Mars must aim for where Mars will be at the time of the spacecraft's arrival, not where Mars is at the time of launch.

In many interplanetary missions, a spacecraft flies past a third planet and uses the planet's gravitational field to bend the craft's trajectory and accelerate it toward its target planet. This is known as a gravitational slingshot maneuver. The first spacecraft to use this technique was the Mariner 10 probe (see Mariner), which flew past Venus on its way to Mercury in 1974.

Navigation and Guidance Most spacecraft depend on a combination of internal automatic systems and commands from ground controllers to keep on the correct path. Normally, ground controllers can communicate with a spacecraft only when it is within sight of an Earth-based receiving station. This poses problems for spacecraft in low Earth orbit—that is, within 2000 km (1200 mi) of the planet’s surface—as such craft are only within sight of a relatively small portion of the globe at any given moment. One way around this restriction is to place special satellites in orbit to act as relays between the orbiting spacecraft and ground stations, allowing continuous communications. NASA has done this for the U.S. space shuttle with the Tracking and Data Relay Satellite System (TDRSS).

At an altitude of about 35,800 km (about 22,200 mi), a satellite's motion exactly matches the speed of the earth's rotation. As a result, the satellite appears to hover over a specific spot on the earth's surface. This so-called stationary, geosynchronous, orbit is ideal for communications satellites, whose job is to relay information between widely separated points on the globe.

Spacecraft on interplanetary trajectories may travel millions or even billions of kilometers or miles from the earth. In these cases their radio signals are so weak that giant receiving stations are necessary to detect them. The largest stations have antenna dishes in excess of 70 m (230 ft) across. NASA and the Jet Propulsion Laboratory operate the Deep Space Network, a system of three tracking stations with several antennas each. The stations are in California, Spain, and Australia, providing continuous contact with distant spacecraft as Earth spins on its axis.

Much of the work of ground controllers involves monitoring a spacecraft's health and flight path. Using a process called telemetry, a spacecraft can transmit data about the functioning of its internal components. In addition, engineers can use a spacecraft's radio signals to assess its flight path. This is possible because of the Doppler effect. Because of the Doppler effect, a spacecraft's motion causes tiny shifts in the frequency of its radio signals—just as the motion of a passing car causes the apparent pitch of its horn to go up as the car approaches an observer and down as the car moves away. By analyzing Doppler shifts in a spacecraft's radio signals, controllers can determine the craft’s speed and direction. Over time, controllers can combine the Doppler shift data with data on the spacecraft’s position in the sky to produce an accurate picture of the craft’s path through space.

The guidance system helps control the craft's orientation in space and its flight path. In the early days of spaceflight, guidance was accomplished by means of radio signals from Earth. The Mercury spacecraft and its Atlas booster utilized such radio guidance signals broadcast from ground stations. During launch, for example, the Atlas received steering commands that it used to adjust the direction of its engines. However, Mercury flight controllers found that radio guidance was limited in accuracy because interference with the atmosphere tends to make the signals weaker.

Beginning with Gemini, engineers used a system called inertial guidance to stabilize rockets and spacecraft. This system takes advantage of the tendency of a spinning gyroscope to remain in the same orientation. A gyroscope mounted on a set of gimbals, or a mechanism that allows it to move freely, can maintain its orientation even if the spacecraft's orientation changes. An inertial guidance system contains several gyroscopes, each oriented along a different axis. When the spacecraft rotates along one or more of its axes, measuring devices tell how far it has turned from the gyroscopes' own orientations. In this way, the gyroscopes provide a constant reference by which to judge the craft's orientation in space. Signals from the guidance system are fed into the spacecraft's onboard computer, which uses this information to control the craft's maneuvers.

The Global Positioning System satellites, which enable ships, airplanes, and even hikers to know his or her position with extreme accuracy, should play a similar role in spacecraft. The space shuttle Atlantis was scheduled to be equipped with GPS receivers during an upgrade in late 1998.

Propulsion Once in orbit, a spacecraft relies on its own rocket engines to change its orientation (or attitude) in space, the shape or orientation of its orbit, and its altitude. Of these three tasks, changes in orientation require the least energy. Relatively small rockets called thrusters control a spacecraft’s attitude. In a massive spacecraft, the attitude control thrusters may be full-fledged liquid-fuel rockets. Smaller spacecraft often use jets of compressed gas. Depending on which combination of thrusters is fired, the spacecraft turns on one or more of its three principal axes: roll, pitch, and yaw. Roll is a spacecraft’s rotation around its longitudinal axis, the horizontal axis that runs from front to rear. (In the case of the space shuttle orbiter, a roll maneuver resembles the motion of an airplane dipping its wing.) Pitch is rotation around the craft’s lateral axis, the horizontal axis that runs from side to side. (On the shuttle, a pitch maneuver resembles an airplane raising or lowering its nose.) Yaw is a spacecraft’s rotation around a vertical axis. (A space shuttle executing a yaw maneuver would appear to be sitting on a plane that is turning to the left or right.) A change in attitude might be required to point a scientific instrument at a particular target, to prepare a spacecraft for an upcoming maneuver in space, or to line the craft up for docking with another spacecraft.

When an orbiting spacecraft needs to drop out of orbit and descend to the surface, it must slow down to a speed less than orbital velocity. The craft slows down by using retrorockets in a process called a deorbit maneuver. On early piloted spacecraft, retrorockets used solid fuel because solid-fueled rockets were generally more reliable than liquid-fueled rockets. Vehicles such as the Apollo spacecraft and the space shuttle have used liquid-fueled retrorockets. In the deorbit maneuver, the retrorocket acts as a brake by firing into the line of flight. The duration of the firing is carefully controlled, because it will affect the path that the spacecraft takes into the atmosphere. The same technique has been used by Apollo lunar modules and by unpiloted planetary landers to leave orbit and head for a planet's surface.

Power Supply
Spacecraft have used a variety of technologies to provide electrical power for running onboard systems. Engineers have used batteries and solar panels since the early days of space exploration. Often, spacecraft use a combination of the two: Solar panels provide power while the spacecraft is in sunlight, and batteries take over during orbital night. The solar panels also recharge the batteries, so the craft has an ongoing source of power. However, solar panels are impractical for many interplanetary spacecraft, which may travel vast distances from the sun. Many of these craft have relied on thermonuclear electric generators, which create power from the decay of radioactive isotopes and have lifetimes measured in years or even decades. The twin Voyager spacecraft, which explored the outer solar system, used generators such as these. Thermonuclear electric generators are controversial because they carry radioactive substances. The radioactivity poses no danger once the spacecraft reaches space, but some people worry that an accident during launch or during an unplanned reentry into Earth’s atmosphere could release harmful radiation into the atmosphere. Concerned groups protested the 1997 launch of the Cassini spacecraft, which carried its radioactive material in explosion-proof graphite containers.

Effects of Space Travel on Humans
Space is a hostile environment for humans. Piloted spacecraft must supply oxygen, food, and water for their occupants. For longer flights, a spacecraft must provide a way to dispose of or recycle wastes. For very long flights, spacecraft will eventually have to become almost totally self-sufficient. For healthy spaceflight, the spacecraft must provide far more than just the core physical needs of astronauts. Exercise equipment, comfortable sleeping and recreation areas, and well-designed work areas are some of the amenities that soften spaceflight’s effects on humans.

Crew Support The effort to save weight is so inherent to spacecraft design that it even affects the food supply. Much of the food eaten by astronauts is dehydrated to save both weight and space. In space, astronauts use a device like a water gun to rehydrate these items. Many food items are also carried in conventional form, ranging from bread to candy to fruit.

On many spacecraft, including the U.S. space shuttle, drinkable water is produced by fuel cells that also provide electrical power. The reaction between hydrogen and oxygen that creates electricity produces water as a byproduct. A small supply of water for emergency use is also carried in onboard storage tanks.

For very long-duration missions aboard space stations, water is recycled. Drinkable water can be extracted from a combination of waste water, urine, and moisture from the cabin atmosphere. This kind of system has been used on the Mir space station, and is planned for the International Space Station. See also Space Station.

Perhaps the question most frequently asked of astronauts is, "How do you go to the bathroom in space?" The answer has changed over the years. On early missions such as Mercury, Gemini, and Apollo, the bathroom facilities were relatively crude. For urine collection, the astronauts, all of whom were men, used a hose with a condomlike fitting at one end. Urine was then dumped overboard. Feces were collected in plastic bags and brought back to Earth for medical analyses. The Skylab space station featured a toilet that used forced air for suction. Mir and the space shuttle use similar toilets, with special fittings for men and women.

Skylab was also the first spacecraft to offer astronauts the chance to bathe in space, by means of a collapsible shower. To prevent globs of water from escaping and floating around inside the cabin, the astronaut sealed the shower once inside. The astronaut used a hand-held nozzle to dispense water, and a small vacuum to remove it. On the space shuttle and on Mir, astronauts and cosmonauts have had to make do with sponge baths. (Mir’s shower malfunctioned and was removed.) The International Space Station will have a shower in its habitation module.

Most piloted spacecraft have carried oxygen in onboard tanks in liquid form at cryogenic (super-cold) temperatures to save space. Liquid oxygen is about 800 times smaller in volume than gaseous oxygen at everyday temperatures. The Russian Mir space station has used an additional source of oxygen: Special generators aboard Mir separate water into oxygen and hydrogen, and the hydrogen is vented overboard.

On Mercury, Gemini, and Apollo, the cabin atmosphere was pure oxygen at about 0.3 kg/cm2 (about 5 lb/sq in). On the space shuttle and on the Mir space station a mixture of oxygen and nitrogen provides a pressure of 1.01 kg/cm2 (14.5 lb/sq in), slightly less than sea level atmospheric pressure on Earth. Shuttle astronauts who go on space walks must pre-breathe pure oxygen to purge nitrogen from their bloodstream. This eliminates the risk of decompression sickness, called the bends, because the shuttle space suit operates at a lower pressure (0.30 kg/cm2 or 4.3 lb/sq in) than inside the cabin. Sudden decompression can cause nitrogen bubbles to form in blood and tissues, a painful and potentially lethal condition. Plans for the International Space Station call for an oxygen-nitrogen atmosphere at a pressure similar to that in the shuttle.

In the past, astronauts on missions of a few days or less have often worked long hours. Some found that their need for sleep was reduced because of the minimal exertion required to move around in microgravity. However, the intense concentration required to complete busy flight plans can be tiring. On longer missions, proper rest is essential to the crew's performance. Even on the moon, astronauts on extended exploration missions—with surface stay times of three days—knew that they could not afford to go without a good night's sleep. Redesigned space suits, which were easier to take off and put on, and hammocks that were strung across the lunar module cabin helped the moon explorers get their rest.

On the Skylab space station, each astronaut had a small sleeping compartment with a sleeping restraint attached to the wall. On Mir, cosmonauts and astronauts have sometimes taken their sleeping bags and moved them to favorite locations inside one module or another. The International Space Station, like Skylab, will have private sleeping quarters.

Recreation is also essential on long missions, and it takes many forms. Weightlessness provides an ongoing source of fascination and enjoyment, offering the opportunity for acrobatics, experimentation, and games. Looking out the window is perhaps the most popular pastime for astronauts orbiting Earth, providing ever-changing vistas of their home planet. On some flights, astronauts and cosmonauts read books, play musical instruments, watch videos, and engage in two-way conversations with family members on the ground.

Work in Space
Humans face many challenges when working in space. These challenges include communicating with Earth and other spacecraft, creating suitable environments for scientific experiments and other tasks, moving around in the microgravity of space, and working within cumbersome spacesuits.

Spacecraft in orbit around the earth cannot communicate continuously with the ground unless special relay satellites provide a link between the spacecraft and ground receiving stations. This problem disappears when astronauts leave Earth orbit. As Apollo astronauts traveled to the moon, they were in constant touch with mission control. However, when they entered lunar orbit, communications were interrupted whenever the spacecraft flew over the far side of the moon, because the moon stood between the spacecraft and Earth. Lunar landing sites were on the near side of the moon, so Earth was always overhead and the astronauts could maintain continuous contact with mission control. For astronauts who venture to other planets, the primary difficulty in communications will be one of distance. For example, radio signals from Mars will take as long as 20 minutes to reach Earth, making ordinary conversations impossible. For this reason, planetary explorers will have to be able to solve many problems on their own, without help from mission control.

The design of spacecraft interiors has changed as more powerful booster rockets have become available. Powerful boosters allow bigger spacecraft with roomier cabins. In Mercury and Gemini, for example, astronauts could not even stretch their legs completely. Their cockpits resembled those of jet fighters. The Apollo command module offered a bit of room in which to move around, and included a lower equipment bay with navigation equipment, a food pantry, and storage areas. The Soviet Vostoks had enough room for their sole occupant to float around, and Soyuz includes both a fairly cramped reentry module and a roomier orbital module. The orbital module is jettisoned prior to the cosmonauts' return to the earth. The space shuttle has two floors—a flight deck with seats, controls, and windows and a middeck with storage lockers and space to perform experiments.

For the Skylab space station, designers had the luxury of creating several different kinds of environments for different purposes. For example, Skylab had its own wardroom, bathroom, and sleeping quarters. Designers have tried several different approaches to work spaces on spacecraft. Most rooms on Skylab were designed like rooms on Earth with a definite floor and ceiling. However, Skylab's multiple docking adaptor had instrument panels on each wall, and each had its own frame of reference. Thanks to weightlessness, this was not a problem: Astronauts reported that they were able to shift their own sense of up and down to match their surroundings. When necessary, ceiling became floor and vice versa. On Salyut and Mir, the ceilings and floors were painted different colors to aid cosmonauts in orienting themselves. Because simulators on the earth were given the same color scheme, the cosmonauts were accustomed to it when they lifted off.

To help astronauts anchor themselves while they work in weightlessness, designers have equipped spacecraft with a variety of devices, including handholds, harnesses, and foot restraints. Foot restraints have taken a number of forms. Skylab crews used special shoes that could lock into a grid-like floor. Apollo astronauts used shoes equipped with strips of Velcro that stuck to Velcro strips on the capsule floor. Space shuttle astronauts have even used strips of tape on the floor as temporary foot restraints.

Astronauts and cosmonauts who perform spacewalks use a variety of devices to aid in mobility and in anchoring the body in weightlessness. Any surface along which astronauts move is fitted with handholds, which the astronauts use to pull themselves along. Foot restraints allow astronauts to remain anchored in one spot, something that is often essential for tasks requiring the use of both hands. During many spacewalks, astronauts use tethers to keep themselves from drifting away from the spacecraft. Sometimes, however, astronauts fly freely as they work by wearing backpacks with thrusters to control their direction and movement.

Astronauts who have conducted spacewalks report that the most difficult tasks are those that involve using their gloved hands to grip or manipulate tools and other gear. Because the suit—including its gloves—is pressurized, closing the hand around an object requires constant effort, like squeezing a tennis ball. After a few hours of this work, forearms and hands become fatigued. The astronauts must also keep careful track of tools and parts to prevent them from floating away. In general, designers of space hardware strive to make any kind of assembly or repair work in space as simple as possible.

THE POLITICS OF SPACE EXPLORATION
Space exploration requires more than just science—it requires an enormous amount of money. The amount of money that a country is willing to invest in space exploration depends on the political climate of the time. During the Cold War, a period of tense relations between the United States and the USSR, both countries poured huge amounts of money into their space programs, because many of the political and public opinion battles were being fought over superiority in space. After the Cold War, space exploration budgets in both countries shrank dramatically.

The Space Race and the Cold War Space exploration became possible at the height of the Cold War, and superpower competition between the United States and the USSR gave a boost to space programs in both nations. Indeed, the primary impact of Sputnik was political—in the United States Sputnik triggered nationwide concern about Soviet technological prowess. When the USSR succeeded in putting the first human into space, it only added to the disappointment and shame felt by many Americans, and especially by President Kennedy. Against this background, Alan Shepard's Mercury flight on May 5, 1961, was a welcome cause for celebration. Twenty days later Kennedy told Congress, "I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth." This was the genesis of the Apollo program. Although there were other motivations for going to the moon—scientific exploration among them—Cold War geopolitics was the main push behind the moon race. Cold War competition also affected the unpiloted space programs of the United States and USSR.

The Moon Race
During the piloted programs of the moon race, the pressure of competition caused Soviet leaders to order a number of "space spectaculars," as much for their propaganda value as for their contributions to space exploration. The first of these was the flight of the first woman in space, Valentina Tereshkova, in 1963. Next came the two Voskhod flights, each of which entailed significant risks to the cosmonauts—the Voskhod 1 crew flew without space suits, while Voskhod 2's Alexei Leonov was almost unable to reenter his craft following his historic space walk. But the space spectacular the Soviets wanted most of all—a piloted mission around the moon in time for the 50th anniversary of the Russian revolution—never came to pass. By December 1968, when the Apollo 8 astronauts flew around the moon, it was clear that victory in the moon race had gone to the United States.

The achievement of Kennedy's goal, with the Apollo 11 lunar landing mission, signaled a new era in space exploration in the United States—but not as NASA had hoped. Instead of accepting NASA's proposals for a suite of ambitious post-Apollo space programs, Congress backed off on space funding, with the space shuttle as the only major space program to gain approval. In time it became clear that the lavish space budgets of the 1960s had been a product of a unique time in history, in which space was the most visible arena for superpower competition.

After the Moon Tensions between the superpowers eased somewhat in the early 1970s, and the United States and USSR joined forces for the Apollo-Soyuz mission in 1975. Nevertheless, Cold War suspicions continued to influence space planners in both nations in the 1970s and 1980s. Both sides continued to spend enormous sums on missiles and nuclear warheads. Missiles of the Cold War arms race were designed to fly between continents on a path that took them briefly into space during their journeys. In the United States, a great deal of research went into a space-based anti-missile system called the Strategic Defense Initiative (known to the public as "Star Wars"), which was never built. The stockpiling of missiles was eventually slowed by the Strategic Arms Limitation Talks (SALT) treaties.

In the USSR, concerns over possible offensive uses of the U.S. space shuttle helped prompt the development of the heavy-lift launcher Energia and the space shuttle Buran. Economic hardships, however, forced the suspension of both programs. The economy worsened after the collapse of the USSR in 1991, threatening the now-Russian space program with extinction.

After the Cold War In 1993 the U.S. government redefined NASA's plans for an international space station to include Russia as a partner, a development that would not have been possible before the end of the Cold War. An era of renewed cooperation in space between Russia and the United States followed, highlighted by flights of cosmonauts on the space shuttle and astronauts on the Mir space station.

Meanwhile, other nations have staged their own programs of unpiloted and piloted space missions. Many have been conducted by the European Space Agency (ESA), formed in 1975, whose 13 member nations include France, Italy, Germany, and the United Kingdom. European astronauts have flown on shuttle missions and have made visits to Mir. Since the late 1970s, a series of European rockets called Ariane have launched a significant percentage of commercial satellites. ESA's activities in planetary exploration have included probes such Huygens, which is scheduled to land on the Saturn’s moon Titan in 2004 as part of NASA's Cassini mission.

The countries of China, Japan, and India have each developed satellite launchers. None have created rockets powerful enough to put piloted spacecraft into orbit. However, Japan has joined Canada, Russia, and the ESA in contributing hardware and experiments to the International Space Station.

The High Cost of Space Exploration One aspect of space exploration that has changed little over time is its cost. To some extent the ability to carry out a vigorous space program is a measure of a nation's economic vitality. For example, Russia has had difficulties in staying on schedule with its contributions to the International Space Station—a reflection of the unstable Russian economy.

Cost has always been a central factor in the political standing of space programs. The enormous expense of the Apollo moon program (roughly $100 billion in 1990s dollars) prompted critics to say that the program could have been carried out far more cheaply by robotic missions. While that claim is oversimplified—no robot has yet equaled the performance of a skilled observer—it reveals how vulnerable space programs are to budget cuts. The reusable space shuttle failed to significantly lower the cost of placing satellites in low Earth orbit, as compared with throw-away launchers like the Saturn V and the Titan. Cost, not scientific potential, is usually the most significant factor for a nation in deciding whether to adopt a major space program. In the United States budgetary process, space funding must compete in a very visible way with expenditures for social programs and other concerns. Congress has steadily trimmed NASA's allotments, forcing the agency to reduce its number of employees to pre-Apollo levels by the year 2000.

In response to the high cost of space access, the late 1990s saw renewed efforts to develop a single-stage, reusable space vehicle. The situation also strengthened arguments that in the future, the most expensive space programs should be carried out by a consortium of nations. Most scientists envision a program for sending humans to Mars as an international one, primarily as a cost-sharing measure. Still, the mix of scientific, political, and other motivations has yet to bring about such a venture, and it may be years or even decades before international piloted interplanetary voyages become reality.

FUTURE OF SPACE EXPLORATION
The future of space exploration depends on many things. It depends on how technology evolves, how political forces shape rivalries and partnerships between nations, and how important the public feels space exploration is. The near future will see the continuation of human spaceflight in Earth orbit and unpiloted spaceflight within the solar system. Piloted spaceflight to other planets, or even back to the moon, still seems far away. Any flight to other solar systems is even more distant, but a huge advance in space technology could propel space exploration into realms currently explored only by science fiction.

Piloted Spaceflight The 1968 film 2001: A Space Odyssey depicted commercial shuttles flying to and from a giant wheel-shaped space station in orbit around Earth, bases on the moon, and a piloted mission to Jupiter. The real space activities of 2001 will not match this cinematic vision, but the 21st century will see a continuation of efforts to transform humanity into a spacefaring species.

The International Space Station was scheduled to become operational in the first years of the new century. NASA plans to operate the space shuttle fleet at least through the year 2012 before phasing in a replacement—possibly a single-stage-to-orbit (SSTO) vehicle. However, some experts predict that the SSTO is too difficult a goal to be achieved that soon, and that a different kind of second-generation shuttle would be necessary—perhaps a two-stage, reusable vehicle much like the current shuttle. In a two-stage launcher, neither stage is required to do all the work of getting into orbit. This results in less stringent specifications on weight and performance than are necessary for an SSTO.

Perhaps the most difficult problem space planners face is how to finance a vigorous program of piloted space exploration, in Earth orbit and beyond. In 1998 no single government or international consortium had plans to send people back to the moon, much less to Mars. Such missions are unlikely to happen until the perceived value exceeds their cost.

Some observers, such as Apollo 11 astronaut Buzz Aldrin, believe the solution may lie in space tourism. By conducting a lottery for tickets on Earth-orbit "vacations," a non-profit corporation could generate revenue to finance space tourism activities. In addition, the vehicles developed to carry passengers might find later use as transports to the moon and Mars. Several organizations are pushing for the development of commercial piloted spaceflight. In 1996 the U.S. X-Prize Foundation announced that it would award $10 million to the first private team to build and fly a reusable spacecraft capable of carrying three individuals to a height of at least 100 km (62 mi). By 1998, 16 teams had registered for the competition, with estimates of first flights in 2001.

One belief shared by Aldrin and a number of other space exploration experts is that future lunar and Martian expeditions should not be Apollo-style visits, but rather should be aimed at creating permanent settlements. The residents of such outposts would have to "live off the land," obtaining such necessities as oxygen and water from the harsh environment. On the moon, pioneers could obtain oxygen by heating lunar soil. In 1998 the Lunar Prospector discovered evidence of significant deposits of ice—a valuable resource for settlers—mixed with soil at the lunar poles. On Mars, oxygen could be extracted from the atmosphere and water could come from buried deposits of ice.

The future of piloted lunar and planetary exploration remains largely unknown. Most space exploration scientists believe that people will be on the moon and Mars by the middle of the 21st century, but how they get there—and the nature of their visits—is a subject of continuing debate. Clearly, key advances will need to be made in lowering the cost of getting people off Earth, the first step in any human voyage to other worlds.

Unpiloted Space flight

The space agencies of the world plan a wide array of robotic missions for the final years of the 20th century and the opening decade of the 21st century. NASA's Mission to Planet Earth (MTPE) Enterprise is designed to study the earth as a global system, and to document the effects of natural changes and human activity on the environment. The first of the so-called Earth Observing System (EOS) spacecraft, which form the cornerstone of the MTPE effort, was slated for launch in the summer of 1998.

Mars will be visited by a succession of landers and orbiters as part of NASA's Discovery Program, of which the Mars Pathfinder lander was a part. If all goes as planned, as early as 2003 NASA will launch a spacecraft to Mars to retrieve a sample of rocks and soil and bring it to Earth.

The Discovery program also includes the Near Earth Asteroid Rendezvous mission (NEAR), which was scheduled to orbit the asteroid Eros beginning in 1999. In 2004 a spacecraft called Stardust is scheduled fly past Comet Wild 2 and gather samples of the comet's dust to bring back to Earth. See also Comet.

Jupiter's moon Europa is also likely to receive increased scrutiny, because of strong evidence for a liquid-water ocean beneath its icy crust. Among the missions being studied is a lander to drill through the ice and explore this suspected ocean. As with Mars, scientists are especially eager to find any evidence of past or present life on Europa. Such investigations will be difficult, but the discovery of any form of life beyond the earth would undoubtedly spur further explorations.

Saturn will be visited by the Cassini orbiter in the summer of 2004. The spacecraft is to deploy a probe called Huygens that will enter the atmosphere of Saturn's largest moon, Titan, in December 2004. During its trip to the surface, Huygens will analyze the cloudy atmosphere, which is rich in organic molecules.

NASA is also considering orbiters to survey Mercury, Uranus, and Neptune. Pluto, the only planet that has never been visited by a spacecraft, is the target for a proposed Pluto Express mission. A pair of lightweight probes would be launched at high speed, reaching Pluto and its moon Charon as early as 2010.

NASA's New Millennium program is aimed at creating new technologies for space exploration and swiftly incorporating them into spacecraft. Its first mission, Deep Space 1, will use solar-electric propulsion to visit an asteroid in January 1999 and a comet in June 2000.

NASA also plans a number of orbiting telescopes, including the Advanced X-Ray Astronomical Facility (AXAF), an X-ray astronomy telescope scheduled for launch from the space shuttle in late 1998. Another program, called Origins, is designed to use ground-based and space-borne telescopes to search for Earthlike planets orbiting other stars.

International Cooperation

Space exploration experts have long hoped that as international tensions have eased, an increasing number of space activities could be undertaken on an international, cooperative basis. One example is the International Space Station. In 1998, however, countries and agencies such as Japan and the European Space Agency (ESA) began to reassess their commitments to space exploration because of economic uncertainty. China has stated that it intends to place two astronauts in orbit by the year 2000. The transportation system for this mission may involve Russian space hardware, such as the Soyuz spacecraft.

In addition to the economic savings that could result from nations pooling their resources to explore space, the new perspective gained by space voyages could be an important benefit to international relations. The Apollo astronauts have said the greatest discovery from our voyages to the moon was the view of their own world as a precious island of life in the void. Ultimately that awareness could help to improve our lives on Earth.

The Russian Sputnik 1, launched on October 4, 1957, was the first artificial satellite put into orbit around the earth. This historic launch kicked off an era of intensive space programs by both the Soviet Union and the United States, a surge of interest sometimes called the "space race." In the next three decades, hundreds of probes, satellites, and other missions would follow Sputnik on the quest to explore both the wonders and the practical potential of space.

My Project On Artificial Satellites.

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