II.

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 Yuri Gagarin became the first human in space just a few years later, in 1961. The decades from the 1950s to the 1990s were full of new “firsts,” new records, and advances in technology.

A.

First Forays into Space

Although artificial satellites and piloted spacecraft are achievements of the last half of the 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.

A.1.

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-fuel rockets became part of the arsenals of Europe. In 1814, during an attack on New Orleans, Louisiana, 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-fuel 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.

A.2.

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 Sergey 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 (1,121 lb) and contained the first space traveler—a dog named Laika, which survived for a few hours aboard Sputnik 2. Laika died from rising temperatures within the satellite 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 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.

B.

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 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, 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 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 3, 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, which 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 tentative evidence 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. Controllers intentionally crashed the probe into the Moon in July 1999, hoping to see signs of water in the plume of debris raised by the impact. Measurements taken by instruments around Earth, however, did not find evidence of water after the crash, nor did they rule out the existence of water.

C.

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.

C.1.

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 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 1999.

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. An international team of scientists and engineers launched the Terra satellite in December 1999. The satellite carries five instruments for observing Earth and monitoring the health of the planet. NASA, a member organization of the team, released the first images taken by the satellite in April 2000.

C.2.

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. NASA launched the Chandra X-Ray Observatory in 1999. Chandra is named after American astrophysicist Subrahmanyan Chandrasekhar and has eight times the resolution of any previous X-ray telescope. The Spitzer Space Telescope was put in orbit in 2003 to study infrared radiation from objects in space, including forming stars and galaxies.

D.

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 Earth every 12 hours and continuously emits navigation signals. Anyone can 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 per hour, and location to within a few meters. In addition to military uses, handheld GPS receivers can be used by hikers, campers, and explorers to locate their positions. GPS receivers are included in many cellular radio telephones, commonly known as cell phones, for possible emergency assistance. Private passenger automobiles often come equipped with a GPS system for navigation.

E.

Planetary Studies

Even as the United States and the USSR raced to explore the Moon, both countries were also readying missions to travel farther 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 20th century, spacecraft had visited every planet in the solar system, except for the outermost planet—tiny, frigid Pluto (now classified as a dwarf planet). In January 2006, however, NASA launched the New Horizons spacecraft from Cape Canaveral, Florida, on a nine-year-long journey to Pluto and then beyond. The earliest the spacecraft was expected to fly by Pluto was 2015. Its mission was to then continue on to explore the outer Kuiper Belt.

E.1.

Mercury

The first spacecraft to visit the solar system’s innermost planet, Mercury, was the U.S. probe Mariner 10. The probe 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. In 2004 NASA launched the MESSENGER spacecraft. MESSENGER flew by Mercury in 2008, with another flyby scheduled in 2009 before it goes into orbit around Mercury in 2011.

E.2.

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 1973 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. The Venus Express spacecraft launched by the European Space Agency (ESA) in 2005 began studying the planet’s thick atmosphere from orbit in 2006.

E.3.

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, 1997, almost 21 years after the Viking 1 lander touched down on the surface of Mars, 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 September 1997.

NASA’s Mars Global Surveyor (MGS) 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, MGS was equipped with a high-resolution camera and instruments to study the planet’s atmosphere, topography and gravity, surface composition, and magnetic field. A problem with an unstable solar panel delayed the start of the MGS mission—mapping the entire planet—for about a year. (In the meantime, Mars Global Surveyor began relaying high-resolution images of select areas in early 1998.) Its mapping operation, slated to last for one Martian year (about two Earth years), began in March 1999. Unlike previous Mars probes, MGS adjusted 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 planned to send to Mars about every 18 months. The next two spacecraft in the series, Mars Climate Orbiter and Mars Polar Lander, began their journeys to Mars in December 1998 and January 1999, respectively. Both probes reached Mars in late 1999, but Mars Climate Orbiter crashed into the planet due to a navigational error, and software defects led to the crash landing of Mars Polar Lander.

Japan launched the spacecraft Nozomi (Japanese for “hope”), destined for Mars, on July 4, 1998. Nozomi contained equipment developed by scientists from around the world, including Canadian space scientists. This was the first time Canada participated in a mission to another planet. After orbiting the Sun, Nozomi was scheduled to reach Mars in 2003, but due to malfunctions Nozomi strayed off course. In December 2003 Japanese space officials announced that they had abandoned efforts to correct its course.

In late 2003 and early 2004, spacecraft launched by the European Space Agency (ESA) and NASA reached Mars. The ESA’s Mars Express orbiter arrived in December 2003 and deployed the British-built Beagle 2 lander. However, the ESA was never able to make contact with the lander. The Mars Express orbiter went on to study the planet in detail, returning important new information about minerals and the existence of water on and below the surface of Mars.

NASA’s Mars Exploration Rover mission sent two spacecraft, Spirit and Opportunity, to Mars in 2003. Spirit landed on the surface of Mars in early January 2004 and nearly two weeks later deployed a rover designed to search for signs that liquid water once existed on the surface of Mars. The Spirit rover could travel farther than the Sojourner rover and carried more scientific instruments, including a high-resolution panoramic camera, a microscopic imager, and a tool that could grind into rocks. Spirit landed in the Gusev Crater near the Martian equator, a crater about the size of the state of Connecticut, which scientists believe is the bed of an ancient lake. The Opportunity spacecraft landed in late January at the Meridiani Planum on the opposite side of Mars with an identical rover. The region contains mineral deposits associated with liquid water. Unlike the previous Pathfinder mission, the Spirit and Opportunity landers were designed so that they ceased to function once they deployed their rovers.

The Mars Reconnaissance Orbiter spacecraft, which uses the most powerful telescope camera ever sent to another planet, went into orbit around Mars in March 2006. It is able to image objects as small as 25 cm (1 ft) across. Its mission is to search for landing sites for future missions to Mars and to conduct studies of the atmosphere, surveys of the subsurface using radar, and mapping of minerals. The Phoenix Mars Lander was launched in 2007 to investigate the north polar region of the planet.

E.4.

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, multicolored atmosphere. Pioneer 10 also investigated Jupiter’s intense magnetic field, and the associated belts of trapped radiation. Acting like a slingshot, 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. Scientists hope both Voyagers will continue sending back data as they reach the edge of the solar system and beyond.

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 of 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 extended the spacecraft’s mission to include measurements taken simultaneously by the Galileo orbiter and by a new spacecraft, Cassini, which visited Jupiter on its way to Saturn. NASA dove the spacecraft into Jupiter’s atmosphere when Galileo’s fuel dwindled in September 2003.

NASA’s Cassini spacecraft set out toward Saturn and Saturn’s moon Titan in October 1997. Cassini reached Jupiter at the end of the year 2000 and reached Saturn in 2004. After reaching Saturn, it released the Huygens probe into Titan’s atmosphere in January 2005. In March 2006 Cassini captured images of geysers on Saturn’s moon Enceladus.

E.5.

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, 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 1,600 km (1,000 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 reached the asteroid 433 Eros and became the first spacecraft to orbit an asteroid in February 2000. The United States launched the spacecraft Deep Space 1 (DS1) in 1998 to prepare for 21st-century missions within the solar system and beyond. In July 1999 DS1 flew by the small asteroid 9969 Braille and discovered that it is composed of the same type of material as the much larger asteroid 4 Vesta. Braille may be a broken piece of Vesta, or it may have simply formed at the same time and place as Vesta in the early solar system. Japan's Hayabusa space probe reached the asteroid Itokawa in 2005 and tried to retrieve a sample that is scheduled to be brought back to Earth in 2010. NASA launched the Dawn spacecraft toward the asteroid belt in 2007. Dawn is expected to orbit Vesta in 2011 and the dwarf planet Ceres in 2015.

Comets are icy wanderers that populate the solar system’s outermost reaches. These “dirty snowballs” are chunks of frozen gases 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 many comets have been observed by a variety of space-borne instruments, spacecraft have made close encounters with relatively few. The most famous comet of all, Halley’s Comet, made its most recent passage through the inner solar system in 1986. In March 1986 three separate spacecraft flew near Halley—the USSR’s Vega 1 and Vega 2 probes, and the Giotto spacecraft of the European Space Agency (ESA). Two Japanese spacecraft—Sakigake and Suisei—observed Halley from great distances. 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.

NASA’s Stardust spacecraft became the first spacecraft to collect dust from a comet when it came within 240 km (149 mi) of the nucleus of Comet Wild 2 in January 2004. The spacecraft used a special device to scoop up dust grains as it traveled through the comet’s coma, a cloud of dust and gas that surrounds the rocky nucleus. The dust grains were then transferred to a canister. Stardust flew past Earth in January 2006 and released the canister, which descended through Earth’s atmosphere, its final descent slowed by parachute. Scientists recovered the canister from a landing site in Utah and calculated that they had recovered millions of microscopic dust grains. These dust grains show evidence that material from the inner and outer regions of the solar system may have mixed as the Sun and planets formed.

In July 2005 NASA’s Deep Impact spacecraft released a small craft known as an impactor that collided with Comet Tempel 1 in the first collision between a comet and a man-made object. The purpose of the collision was to release chemical compounds from within the comet’s nucleus for scientific study. The highly successful mission scored a near-perfect hit with the comet, which was about 134 million km (83 million mi) from Earth, without disturbing the comet’s orbit.

F.

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.

F.1.

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 Yuri 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 the 34-hour mission flown by Gordon Cooper 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 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 2,500 kg (5,500 lb)—a reflection of the greater lifting power of the R-7 compared with the U.S. Redstone and Atlas rockets.

F.2.

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 spacewalks 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 multiperson 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 ten 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 1,370 km (850 mi). The final mission of the series, Gemini 12 in November 1966, saw Buzz Aldrin make a record five hours of spacewalks. At the conclusion of the Gemini program, the United States held a clear lead in the race to the Moon.

F.3.

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 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 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 reignited 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 Beregovoi 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.

F.4.

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 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. 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.

F.5.

Salyut Space Stations

Even before the first human spaceflights, planners in the United States and the USSR envisioned space stations in orbit around 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 Yuri 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 (3,500 cu ft) of living space. Cosmonauts traveled between 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 spaceflight. 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 spaceflight, 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.

F.6.

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 spacewalks 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 (7,400 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 spacewalk 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 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 spaceflight.

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.

F.7.

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 cubic meters (about 13,600 cubic feet).

Cosmonauts lived aboard Mir even longer than their Salyut predecessors lived in space. In 1987 and 1988 Mir cosmonauts Vladimir Titov and Musa Manarov achieved the first yearlong mission. In 1995 physician-cosmonaut Valeriy Polyakov completed a record 14 months aboard the station. Such long-duration missions 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 took over the Soviet space program after the collapse of the USSR in 1991.) The joint missions paved the way for the International Space Station (ISS; discussed below). United States 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. spaceflight 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 planned to abandon Mir and cause it to reenter Earth’s atmosphere in the summer of 2000, but the station was temporarily rescued by a private company called Mircorp. Mircorp planned to turn the station into a commercial venture. The company funded a mission in April 2000 that sent two cosmonauts to Mir to make repairs and conduct experiments, but it could not attract enough investors to keep Mir in orbit. Russian ground controllers sent the station plunging into a remote area of the South Pacific Ocean in March 2001.

F.8.

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 (ISS). In addition to the United States, many other nations have joined the project. Russia, Japan, Canada, and the European Space Agency have produced hardware for the station.

Launch of the first ISS element, a Russian-built module called Zarya, occurred in November 1998. Zarya provides the power and propulsion needed during the ISS’s assembly. Once the ISS is complete, Zarya will be used mostly for storage. The Unity module, built by the United States, was launched in December 1998. Unity acts as a passage from Zarya to other parts of the station. The first habitable part of the ISS—the Russian-made Zvezda service module—was launched in July 2000, and the first long-term crew arrived in November 2000. Planned for completion around 2010, the ISS is designed to be continuously occupied by up to three crew members. It is envisioned as a world-class research facility, where scientists can study Earth and the heavens, as well as explore the medical effects of long-duration spaceflight, the behavior of materials in a weightless environment, and the practicality of space manufacturing techniques.

F.9.

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 cargo bay 20 m (60 ft) long. 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. A total of six 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 spacewalks, 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, “throwaway” 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 began taking crews into orbit to assemble the International Space Station. On October 29, 1998, John Glenn, the first American to orbit Earth, returned to space aboard the space shuttle Discovery at the age of 77. He is the oldest person ever to fly in space.

The shuttle program’s 100th mission took place in 2000, and shuttle orbiters were expected to keep flying during the first decades of the 21st century. On February 1, 2003, however, disaster struck the 113th shuttle mission. The shuttle Columbia disintegrated and burned up while reentering Earth’s atmosphere after successfully completing a series of scientific experiments. The seven crewmembers, including the first Israeli astronaut, all died. Engineers immediately suspected that problems with Columbia’s left wing caused the disaster. Onboard sensors recorded abnormally high temperatures in the wing just before all contact with the shuttle was lost. An investigation determined that the wing was damaged during takeoff, when a piece of foam insulation for the external fuel tank came loose and struck the wing. Shuttle launches were canceled pending the implementation of safety measures recommended by the investigation. In 2004 President George Bush announced that the space shuttle would be retired in 2010 after completion of the International Space Station.

In July 2005 the shuttle Discovery returned to space. However, during its launch, foam debris again came loose. Although it did not appear to cause any damage, NASA suspended further shuttle flights until July 2006 when the Discovery returned to orbit and docked with the International Space Station.

F.10.

China’s Space Program

In 2003 China became the third nation to send a piloted spacecraft into an Earth orbit. Astronaut Yang Liwei was launched into space on October 15 aboard the spacecraft Shenzhou 5 (Divine Vessel 5). The spacecraft orbited the Earth 14 times in 21 hours before landing. The successful mission signaled that China, with a budget for its space program comparable to Russia’s, had become a significant player in space exploration. China also announced plans to send an unpiloted spacecraft to the Moon. China successfully launched a second piloted spacecraft into Earth orbit in 2005.

F.11.

Private Spaceflight

The first privately developed and funded piloted spacecraft, SpaceShipOne, carried test pilot Mike Melvill into space in June 2004. The craft rose to a height of 100 km (62 mi), the internationally accepted boundary of space, before gliding back to Earth. SpaceShipOne was designed by aviation engineer Burt Rutan, funded by Microsoft cofounder Paul Allen, and built by the California-based company Scaled Composites. The project proved that launching people into space was no longer a feat exclusively reserved for governments.

SpaceShipOne flew twice more in October 2004 to win the Ansari X Prize. The prize offered $10 million to the first private team to build and fly a reusable spacecraft capable of carrying three individuals into space twice within two weeks. The prize was offered by private donors to encourage the development of private spacecraft.