History of Space Exploration

Yuri Gagarin Photo credit: NASA,

1961, at the age of 27, Gagarin left the earth. It was April the 12th, 9.07 Moscow time (launch-site, Baikonur). 108 minutes later, he was back .

The 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.

Photo credit: G.L. Technologies

Following the battle of Kai-Keng, the Mongols produced rockets of their own and may have been responsible for the spread of rockets to Europe.

Photo credit: G.L. Technologies

The Chinese began experimenting with the gunpowder-filled tubes. At some point, they attached bamboo tubes to arrows and launched them with bows.

Konstantin Tsiolkovsky Soviet Space Scientist 1857 - 1935 Photo credit: www.informatics.org/museum

Konstantin Tsiolkovsky is generally considered the father of astronautics and rocket dynamics.

    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,

Robert Goddard Photo: NASA,

By 1926, Goddard had constructed and tested successfully the first rocket using liquid fuel.

V-2 Rocket Photo: NASA,

The A-4, later called the V-2, was a single-stage rocket fueled by alcohol and liquid oxygen.

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”).

Photo credit: NASA,

History changed on October 4, 1957, when the Soviet Union successfully launched Sputnik I. The world's first artificial satellite was about the size of a basketball, weighed only 183 pounds, and took about 98 minutes to orbit the Earth on its elliptical path.

    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.

Vanguard 1 Photo credit: NASA,

    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.

The Pioneer Venus Orbiter Photo credit: NASA,

The Pioneer Venus Orbiter was inserted into an elliptical orbit around Venus on December 4, 1978.

Luna 1 Photo credit: NASA,

Luna 1 was the first spacecraft to reach the Moon.

Ranger (1961 - 1965) Photo credit: NASA,

The Ranger series was the first U.S. attempt to obtain close-up images of the Lunar surface.

    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.

Surveyor Photo credit: NASA,

The Surveyor spacecraft was designed to attain the engineering objectives of the Surveyor program, which included the first lunar soft landing.

    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.

Lunar Orbiter (1966 - 1967) Photo credit: NASA,

Five Lunar Orbiter missions were launched in 1966 through 1967 with the purpose of mapping the lunar surface before the Apollo landings.

    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.

Clementine Photo credit: NASA,

The spacecraft (175K GIF) weighed 3400 lbs at launch including the solid rocket moter used for injection towards 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.

Scientific Satellites Photo credit: NASA,

Solar system observation satellites are used for observing the Sun and the planets.

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).

Hurricane Fran Photo credit: NASA,

As seen in Time magazine ( Sept. 16, 1996, p.70) and in Life magazine (Year in Pictures 1996).

    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 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.

Lagoon Nebula Photo credit: NASA,

This Hubble telescope snapshot unveils a pair of one-half, light-year-long interstellar "twisters".

OAO 1

Hubble Space Telescope Photo credit: NASA,

The Hubble Space Telescope (HST) is an orbiting observatory that was launched into orbit by the space shuttle Discovery in 1990.

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. 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 per hour, and location to within a few meters. 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 handheld GPS receivers, and some private passenger automobiles now come equipped with a GPS system

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 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.

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.

    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 stronauts 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 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 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.

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.

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.

    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). 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. 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.

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 in 2006, the ISS is designed to be continuously occupied by up to seven 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.

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. The shuttle program's 100th mission is slated to take place early in 2001, and shuttle orbiters are expected to keep flying during the first decades of the 21st century.