Space Exploration
...prisingly Earthlike in others. The USSR’s Mars probes were stymied by technical malfunctions. In November 1971 the Mars 2 spacecraft (see Mars (space program)) went into orbit around the planet and released a landing capsule that crashed without returning any data. Mars 2 became the first artificial object to reach the Martian surface. In December 1971 a lander released by the Mars 3 orbiter reached the surface successfully. However, it sent back only 20 seconds of video signals that included no data. In 1973 two more landing missions also failed. In 1988 the USSR made two unsuccessful attempts to explore the Martian moon Phobos. Contact with the spacecraft Phobos 1 (see Phobos (space program)) was lost due to an error by mission controllers when the spacecraft was on its way to Mars. Phobos 2 reached Martian orbit in January 1989 and sent back images of the planet, but failed before its planned rendezvous with Phobos. The U.S. Viking probes made the first successful Mars landings in 1976. Two Viking spacecraft, each consisting of an orbiter and lander, left Earth in August and September 1975. Viking 1 went into orbit around Mars in June 1976, and after a lengthy search for a relatively smooth landing site, the Viking 1 lander touched down safely on Mars’s Chryse Planitia (Plain of Gold) on July 20, 1976. The Viking 2 lander reached Mars’s Utopia Planitia (Utopia Plain) on September 3, 1976. Each lander sent back close-up pictures of a dusty surface littered with rocks, under a surprisingly bright sky (due to sunlight reflecting off of airborne dust). The landers also recorded changes in atmospheric conditions at the surface. They searched, without success, for conclusive evidence of microbial life. The landers continued to send back data for several years, while the orbiters took thousands of high-resolution photographs of the planet. On July 4, 1996, 20 years after Viking 1 arrived, NASA’s Mars Pathfinder spacecraft landed in Mars's Ares Vallis (Mars Valley). Pathfinder used a new landing system featuring pressurized airbags to cushion its impact. The next day, Pathfinder released a 10-kg (22-lb) rover called Sojourner, which became the first wheeled vehicle to operate on another planetary surface. While Pathfinder sent back images, atmospheric measurements, and other data, Sojourner examined rocks and soil with a camera and an Alpha Proton X-ray Spectrometer, which provided data on chemical compositions by measuring how radiation bounced back from rocks and dust. The mission ended when the spacecraft ceased responding to commands from Earth in October 1997. NASA’s Mars Global Surveyor went into orbit around Mars in September 1997. Designed as a replacement for NASA’s Mars Observer probe, which failed before reaching Mars in 1993, Mars Global Surveyor is equipped with a high-resolution camera and instruments to study the planet’s atmosphere, topography and gravity, surface composition, and magnetic field. Global Surveyor reached orbit around Mars in the fall of 1997, but a problem with an unstable solar panel delayed the start of its 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, Mars Global Surveyor 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 plans 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 contains equipment developed by scientists from around the world, including Canadian space scientists. This is the first time Canada has participated in a mission to another planet. Nozomi is scheduled to reach Mars in 2003. E4 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. On February 17, 1998, Voyager 1 became the most distant human-made object, reaching a distance of 10.5 billion km (6.5 billion mi) from Earth. Scientists hope it will continue sending back data well into the 21st century. NASA’s Galileo orbiter reached Jupiter in December 1995. The spacecraft deployed a probe that entered Jupiter’s atmosphere on December 7, 1995, radioing data for 57 minutes before succumbing to intense pressures. The probe sent back the first measurements 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 through the year 2003. The extended mission included measurements taken simultaneously by the Galileo orbiter and by a new spacecraft, Cassini, which visited Jupiter on its way to Saturn. 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 is scheduled to reach Saturn in 2004. After reaching Saturn, it should release a probe into Titan’s atmosphere. E5 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. 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 several comets have been observed by a variety of space-born instruments, only one has been visited by spacecraft. The most famous comet of all, Halley’s Comet, made its most recent passage through the inner solar system in 1986. In March 1986 five separate spacecraft flew past Halley, including the USSR’s Vega 1 and Vega 2 probes, the Giotto spacecraft of the European Space Agency, and Japan’s Sakigake and Suisei probes. These encounters produced valuable data on the composition of the comet’s gas and dust tails and its solid nucleus. Vega 1 and 2 returned the first close-up views ever taken of a comet’s nucleus, followed by more detailed images from Giotto. Giotto went on to make a close passage to Comet P/Grigg-Skjellerup on July 10, 1992. 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. F1 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. F2 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. F3 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. F4 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. F5 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. F6 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. F7 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. F8 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. F9 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 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. The wing may have been damaged during takeoff, when a piece of external fuel tank insulation came loose and fell on the wing. Shuttle launches have been cancelled pending the final results of an investigation. III SCIENCE OF SPACE EXPLORATION Space is a harsh environment for humans and human-made machines. Radiation from the Sun and other cosmic sources can weaken material and harm the human body. In the vacuum of space, objects become boiling hot when exposed to the Sun and freezing cold when in the shadow of Earth or some other body. Scientists, engineers, and designers must make spacecraft that can withstand these extreme conditions and more. A General Principles of Spacecraft Design The challenges that spacecraft designers face are daunting. Each component of a spacecraft must be durable enough to withstand the vibrations of launch, and reliable enough to function in space on time spans ranging from days to years. At the same time, the spacecraft must also be as lightweight as possible to reduce the amount of fuel required to boost it into space. Materials such as Mylar (a metal-coated plastic) and graphite epoxy (a construction material that is strong but lightweight) have helped designers and manufactures meet the requirements of durability, reliability, and lightness. Spacecraft designers also conserve space and weight by using miniaturized electronic components; in fact, the space program has fueled many advances in the field of miniaturization. Since the early 1990s, budgetary restrictions have motivated NASA to plan projects that are better, faster, and cheaper. In this approach, space missions requiring single large, complex, and expensive spacecraft are replaced with more limited missions using smaller, less expensive craft. Although this new approach was successful with spacecraft such as the Mars Pathfinder lander and Mars Global Surveyor 96, budgetary constraints may have contributed to the loss of two other Mars spacecraft, Mars Climate Orbiter and Mars Polar Lander, in 1999. The approach is also difficult to apply to piloted spacecraft, in which the overriding concern is crew safety. However, engineers are always looking for new technologies to make spacecraft lighter and less expensive. B Getting into Space One of the most difficult parts of any space voyage is the launch. During launch, the craft must attain sufficient speed and altitude to reach Earth orbit or to leave Earth’s gravity entirely and embark on a path between planets. Scientists sometimes find it helpful to think of Earth’s gravitational field as a deep well, with sides that are steepest near the planet’s surface. The task of the launch vehicle or booster rocket is to climb out of this well. Although some launch vehicles consist of just a single rocket, many are composed of a series of individual rockets, or stages, stacked atop one another. Such multistage launch vehicles are used especially for heavier payloads. With a multistage rocket, each stage fires for a period of time and then falls away when its fuel supply is used up. This lightens the load carried by the remaining stages. In some liquid-fuel boosters, strap-on solid-fuel rockets are used to provide extra thrust during the initial portion of ascent. For example, the Titan III booster has two liquid-fuel core stages and two strap-on solid-fuel motors. The largest example of a successful multistage booster was the Saturn V Moon rocket, which had three liquid-fuel stages and measured 111 m (363 ft), including the Apollo spacecraft, in length. Despite their utility, most multistage boosters are not reusable, which makes them expensive. Cost-conscious engineers have focused on creating a single-stage-to-orbit (SSTO) vehicle. In an SSTO, the entire spacecraft and booster would be integrated into one fully reusable unit. If successful, this approach would reduce the costs of reaching Earth orbit. However, the technical challenge is enormous: A full 89 percent of an SSTO’s total weight must be reserved for fuel, a much higher proportion than any previous launch vehicle. The payload, the crew, and the weight of the vehicle itself must make up only 11 percent of the SSTO’s total weight. C Navigation in Space Spaceflight requires very detailed planning and measurement to get a spacecraft into place or to send it on its proper path. Some of the Apollo spacecraft were able to travel from Earth to the Moon (a distance of almost 390,000 km, or almost 240,000 mi) and land on the lunar surface within a few dozen meters (several dozen feet) of their target. Careful planning allowed the Mars Pathfinder spacecraft to fly from Earth to Mars, traveling more than 500 million km (300 million mi), and land just 19 km (12 mi) from the center of its target area. C1 Flight Paths To launch a spacecraft into orbit around Earth, a booster rocket must do two things. First it must raise the spacecraft above the atmosphere—roughly 160 km (100 mi) or more. Second it must accelerate the spacecraft until its forward speed—that is, its speed parallel to Earth’s surface—is at least 28,200 km/h (17,500 mph). This is the speed, called orbital velocity, at which the momentum of the spacecraft is strong enough to counteract the force of gravity. Gravity and the spacecraft’s momentum balance so that the spacecraft does not fall straight down or move straight ahead—instead it follows a curved path that mimics the curve of the planet itself. The spacecraft is still falling, as any object does when it is released in a gravitational field. But instead of falling toward Earth, it falls around it. See Orbit. Using its own thrusters, a spacecraft can raise or lower its orbit by adding or removing energy, respectively. To add energy, the spacecraft orients itself and fires its thrusters so that it accelerates in its direction of flight. To subtract energy, the craft fires its engines against the direction of flight. Any change in the height of a spacecraft’s orbit also produces a change in its speed and vice versa. The craft moves more slowly in a higher orbit than it does in a lower one. By firing its rockets perpendicular to the plane of its orbit, the craft can change the orientation of its orbit in space. To travel from one planet to another, a spacecraft must follow a precise path, or trajectory, through space. The amount of energy that a spacecraft’s launch rocket and onboard thrusters must provide varies with the type of trajectory. The trajectory that requires the least amount of energy is called a Hohmann transfer. A Hohmann transfer follows the shape of an ellipse, or a flattened circle, whose sides just touch the orbits of the two planets. The trajectory must also take into account the motion of the planets around the Sun. For example, a probe traveling from Earth to Mars must aim for where Mars will be at the time of the spacecraft’s arrival, not where Mars is at the time of launch. In many interplanetary missions, a spacecraft flies past a third planet and uses the planet’s gravitational field to bend the craft’s trajectory and accelerate it toward its target planet. This is known as a gravitational slingshot maneuver. The first spacecraft to use this technique was the Mariner 10 probe (see Mariner), which flew past Venus on its way to Mercury in 1974. C2 Navigation and Guidance Most spacecraft depend on a combination of internal automatic systems and commands from ground controllers to keep on the correct path. Normally, ground controllers can communicate with a spacecraft only when it is within sight of an Earth-based receiving station. This poses problems for spacecraft in low Earth orbit—that is, within 2,000 km (1,200 mi) of the planet’s surface—as such craft are only within sight of a relatively small portion of the globe at any given moment. One way around this restriction is to place special satellites in orbit to act as relays between the orbiting spacecraft and ground stations, allowing continuous communications. NASA has done this for the U.S. space shuttle with the Tracking and Data Relay Satellite System (TDRSS). At an altitude of about 35,800 km (about 22,200 mi), a satellite’s motion exactly matches the speed of Earth’s rotation. As a result, the satellite appears to hover over a specific spot on Earth’s surface. This so-called stationary, or geosynchronous, orbit is ideal for communications satellites, whose job is to relay information between widely separated points on the globe. Spacecraft on interplanetary trajectories may travel millions or even billions of kilometers from Earth. In these cases their radio signals are so weak that giant receiving stations are necessary to detect them. The largest stations have antenna dishes in excess of 70 m (230 ft) across. NASA and the Jet Propulsion Laboratory operate the Deep Space Network, a system of three tracking stations with several antennas each. The stations are in California, Spain, and Australia, providing continuous contact with distant spacecraft as Earth spins on its axis. Much of the work of ground controllers involves monitoring a spacecraft’s health and flight path. Using a process called telemetry, a spacecraft can transmit data about the functioning of its internal components. In addition, engineers can use a spacecraft’s radio signals to assess its flight path. This is possible because of the Doppler effect. Because of the Doppler effect, a spacecraft’s motion causes tiny shifts in the frequency of its radio signals—just as the motion of a passing car causes the apparent pitch of its horn to go up as the car approaches an observer and down as the car moves away. By analyzing Doppler shifts in a spacecraft’s radio signals, controllers can determine the craft’s speed and direction. Over time, controllers can combine the Doppler shift data with data on the spacecraft’s position in the sky to produce an accurate picture of the craft’s path through space. The guidance system helps control the craft’s orientation in space and its flight path. In the early days of spaceflight, guidance was accomplished by means of radio signals from Earth. The Mercury spacecraft and its Atlas booster utilized such radio guidance signals broadcast from ground stations. During launch, for example, the Atlas received steering commands that it used to adjust the direction of its engines. However, Mercury flight controllers found that radio guidance was limited in accuracy because interference with the atmosphere tends to make the signals weaker. Beginning with Gemini, engineers used a system called inertial guidance to stabilize rockets and spacecraft. This system takes advantage of the tendency of a spinning gyroscope to remain in the same orientation. A gyroscope mounted on a set of gimbals, or a mechanism that allows it to move freely, can maintain its orientation even if the spacecraft’s orientation changes. An inertial guidance system contains several gyroscopes, each oriented along a different axis. When the spacecraft rotates along one or more of its axes, measuring devices tell how far it has turned from the gyroscopes’ own orientations. In this way, the gyroscopes provide a constant reference by which to judge the craft’s orientation in space. Signals from the guidance system are fed into the spacecraft’s onboard computer, which uses this information to control the craft’s maneuvers. The Global Positioning System satellites, which enable ships, airplanes, and even hikers to know their positions with extreme accuracy, should play a similar role in spacecraft. The space shuttle Atlantis was equipped with GPS receivers during an upgrade in late 1998. C3 Propulsion Once in orbit, a spacecraft relies on its own rocket engines to change its orientation (or attitude) in space, the shape or orientation of its orbit, and its altitude. Of these three tasks, changes in orientation require the least energy. Relatively small rockets called thrusters control a spacecraft’s attitude. In a massive spacecraft, the attitude control thrusters may be full-fledged liquid-fuel rockets. Smaller spacecraft often use jets of compressed gas. Depending on which combination of thrusters is fired, the spacecraft turns on one or more of its three principal axes: roll, pitch, and yaw. Roll is a spacecraft’s rotation around its longitudinal axis, the horizontal axis that runs from front to re...