Space Exploration

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4

The Outer Planets

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

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

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

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

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Voyager 2 made its own flybys of Jupiter in July 1979 and of Saturn in August 1981. It continued outward to make the first spacecraft visits to Uranus in January 1986 and Neptune in August 1989. Like Pioneer 10 and 11, the Voyagers are now headed for interstellar space. Scientists hope both Voyagers will continue sending back data as they reach the edge of the solar system and beyond.

NASA’s Galileo orbiter reached Jupiter in December 1995. The spacecraft deployed a probe that entered Jupiter’s atmosphere on December 7, 1995, radioing data for 57 minutes before succumbing to intense pressures. The probe sent back the first measurements of the composition and structure of Jupiter’s atmosphere from within the atmosphere. The Galileo spacecraft then began a long-term mission to study Jupiter’s atmosphere, magnetosphere, and moons from an orbit around the planet. NASA extended the spacecraft’s mission to include measurements taken simultaneously by the Galileo orbiter and by a new spacecraft, Cassini, which visited Jupiter on its way to Saturn. NASA dove the spacecraft into Jupiter’s atmosphere when Galileo’s fuel dwindled in September 2003.

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

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Other Solar System Missions

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

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

Asteroids are chunks of rock that vary in size from dust grains to tiny worlds, the largest of which is more than a third the size of Earth’s Moon. These rocky bodies, composed of debris left over from the formation of the solar system, are among the latest solar system objects to be visited by spacecraft. The first such encounter was made by the Galileo spacecraft, which passed through the solar system’s main asteroid belt on its way to Jupiter. Galileo flew within 1,600 km (1,000 mi) of the asteroid Gaspra on October 29, 1991. Galileo’s images clearly showed Gaspra's irregular shape and a surface covered with impact craters. On August 28, 1993, Galileo passed close by the asteroid 243 Ida and discovered that it is orbited by another, smaller asteroid, subsequently named Dactyl. Ida is the first asteroid known to possess its own moon. On June 27, 1997, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft flew past asteroid 253 Mathilde. NEAR reached the asteroid 433 Eros and became the first spacecraft to orbit an asteroid in February 2000. The United States launched the spacecraft Deep Space 1 (DS1) in 1998 to prepare for 21st-century missions within the solar system and beyond. In July 1999 DS1 flew by the small asteroid 9969 Braille and discovered that it is composed of the same type of material as the much larger asteroid 4 Vesta. Braille may be a broken piece of Vesta, or it may have simply formed at the same time and place as Vesta in the early solar system. Japan's Hayabusa space probe reached the asteroid Itokawa in 2005 and tried to retrieve a sample that is scheduled to be brought back to Earth in 2010. NASA launched the Dawn spacecraft toward the asteroid belt in 2007. Dawn is expected to orbit Vesta in 2011 and the dwarf planet Ceres in 2015.

Comets are icy wanderers that populate the solar system’s outermost reaches. These “dirty snowballs” are chunks of frozen gases and dust. When a comet ventures into the inner solar system, some of its ices evaporate. The comet forms tails of dust and ionized gas, and many have been spectacular sights.

Because they may contain the raw materials that formed the solar system, comets hold special fascination for astronomers. Although many comets have been observed by a variety of space-borne instruments, spacecraft have made close encounters with relatively few. The most famous comet of all, Halley’s Comet, made its most recent passage through the inner solar system in 1986. In March 1986 three separate spacecraft flew near Halley—the USSR’s Vega 1 and Vega 2 probes, and the Giotto spacecraft of the European Space Agency (ESA). Two Japanese spacecraft—Sakigake and Suisei—observed Halley from great distances. These encounters produced valuable data on the composition of the comet’s gas and dust tails and its solid nucleus. Vega 1 and 2 returned the first close-up views ever taken of a comet’s nucleus, followed by more detailed images from Giotto. Giotto went on to make a close passage to Comet P/Grigg-Skjellerup on July 10, 1992.

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

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

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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.

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1

Vostok and Mercury

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

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

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

By today’s standards, Vostok and Mercury were simple spacecraft, though they were considered advanced at the time. Both were designed for the basic mission of keeping a single pilot alive in the vacuum of space and providing a safe means of return to Earth. Both were equipped with small thrusters that allowed the pilot to change the craft’s orientation in space. There was no provision, however, for altering the craft's orbit—that capability would have to wait for the next generation of spacecraft. Compared to Mercury, Vostok was both roomier and more massive, weighing 2,500 kg (5,500 lb)—a reflection of the greater lifting power of the R-7 compared with the U.S. Redstone and Atlas rockets.

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2

Voskhod and Gemini

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

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

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

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

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

Ten piloted Gemini missions took place in 1965 and 1966, accomplishing all of the program’s objectives. In March 1965 Gus Grissom and John Young made Gemini's piloted debut and became the first astronauts to alter their spacecraft's orbit. In June, Gemini 4’s Ed White became the first American to walk in space. Gemini 5’s Gordon Cooper and Pete Conrad captured the space endurance record with an eight-day mission. Gemini 7’s Frank Borman and Jim Lovell stretched the record to 14 days in December 1965. During their flight they were visited by Gemini 6’s Wally Schirra and Tom Stafford in the world’s first space rendezvous. Neil Armstrong and Dave Scott succeeded in making the first space docking by mating Gemini 8 to an unpiloted Agena rocket in March 1966, but their flight was cut short by a nearly disastrous episode with a malfunctioning thruster. On Gemini 11 in September 1966 Pete Conrad and Dick Gordon reached a record altitude of 1,370 km (850 mi). The final mission of the series, Gemini 12 in November 1966, saw Buzz Aldrin make a record five hours of spacewalks. At the conclusion of the Gemini program, the United States held a clear lead in the race to the Moon.

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