Prior to missions to the Moon and the planets in the solar system our knowledge of what lay beyond Earth was minimal. Five millennia of astronomical observation had produced an incomplete picture of the solar system. Although the Moon and planets were neighbors, there was only so much that could be learned from even the best telescopes. Only by sending spacecraft and astronauts on programs of exploration could we examine our neighbors in space more closely.
The first objective for both the United States and the Soviet Union was reaching the Moon. In September 1959 the Soviet probe Luna 2 struck the Moon. Three weeks later, Luna 3 sent back the first grainy images of the Moon's farside. For the United States, the Ranger project of the 1960s marked the first effort to launch probes toward the Moon. A variety of difficulties plagued the first several Ranger missions, and it was not until Ranger 7, in July 1964, that the program achieved complete success. Two more Ranger spacecraft were launched, including Ranger 8, which took 7,300 images before crash-landing in the Sea of Tranquility, where the Apollo 11 astronauts would land four and a half years later.
The Ranger program was the first of three intermediate steps leading to Apollo. Next came the Lunar Orbiter program, which photographed potential Apollo landing sites. Altogether, five Lunar Orbiter spacecraft were launched from 1966 to 1967. By the end of the fourth mission, Lunar Orbiter probes had surveyed 99 percent of the front and 80 percent of the backside of the Moon. While Lunar Orbiters snapped photographs overhead, the Soviets and Americans perfected soft landing techniques. In February 1966, a 100-kilogram Soviet probe, shaped like a beach ball, touched down on the Moon and returned the first images of the lunar surface.
The Americans countered the Soviet success with a program called Surveyor. Once on the surface, the tripod-shaped Surveyors evaluated the lunar soil and environment. Surveyor 1 made a successful soft landing in three centimeters of dust in the Ocean of Storms in June 1966. Surveyors 3, 5, 6 and 7 landed at different sites and carried out experiments on the surface, including analyzing the chemical composition of the lunar soil. All told, Surveyors acquired almost 90,000 images from five landing sites. The success of the Ranger and Surveyor programs and that of the five Lunar Orbiters gave the National Aeronautics and Space Administration (NASA) the confidence that humans could go the Moon.
In July 1969, Apollo 11 became the first mission to land humans on the Moon when Neil Armstrong and Edwin "Buzz" Aldrin piloted their lunar module "Eagle" to the Sea of Tranquility. Four months later, Apollo 12 landed at the site where Surveyor 3 had touched down in the Sea of Storms. Four more Apollo missions visited the Moon through December 1972. By the end of the program, Apollo astronauts had returned nearly 380 kilograms of samples from the Moon. Besides the samples, data from lunarorbital experiments and information from lunar surface experiments were returned. Over the same time period, the Soviet Union retrieved several hundred grams of lunar material using Luna probes.
Beyond the Moon
As plans were getting under way to explore the Moon, NASA also focused on the rest of the solar system. The Mariner series of missions were designed to be the first U.S. spacecraft to reach other planets. Mariner 2 became Earth's first interplanetary success. After a flawless launch, the Mariner 2 spacecraft encountered Venus at a range of 35,000 kilometers in December 1962. As it flew by, Mariner 2 scanned the planet and revealed that Venus has an extremely hot surface. Mariner 2 also measured the solar wind, a constant stream of charged particles flowing outward from the Sun.
In July 1965, Mariner 4 provided the first close look at Mars. The twenty-two fuzzy images returned by Mariner 4 revealed a planet pocked with craters. Four years later, Mariner 6 and 7 provided 200 more images of the Red Planet. Late in 1971, Mariner 9 went into orbit around Mars and a new era of Mars exploration dawned. Previous missions had been flybys , but Mariner 9 became the first artificial satellite of Mars. Upon arrival, a dust storm obscured the entire planet, but after the dust cleared, Mariner 9 revealed a place of incredible diversity that included volcanoes and a canyon stretching 4,800 kilometers. More surprisingly, Mariner 9 radioed back images of ancient riverbeds carved in the landscape.
Mariner 9 was followed in 1976 by Viking 1 and 2, each consisting of a lander and an orbiter. Each orbiter-lander pair entered Mars orbit; then the landers separated and descended to the planet's surface. The Viking 1 lander touched down on the western slope of Chryse Planitia ("Plains of Gold") on July 20, 1976, the seventh anniversary of the Apollo 11 Moon landing. Within an hour of landing, the first photos of Mars' surface were radioed back to Earth. Besides taking photographs, both Viking 1 and 2 landers conducted biology experiments to look for signs of life. These experiments discovered unusual chemical activity in the Martian soil, but provided no clear evidence for the presence of microorganisms. However, both landers provided a wealth of data about the Martian surface, and the Viking 1 and 2 orbiters took thousands of images from above.
While the United States focused much attention on Mars throughout the 1960s and 1970s, the Soviet Union flew a series of missions to Venus. Venera 4 became the first mission to place a probe into the Venusian atmosphere in June 1967. In June 1975, probes released by Venera 9 and 10 transmitted the first black and white images of Venus' surface. Other missions followed, including Venera 15 and 16, which produced radar images of the Venusian surface. Venus was also a target of NASA's Mariner 10 mission in 1973 to 1974, which used a "gravity assist" to send the spacecraft on to Mercury. Gravity assist techniques were to play a crucial role in NASA's next phase of planetary exploration—journeys to Jupiter and beyond.
Pioneer 10 and 11 were the first spacecraft to venture beyond the asteroid belt into the realm of the outer planets. Pioneer 11 safely passed through the asteroid belt and passed 42,000 kilometers (26,098 miles) below Jupiter's south pole in December 1974, exactly a year after Pioneer 10's closest approach. Using Jupiter's immense gravity like a slingshot, Pioneer 11 encountered Saturn in September 1979. After passing Saturn, Pioneer 11 plunged into deep space, carrying a plaque similar to that aboard Pioneer 10 in the hope that intelligent life would someday find it.
NASA mission designers recognized that the giant outer planets—Jupiter, Saturn, Uranus and Neptune—would soon align in such a way that a single spacecraft might be able to use gravity assists to hop from one planet to the next. Taking advantage of this alignment, NASA approved the Voyager Project. Voyager 1 made its closest approach to Jupiter in March 1979, and Voyager 2 came within 570,000 kilometers (354,182 miles) of Jupiter in July 1979. Voyager 1 and 2 flybys of Saturn occurred nine months apart, with the closest approaches occurring in November 1980 and August 1981. Voyager 1 then headed out of the orbital plane of the planets. However, Voyager 2 continued onward for two more planetary encounters, coming within 81,500 (50,642 miles) kilometers of Uranus' cloud tops in January 1986, and making a flyby of Neptune in August 1989. Both Voyagers continue to operate and are approaching interstellar space.
While the Voyager missions were highly successful, the pace of planetary exploration slowed in the 1980s. One of the few new missions was Magellan, which went into orbit around Venus in August 1990. Over the next four years Magellan used radar to map 99 percent of the Venusian surface. After concluding its radar mapping, Magellan made global maps of Venus's gravity field. Flight controllers also tested a new maneuvering technique called aerobraking, which uses a planet's atmosphere to slow a spacecraft.
NASA managers also followed up the initial reconnaissance of Jupiter with the Galileo mission. En route to Jupiter, Galileo flew by two asteroids—Gaspra and Ida—the first such visits by any spacecraft. Galileo arrived at Jupiter in December 1995, and dropped an instrumented probe into the giant planet's atmosphere. Since then, Galileo has made dozens of orbits of Jupiter, usually flying close to one of its four major moons. Among its discoveries, Galileo uncovered strong evidence that Jupiter's moon Europa has a saltwater ocean beneath its surface.
In the mid-1980s, the European Space Agency launched its first deep space mission, part of an ambitious international mission to Comet Halley. The plan was to send an armada of five spacecraft—two Soviet (Vega 1 and 2), two Japanese (Sakigake and Suisei) and one European (Giotto)—towards the comet in 1986. A series of images sent back by Giotto revealed the comet nucleus to be a dark, peanut-shaped body, about 15 kilometers long. NASA did not send a mission to Comet Halley for budgetary reasons. Instead, NASA planed to return to Mars after a seventeen-year pause. In September 1992, the United States launched Mars Observer, but the mission ended in failure when contact was lost with the spacecraft as it approached Mars.
A New Strategy: Faster, Better, Cheaper
The loss of Mars Observer prompted NASA to rethink its strategy for planetary exploration. The few large and expensive missions that characterized the preceding fifteen years were replaced by greater numbers of focused, cheaper missions, a strategy described as "faster, better, cheaper." Ironically, it was a joint project between the U.S. military and NASA, called Clementine, which underscored the potential of this concept. Clementine was launched in January 1994 and mapped the lunar surface, providing preliminary evidence of ice at the Moon's poles.
In the early 1990s, NASA established the Discovery program to select low-cost solar system exploration missions with focused science goals. The first Discovery mission was the Near Earth Asteroid Rendezvous (NEAR) mission. NEAR entered orbit around the asteroid Eros in February 2000, beginning a yearlong encounter. The car-sized spacecraft gathered ten times more data during its orbit than originally planned, and completed all the mission's science goals before becoming the first spacecraft to land on the surface of an asteroid.
Mars Pathfinder, the second Discovery class mission, landed on Mars on July 4, 1997, assisted by airbags to cushion the impact. The landing site, known as Ares Vallis, was chosen because scientists believed it was the site of an ancient catastrophic flood. Onboard Pathfinder was a six-wheeled rover named Sojourner. From landing until the last transmission in September 1997, Mars Pathfinder returned more than 16,500 images from the lander and 550 images from the rover, in addition to chemical analyses of rocks and soil plus data on winds and other weather phenomena.
Coinciding with Pathfinder's mission was the arrival of the Mars Global Surveyor (MGS) spacecraft in orbit around the Red Planet in September 1997. Although not a Discovery mission, MGS applied the aerobraking skills pioneered by Magellan. After a year and a half trimming its orbit, MGS began its prime mapping mission in March 1999. From orbit, MGS took pictures of gullies and debris features that suggest there may be current sources of liquid water at or near the planet's surface. In addition, magnetometer readings showed that the planet's magnetic field is localized in particular areas of the crust. MGS completed its primary mission in January 2001, but continues to operate in an extended mission phase.
Since Mars Pathfinder, more Discovery class missions have been launched, including Lunar Prospector, Stardust, and Genesis. The small spin-stabilized Lunar Prospector spacecraft lifted off in January 1998 and spent almost two years measuring the Moon's magnetic and gravitational fields and looking for natural resources, such as minerals and gases, which could be used to sustain a human lunar base or manufacture fuel. Mission scientists believe that Lunar Prospector detected between 10 to 300 million tons of water ice scattered inside craters at the lunar poles.
Stardust, the fourth Discovery mission, was launched in February 1999 and is slated to fly through the cloud of dust that surrounds Comet Wild-2, and bring cometary samples back to Earth in January 2006. Stardust will be the first mission to return extraterrestrial material from outside the orbit of the Moon. Launched in August 2001, the Genesis spacecraft is headed toward an orbit around Lagrangian 1 (L1), a point between Earth and the Sun where the gravity of both bodies is balanced. Once it has arrived, Genesis will begin collecting particles of solar wind that imbed themselves in specially designed high purity wafers. After two years, the sample collectors will be returned to Earth.
In addition to the Discovery program, the U.S. space agency has embarked on a series of New Millennium missions to test advanced technologies. The first New Millennium mission was Deep Space 1, which validated ion propulsion and tested other new technologies, such as autonomous optical navigation, several microelectronics experiments, and software to plan and execute onboard activities with only general direction from the ground.
While the trend in planetary exploration has been toward cheaper, smaller missions, the joint American/European Cassini mission to Saturn represents the opposite approach. Launched in October 1997, Cassini is the most ambitious effort in planetary space exploration ever mounted and involves sending a sophisticated robotic spacecraft to orbit the ringed planet and study the Saturnian system over a four-year period. Onboard Cassini is a scientific probe called Huygens that will parachute through the atmosphere to the surface of Saturn's largest moon, Titan. Cassini will enter Saturn orbit in July 2004, and the Huygens probe will descend to the surface of Titan in November of that year. Building on the spectacular success of exploration programs over the past forty years, future missions are planned to the Moon, Mars, Mercury, Jupiter's moons and beyond.
see also Apollo (volume 3); Apollo Lunar Landing Sites (volume 3); Astrobiology (volume 4); Life in the Universe, Search for (volume 2); Planetary Exploration (volume 1); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2).
John F. Kross
Compton, William D. Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions. Washington, DC: NASA Historical Series (NASA SP-4214), 1989.
Dewaard, E. John, and Nancy Dewaard. History of NASA: America's Voyage to the Stars. New York: Exeter Books, 1984.
Washburn, Mark. Distant Encounters. The Exploration of Jupiter and Saturn. New York:Harcourt Brace Jovanovich, 1983.
Yenne, Bill. The Encyclopedia of US Spacecraft. New York: Exeter Books, 1988.
Deep Space 1. Jet Propulsion Laboratory. <http://nmp.jpl.nasa.gov/ds1/>.
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Galileo Program. Jet Propulsion Laboratory/NASA Current Mission Series. <http://www.jpl.nasa.gov/missions/current/galileo.html>.
Genesis Program. Jet Propulsion Laboratory/NASA Current Mission Series. <http://www.jpl.nasa.gov/missions/current/genesis.html>.
Lunar Prospector. NASA Ames Research Center. <http://lunar.arc.nasa.gov/>.
Mars Pathfinder Program. Jet Propulsion Laboratory/NASA History Series. <http://www.jpl.nasa.gov/missions/past/marspathfinder.html>.
Near Earth Asteroid Rendezvous (NEAR). The John Hopkins University Applied Physics Laboratory. <http://near.jhuapl.edu/>
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