- Grades: 6–8, 9–12
The exploration of other bodies in the solar system began within a few years of the first satellites. Both U.S. and Soviet space engineers set their sights on the Moon. Early Soviet launches in 1958 all failed and were never announced. Several U.S. launches also failed, although two of them ( Pioneer 1 and 3) reached nearly 100,000 km into space before falling back to Earth. The first probe to escape Earth's gravity was the Soviet Luna 1, launched on Jan. 2, 1959, which passed the Moon and continued into interplanetary space. The U.S. probe Pioneer 4, launched two months later, followed the same path. Later Soviet probes either hit the Moon or passed it and took photographs of the hidden far side, relaying them back to Earth.
In the mid-1960s three NASA programs pursued the lunar objective. Ranger probes crashed into the Moon's surface but succeeded in sending high-resolution photographs prior to impact. Surveyor probes soft-landed on the Moon and analyzed its surface, while Lunar Orbiter probes circled the Moon and sent back pictures both of potential landing sites for astronauts and of areas of general scientific interest. Soviet efforts proceeded along similar lines and achieved limited successes shortly before their U.S. equivalents (Luna 9 made the first survivable hard lunar landing in 1966, and Luna 10 became the first probe to enter lunar orbit a few months later). The Soviets later operated heavy automatic probes that retrieved small amounts of lunar soil, deployed wheeled rovers called Lunokhods, and made lunar-orbit surveys.
The USSR also secretly aimed two Soviet rockets toward Mars in October 1960, but both failed and were not announced. Other shots in the Soviet series also failed, including later landing attempts. The first successful Mars probe was the U.S. Mariner 4, in 1964. Two more Mars flyby missions and an orbital photographic flight followed before the landings of Viking spacecraft in 1976. The USSR sent two probes to reach the Martian moon Phobos in 1989, but one failed and contact was lost with the other. Contact was also lost with the U.S. Mars Observer in 1993. In 1997 the U.S. Mars Pathfinder conducted a successful mission on the Martian surface, while the orbiting Global Surveyor prepared to conduct a lengthy mapping program.
Soviet probes toward Venus also failed in early attempts, and the first spacecraft to reach the planet successfully was NASA's Mariner 2, in 1962. Later Soviet Venera atmospheric probes eventually returned some basic data, and Mariner 5 provided sophisticated measurements during a flyby. A Soviet probe survived briefly on the surface of Venus in 1971, with more advanced landers following in 1975 and later. Two NASA Pioneer missions in 1978 provided additional atmospheric and topographical data, while a pair of Soviet probes radar-mapped the northern hemisphere in 1983. Two Soviet spacecraft dropped instrumented balloons into the upper atmosphere in 1985. The U.S. Magellan probe attained orbit around Venus for an extended mapping mission in 1990.
The only mission to Mercury was the triple flyby performed by Mariner 10 in 1974-75. The craft returned detailed photographs and environmental measurements of the planet.
Among the giant outer planets the first missions to Jupiter were NASA's Pioneer 10 and Pioneer 11 probes, launched by Atlas-Centaur boosters in 1972-73. They each took more than two years to reach their destination before continuing on toward the outer solar system. Their measurements of the environment of Jupiter and particularly of its severe radiation belts paved the way for the Voyager probes launched in 1977. Voyager 1 and Voyager 2 discovered new features of the Jupiter system, such as active volcanism on Jupiter's satellite Io. In 1989 the U.S. Galileo probe, launched from a Space Shuttle, was sent toward Jupiter. After photographing the asteroids Ida and Gaspra, the craft reached Jupiter, dropped a probe into its atmosphere in December 1995, and began orbiting the planet to observe its moons.
Saturn was first visited by the Pioneer 11 spacecraft in 1979, and in the following year both Voyager probes passed the planet, measuring its environment and observing its atmosphere, satellites, and impressive system of rings. Voyager 2 went on to become the first probe to reach the planets Uranus, in 1986, and Neptune, in 1989. The U.S. Cassini probe should reach Saturn and its largest moon Titan early in the 21st century.
The first probe to a comet was the International Sun-Earth Explorer launched by NASA in 1978 into a high lunar orbit and then diverted five years later for an interception of Comet Giacobini-Zinner. In 1986 four probes from Earth - two Soviet Vega missions, ESA's Giotto, and a Japanese payload - passed near Halley's comet.
Many early Pioneer probes were launched to observe the Sun from different angles. Two German spacecraft, Helios 1 and Helios 2, were sent well within the orbit of Mercury to measure the Sun's magnetic field and other environmental features. In the mid-1990s a European probe named Ulysses observed the polar regions of the Sun.