The easiest and least expensive missions to other planetary bodies were those in which the spacecraft hit or just missed (flew by) the planet. Because such a spacecraft spends less than a few hours near the planet and observes only a small area, the amount of information these missions send back to Earth is limited. Still, this information is richer in detail than our Earth-bound views.
Luna 3 was the first spacecraft to view the far side of the Moon. Photographs suggested that the far side was very different from the familiar Earth-facing near side of the Moon. Dark lava-filled mare are rare on the Moon's heavily cratered far side.
The mission of Rangers VI through IX was to take close-up photographs of the lunar surface. These spacecraft were launched between January 30, 1964, and March 21, 1965. Each Ranger spacecraft carried six television cameras. Pictures were transmitted to Earth as the spacecraft plunged toward the Moon at about 2.5 kilometers per second (6000 miles per hour).
Mariner 4 was launched November 28, 1964, to take the first close-up observations of the planet Mars. Instruments on board Mariner found that the Martian atmosphere is less than one-tenth as dense as Earth's. As Mariner 4 passed near Mars, it took 22 photographs of the surface. The only landforms seen were craters, and Mars appeared very moonlike. The later flybys, Mariners 6 and 7, viewed only cratered terrain. We now know that Mars is not moonlike at all, but has volcanoes, vast canyons, and geologic evidence of surface water in ancient times.
Pioneer 10 and 11 and the Voyager 1 and 2 spacecraft were the first probes designed to study the outer gas giants (Jupiter, Saturn, Neptune, Uranus). Previous spacecraft used solar panels to generate power, but at the distances traveled by Pioneer and Voyager spacecraft, a long-term energy source created by radioactive decay of plutonium was required. Voyager used a 3.7 meter (10 foot) antenna to transmit data from up to 3 billion kilometers (1.9 billion miles) away from Earth. These spacecraft also carried numerous instruments and experiments to function during the multiple year mission. Voyager 2 provided the only close-up views of Neptune and Uranus available to date. Pioneer 10 became the first spacecraft to travel beyond our solar system.
The next large probe similar to Pioneer and Voyager in design and scope is Cassini. On its journey to Saturn, Cassini joined the Galileo spacecraft in a three month study of Jupiter and its moons during a flyby in December 2000. The result of this partnership was the observation of a new volcanic plume near the north pole of Jupiter's moon, Io. Information from both spacecraft indicated that the new plume was nearly 400 kilometers (250 miles) high and about the same size as a long-lived plume produced by a volcano on Io named Pele. The presence of such a large plume near the pole came as a surprise, since all previously detected plumes have been over equatorial regions and no others have approached Pele's in size.