The Pioneer series of U.S. space probes was equipped with cameras and instruments to detect subatomic particles, meteorites, and electric and magnetic fields in the solar system and interstellar space.
The giant gaseous world Jupiter, the solar systemís largest planet, had its first visit from a spacecraftóPioneer 10óon December 1, 1973. Pioneer 10 flew past Jupiter 21 months after launch and sent back images of the planetís turbulent, multicolored atmosphere. Pioneer 10 also investigated Jupiterís intense magnetic field, and the associated belts of trapped radiation. Acting like a slingshot, Jupiterís 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 probeís 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 planetís cloud-swirled atmosphere and its family of moons. Other sensors probed the planetís 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 planetís rings. It also photographed the Saturnian moons, including planet-sized Titan. Voyager 1 found Titanís 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.
NASAís Galileo orbiter reached Jupiter in December 1995. The spacecraft deployed a probe that entered Jupiterís 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 Jupiterís atmosphere from within the atmosphere. The Galileo spacecraft then began a long-term mission to study Jupiterís atmosphere, magnetosphere, and moons from an orbit around the planet. NASA extended the spacecraftís 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.