When NASA’s Juno spacecraft reaches Jupiter in several years’ time, it will be only the second probe designed mainly to explore Jupiter. The first, Galileo, arrived in the 1990s and was destroyed when it was deliberately crashed into Jupiter’s atmosphere more than a decade ago. However, several past spacecraft have explored Jupiter briefly while they sped by towards more distant destinations, including Pioneer 10, Pioneer 11, and the two Voyager space probes.
Pioneer 10 and Pioneer 11 (1973)
NASA first aimed to explore the outer solar system with Pioneer 10 and Pioneer 11. Pioneer 10 reached Jupiter in December 1973 and took the first close-by color images of the planet ever made by humans. It then continued on towards Neptune, and is now one of the farthest human-built objects from the Sun. The last successful contact with Pioneer 10 was in 2003 and it is now considered dead or inaccessible.
Pioneer 11 reached Jupiter the following year, and passed by at a distance of 21,000 miles. It sent back breathtaking new pictures of the Great Red Spot, the enormous equatorial storm which seems to circle Jupiter permanently. Pioneer 11 was also able to study the Galilean moon Callisto and estimate its mass for the first time. Pioneer 11 is also now headed out of the solar system, and has not been contacted since the mid-1990s.
Voyager 1 (1979)
Launched in 1977, the first of the Voyager probes reached Jupiter in the beginning of 1979. It closed to about a quarter of a million miles with Jupiter, and took pictures over almost three months, both on its approach and on its departure.
Voyager 1 is currently operational, but, as it heads out of the solar system, its nuclear power generator is nearly exhausted. It will lose the ability to record data in a few years, and the last scientific instruments will be disabled by the early 2020s.
Voyager 2 (1979)
The second of the Voyager probes also reached Jupiter in 1979. It crossed within a little over a third of a million miles from Jupiter’s atmosphere, in the process sending back important new images of Jupiter’s rings, its famous Great Red Spot storm, and several of Jupiter’s moons. Several new moons were discovered, including Adrastea, Metis and Thebe. It was Voyager 2’s photographs which first revealed numerous active volcanoes on Io’s surface. Its photographs of Europa were even more mysterious, revealing a bizarre cracked surface.
Voyager continued past the final three gas giants and is now leaving the solar system. It will remain operational for another decade or so before its nuclear power generator is fully exhausted.
Galileo (1995-2003)
The most important mission to explore Jupiter so far, and the only probe actually designed for that purpose, was Galileo. NASA launched this probe in 1989, and it arrived in 1995, closing just in time to see Comet Shoemaker-Levy 9 crash into Jupiter’s upper atmosphere.
During its eight years in orbit, Galileo conducted a variety of different missions, studying both Jupiter and many of its moons. By 2002, plunging through Jupiter’s vast radiation belts had permanently damaged many of its main systems. NASA conducted a few final orbits, and then ended the probe’s life by sending it down into Jupiter’s atmosphere, where it was destroyed.
One of the highlights of the Galileo mission was the despatch of a special atmospheric probe upon the spacecraft’s arrival in 1995 – the first spacecraft ever to enter Jupiter’s atmosphere. The probe managed to descend over one hundred miles before failing due to high temperatures.
Since Galileo, no space probe has explored Jupiter in detail. Several probes en route to other destinations, like Cassini (going to Saturn), Ulysses (studying the Sun), and New Horizons (headed to Pluto and the Kuiper Belt) took pictures of Jupiter as they passed by, but did not add much to astronomers’ knowledge of the planet from previous missions.
