So far space probes have visited every planet in the Solar System except Pluto. Venus was the first planet to be investigated as Mariner 2 flew by in 1962. Mariner 10 orbited Mercury in 1975. Mars is the most visited planet in the Solar System, with over five probes landing on its surface. The Pioneer and Voyager probes, launched between 1979 and 1989, investigated the outer planets Jupiter, Saturn, Uranus and Neptune. The Pluto-Kuiper Express aims to visit Pluto between 2006 and 2008.
NASA launched the Voyager spacecraft in 1977 to take advantage of a rare alignment among the outer four planets (Jupiter, Saturn, Uranus and Neptune) that would not take place for another 175 years. A spacecraft visiting each planet could use a gravitational assist to fly on to the next one, saving on fuel. The original plan called for launching two spacecraft pairs — one pair to visit Jupiter, Saturn and Pluto, and the other to look at Jupiter, Uranus and Neptune. The plan was cut back due to budgetary reasons, resulting in two spacecraft: Voyager 1 and Voyager 2.
The primary five-year mission of the Voyagers included the close-up exploration of Jupiter and Saturn, Saturn’s rings and the larger moons of the two planets. The mission was extended after a succession of discoveries. After passing by Saturn in 1980, Voyager 1 made a sharp turn out of the plane of the solar system. Voyager 2’s trajectory, however, was planned to take it past Uranus and Neptune. While the initial budget for Voyager 2 didn’t guarantee it would last long enough to transmit pictures from those two planets, it thrived and made successful flybys of Uranus in 1986 and Neptune in 1989.
Between them, the two spacecraft have explored all the giant outer planets of our solar system — Jupiter, Saturn, Uranus and Neptune — as well as 49 moons, and the systems of rings and magnetic fields those planets possess.
The current mission, the Voyager Interstellar Mission, was planned to explore the outermost edge of our solar system and eventually leave our sun’s sphere of influence to enter interstellar space — the space between the stars. Since passing the boundary of interstellar space in 2012, Voyager 1 is examining the intensity of cosmic radiation, and also looking at how the sun’s charged particles are interacting with particles from other stars, according to Voyager project scientist Ed Stone. Voyager 2 is still traveling within the solar system, but is expected to breach interstellar space in the next few years.