Diagram of the Solar System and heliosphere showing the trajectories of the unmanned space probes Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2. These probes were the first to study the outer Solar System, taking advantage of a rare alignment of the gas giant planets to visit them all. The heliosphere is a space in the interstellar medium formed by the Sun and its solar wind. Pioneer 10 was launched on 2nd March 1972, with Pioneer 11 following on 6th April 1973. Voyager 2 was launched on 20th August 1977, followed by Voyager 1 on 5th September 1977. In December 1973 Pioneer 10 was the first probe to encounter Jupiter. In September 1979 Pioneer 11 became the first probe to visit Saturn. Voyager 2 is the only probe to have visited Uranus and Neptune, in 1985 and 1989 respectively. In August 2012 Voyager 1 became the first spacecraft to enter interstellar space.