Jupiter is the fifth planet from the Sun and the largest planet in the Solar System. It is a gas giant with mass one-thousandth of that of the Sun but is two and a half times the mass of all the other planets in the Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian or outer planets. The planet was known by astronomers of ancient times, and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough to cast shadows, and making it on average the third-brightest object in the night sky after the Moon and Venus. (Mars can briefly match Jupiter's brightness at certain points in its orbit.)
The first spacecraft to visit Jupiter was Pioneer 10 in 1973, followed a few months later by Pioneer 11 in 1974. Aside from taking the first close-up pictures of the planet, the probes discovered its magnetosphere and its largely fluid interior. These missions
The Voyager 1, Voyager 2 probes visited the planet in 1979, while Voyager 3 and Voyager 4 flew by in 1981. They studied its moons and the ring system, discovering the volcanic activity of Io and the presence of water ice on the surface of Europa. These missions launched in 1977 from the Space Shuttle and two Chemical Space Tug stages (one of which was discarded while one was reused). They were among the earliest probes to depart from the 12-Person Space Station-III ("Space Base").
In 1982 two Mariner-Class Jupiter flyby probes encountered the Planet and performed a simple Jupiter Flyby. While they each performed a variety of scientific experiments and measurements relating to Jupiter's clouds, composition and magnetic field the spacecraft also deployed two Probes to enter the atmosphere of Jupiter for InSitu study of the environment.
In 1983 two Mariner-Class Saturn Orbiters flew past Jupiter studying the planet as they received their Gravity slingshot, necessary to reach Saturn. Significant information was gathered about Jupiter and its Moons from the advanced imaging hardware.
In July 1985 Ulysses and Telemachus flew over Jupiter's poles in order to gain a gravity assist to depart the ecliptic plane and study the Sun. Like with the previous flybys Galileo and the two flyby craft cooperated and coordinated their efforts during the encounter.