Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, and slightly less than one one-thousandth the mass of the Sun. Jupiter is the third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after Jupiter, the chief deity of ancient Roman religion. Jupiter is primarily composed of hydrogen, followed by helium, which constitutes a quarter of its mass and a tenth of its volume. The ongoing contraction of Jupiter's interior generates more heat than the planet receives from the Sun. Because of its rapid rotation rate of 1 rotation per 10 hours, the planet's shape is an oblate spheroid: it has a slight but noticeable bulge around the equator. The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries. A prominent result of this is the Great Red Spot, a giant storm which has been observed since at least 1831. Jupiter is surrounded by a faint planetary ring system and has a powerful magnetosphere. The planet's magnetic tail is nearly 800 million kilometres (5.3 astronomical units; 500 million miles) long. Jupiter has 92 known moons and probably many more, including the four large moons discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto. Ganymede, the largest of the four, is larger than the planet Mercury. Callisto is the second largest; Io and Europa are approximately the size of Earth's moon. Pioneer 10 was the first spacecraft to visit Jupiter, making its closest approach to the planet in December 1973. Jupiter has since been explored by multiple robotic spacecraft, beginning with the Pioneer and Voyager flyby missions from 1973 to 1979. The Galileo orbiter arrived in orbit around Jupiter in 1995. In 2007, New Horizons visited Jupiter for a gravity assist to increase its speed and bend its trajectory on the way to Pluto. The latest probe to visit Jupiter, Juno, entered its orbit in July 2016. Future targets for exploration in the Jupiter system include its moon Europa, which probably has an ice-covered liquid ocean which scientists think could sustain life.

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