1. Characteristics ↑
Jupiter is the fifth closest planet to the Sun. It takes over 4000 days to orbit the Sun, this is almost 12 years, and one day on Jupiter is just under 10 hours long. Jupiter is over twice as massive as all the other planets in the Solar System combined. It is visible from Earth, and sometimes appears as bright as Mars. As with Mercury, Venus, and Mars, Jupiter is named after a Roman god, the god of the sky, known to the Greeks as Zeus[2a].
Jupiter's southern pole, image taken by Cassini on its way to Saturn. Image credit: NASA/JPL/Space Science Institute/Public domain.
Jupiter is thought to have a solid core, which is surrounded by hydrogen and helium gas[2b]. Jupiter's Great Red Spot, which was first observed by English natural philosopher Robert Hooke in 1664, is now known to be a storm that is over three times the size of Earth.
2. Moons ↑
Jupiter has a faint ring system and 67 moons, most of which are less than 10 km in diameter. The largest of Jupiter's moons are the Galilean satellites - Ganymede, Callisto, Io, and Europa - these were discovered by Italian natural philosopher Galileo Galilei in 1610.
A composite image of Jupiter and the Galilean moons from left to right: Io, Europa, Ganymede, and Callisto (to scale). Image credit: NASA/JPL/DLR/Public domain.
2.1 Io ↑
Io is the closest Galilean moon to Jupiter and has the third largest mass of the Galilean moons[7a]. The strong gravitational pull of Jupiter causes it to be geologically active, and it's thought to be the most geologically active object in the Solar System, with over 500 volcanic centres and over 100 mountains, some almost twice the height of Everest.
Io is mostly composed of silicate rock. Its surface is covered with sulfur, and sulfur dioxide frost, and it has a molten core made of iron and iron sulfide.
2.2 Europa ↑
Europa is the second Galilean moon from Jupiter, it's also the least massive of the Galilean moons and has a diameter similar to that of the Earth's Moon[7b]. It has a thin oxygen atmosphere and a surface composed of frozen water. Europa's interior is mostly composed of silicate rock, with an iron core. Europa is also expected to contain a salt-water ocean, which extends for about 100 km below its frozen surface.
Artist's impression of Europa, with Jupiter and Io in the background. Image credit: NASA/JPL-Caltech/Public domain.
2.3 Ganymede ↑
Ganymede is the third Galilean moon from Jupiter. It's the largest and most massive moon in the Solar System. It's also larger than Mercury, although not as massive[7c]. Ganymede is primarily composed of silicate rock and frozen water[15a], but a saltwater ocean is thought to exist nearly 200 km below the surface, between two layers of ice.
The surface of Ganymede is covered in dark cratered regions and lighter regions, which are covered in ridges and grooves. Ganymede has a thin oxygen atmosphere and a liquid iron core[15b]. It's the only moon in the Solar System known to have a magnetosphere[7d].
Artist's impression of Ganymede's interior. Image credit: NASA, ESA, and A. Feild (STScI)/Public domain.
2.4 Callisto ↑
Callisto is the furthest of the Galilean moons. It has the second largest mass of the Galilean moons and is about the same size as Mercury[7e].
The surface of Callisto is composed of silicate rock and frozen water. Callisto does not feel Jupiter's tidal effects as strongly as the other moons, and so it has no active volcanoes. This means that there is nothing to periodically fill in old craters, and so Callisto is the most heavily cratered object in the Solar System.
Callisto has a thin atmosphere composed of carbon dioxide. It's thought to have a small silicate core, and a large icy mantle, with an ocean of liquid water between the mantle and the surface.
3. Missions to Jupiter ↑
The first spacecraft to observe Jupiter, as well as all the Galilean moons, was NASA's Pioneer 10 probe in 1973. This was followed by NASA's Pioneer 11 in 1974, and Voyager 1 and Voyager 2, both in 1979. All of these probes were on their way out of the Solar System. Pioneer 11 came closest to Jupiter, flying within 34,000 km, and Voyager 2 was furthest coming within 570,000 km.
In 1990, NASA and the European Space Agency (ESA) launched Ulysses, which came within 400,000 km of Jupiter in 1992, and within 120 million km in 2004.
NASA's Galileo spacecraft launched in 1989. The Galileo orbiter went into orbit around Jupiter in 1995, after flying by all of the Galilean moons. It released an atmospheric probe, which lasted for just under an hour. The orbiter was deliberately steered into Jupiter and destroyed in 2003, to avoid it contaminating any moons that may harbour life.
NASA launched Juno in 2011, and it entered into orbit around Jupiter in July 2016. Juno will observe Jupiter's gravitational and magnetic fields, and the composition and dynamics of its atmosphere, in order to help us understand how planets like Jupiter form.
The ESA plan to launch a mission to the Galilean moons in 2022, known as Jupiter Icy Moon Explorer (JUICE). The JUICE spacecraft arrive at Jupiter in 2030. It will then fly by Callisto and Europa, before orbiting Ganymede.
NASA also plans to launch a mission to the Galilean moons in the 2020s, which will focus on looking for evidence for life on Europa. This is known as the Europa Multiple-Flyby Mission.
Artist's impression of people exploring Jupiter. Image credit: NASA/JPL/Public domain.