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Jupiter is the largest of the planets in the solar system, and the first of the giants.

    Vital statistics
  • Mass: 1.9 x 1027 kg = 318 Earths
  • Diameter: 140 000 km = 11.2 Earth's
  • Density: 1.33 g/cm3 = 0.24 Earth's density
  • Distance from Sun: 741 - 817 million km (778.5 Mkm semi-major axis) = 5.0 - 5.5 AU (Earth-Sun distances)
  • Period of orbit: 11.9 Earth years
  • Period of rotation: 9.93 hours
  • No. of moons: 67 (51 of these smaller than 10 km)
  • Atmosphere: Cloud layer, 20-100 kPa 90% hydrogen (H2), 10% He, and trace gases, including methane
Jupiter is only one-thousandth the mass of the Sun, but its mass makes up 72% of all the mass of the planets in the solar system.

Moons of Jupiter

Jupiter has 67 known satellites. Satellites are any objects that orbit another body in space. But only a few of these stellites are large enough to be worthy of the title 'moon'.

Moons of Jupiter
In 1610 Galileo Galilei astonished the world by revealing to the world with his telescope that Jupiter had moons.

When Galileo first developed his telescope in 1610, and used it to look at the night sky, he was astonished to discover that Jupiter had moons. Galileo's telescope was not very powerful, so he could only see 4 of these moons. He named them 'Medici Moons', because his patron (the man paying him) was a duke from the powerful Medici family in Florence.

Today, we think more of Galileo than the Medicis, so we prefer to call these moons 'Galilean Moons'. These are: Io, Europa, Ganymede, and Callisto.

Galilean moon orbits
The orbital periods in Earth days, and relative sizes, of the moons of Jupiter.

The discovery of moons around Jupiter provided proof that not all things in the universe rotated around the Earth - a step towards having the Copernican heliocentric theory accepted. At the time, the Church violently opposed anyone challenging its authority, including scientists. Poor Galileo was persecuted for opening everyone's eyes to the truth about the universe.

Planet sizes

Missions to Jupiter


The first orbiter around Jupiter, this US/German probe was operational from 1995 - 2003. Launched on October 18, 1989, it used gravity slingshot assist flybys from Venus and the Earth, to enter Jovian orbit on December 7, 1995. there have been many flybys of Jupiter, but Galileo was the first orbiter. It dropped a probe into the atmpsphere. Galileo also made an asteroid flyby (951 Gaspra). Galileo observed the impact of comet Shoemaker-Levy 9 into Jupiter in 1994.


Juno is a NASA probe, launched on August 5, 2011, and due to arrive at Jupiter on July 4, 2016. Its mission is a polar orbit, at an altitude of 559 km, to study Jupiter's magnetic and gravitational fields, and the polar magnetosphere. It will attempt to gather information about the structure and composition of the planet, to help advance our understanding of how the planet formed. Instead of radioisotope thermoelectric generators (used on outer solar system probes), Juno relies on 3 large solar cell array wings.

The 2-year orbital phase of the mission forsees 37 orbits, each 14 Earth-days long, before allowing the craft to burn up in the atmosphere. It will measure the abundances of water and oxygen. The mission is controlled by JPL, with Scott Bolton the chief investigator in charge. Total cost is 1.1 billion dollars.

One of the goals of the mission is to study orbital frame-dragging (Lense-Thirring precession), which will test General Relativity effects related to the angular momentum of Jupiter's rotation.

The craft carries a 35cm2 plaque with a dedication to Galileo Galilei, with a portrait of the great discoverer of three of the moons of Jupiter, and a text in his own handwriting, made in 1610.

Juno's investigation of the formation of Jupiter

It is not known whether Jupiter has a solid, rocky core, or whether it is a cloud of mainly hydrogen and helium all the way through. How the solar system was created has several mysteries. One of which is how did the outer planets form where they are, if indeed they are in the location of their genesis.

Kevin Walsh of Southwest Research Institute, Boulder Colorado, a specialist in planet formation, believes the most likely scenario for the formation of Jupiter was a solid, rocky core (primarily ice), ten times of the mass of the Earth, which pulled in the blanket of cloud around it. An alternative is a rapid compaction of the hydrogen and helium 'dust', in a similar process to the Sun, and indeed the other gas giants.Planned Missions

Jupiter Icy Moon Explorer

Due for launch 2022, orbit insertion of Jupiter 2030, orbit of Ganymede 2033. Flybys of Callisto and Europe.

Content © All rights reserved. Created : February 11, 2014 Last updated :April 2, 2016

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