The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope enabled the discovery of the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.
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 2.5 times that of all the other planets in the Solar System combined, and slightly less than one-thousandth the mass of the Sun. Its diameter is eleven times that of Earth, and a tenth that of the Sun. Jupiter orbits the Sun at a distance of 5.20 AU (778.5 Gm), with an orbital period of 11.86 years. It is the third brightest natural object in the Earth's night sky, after the Moon and Venus, and has been observed since prehistoric times. Its name derives from that of Jupiter, the chief deity of ancient Roman religion.
Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and as large as the smallest planet Mercury. Callisto is, with a diameter of 4,821 km, roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1,883,000 km, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars.
Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 95 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered independently by Simon Marius and Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.
Ganymede, or Jupiter III, is the largest and most massive natural satellite of Jupiter and in the Solar System. It is the largest Solar System object without a substantial atmosphere, despite being the only moon in the Solar System with a substantial magnetic field. Like Titan, Saturn's largest moon, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three.
The Jupiter Icy Moons Orbiter (JIMO) was a proposed NASA spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, where an ocean of liquid water may harbor alien life. Ganymede and Callisto, which are now thought to also have liquid, salty oceans beneath their icy surfaces, were also targets of interest for the probe.
There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. All together, Jupiter's moons form a satellite system called the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 91 known moons and the rings together comprising just 0.003% of the total orbiting mass.
Juno is a NASA space probe orbiting the planet Jupiter. It was built by Lockheed Martin and is operated by NASA's Jet Propulsion Laboratory. The spacecraft was launched from Cape Canaveral Air Force Station on August 5, 2011 UTC, as part of the New Frontiers program. Juno entered a polar orbit of Jupiter on July 5, 2016, UTC, to begin a scientific investigation of the planet. After completing its mission, Juno was originally planned to be intentionally deorbited into Jupiter's atmosphere, but has since been approved to continue orbiting until contact is lost with the spacecraft.
The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2023, has continued with eight further spacecraft missions in the vicinity of Jupiter. All of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but two were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On 5 July 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult, mostly due to large fuel requirements and the effects of the planet's harsh radiation environment.
The Europa Orbiter was a planned NASA mission to Jupiter's Moon Europa, that was cancelled in 2002. Its main objectives included determining the presence or absence of a subsurface ocean and identifying candidate sites for future lander missions. Europa Orbiter received pre-project funding in 1998, and resulted from NASA's Fire and Ice project.
The Jovian Europa Orbiter (JEO) was a feasibility study by the European Space Agency for a mission to Jupiter's moon Europa. JEO would be capable of collecting information about Europa by orbiting it, and would have worked together with the Jovian Relay Spacecraft (JRS) and the Jovian Minisat Explorer (JME).
In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surrounded their primary, usually the aftermath of a large collision or leftover material accumulated from the protoplanetary disc. Young regular moons then begin to accumulate material within the circumplanetary disc in a process similar to planetary accretion, as opposed to irregular moons, which formed independently before being captured into orbit around the primary.
The Europa Jupiter System Mission – Laplace (EJSM-Laplace) was a proposed joint NASA/ESA uncrewed space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere. The mission would have comprised at least two independent elements, NASA's Jupiter Europa Orbiter (JEO) and ESA's Jupiter Ganymede Orbiter (JGO), to perform coordinated studies of the Jovian system.
The Jupiter Magnetospheric Orbiter is a cancelled space probe proposed by the Japanese Aerospace Exploration Agency (JAXA), to undertake detailed in situ studies of the magnetosphere of Jupiter as a template for an astrophysical magnetised disk.
JunoCam is the visible-light camera/telescope onboard NASA's Juno spacecraft currently orbiting Jupiter. The camera is operated by the JunoCam Digital Electronics Assembly (JDEA). Both the camera and JDEA were built by Malin Space Science Systems. JunoCam takes a swath of imaging as the spacecraft rotates; the camera is fixed to the spacecraft, so as it rotates, it gets one sweep of observation. It has a field of view of 58 degrees with four filters.
Laplace-P was a proposed orbiter and lander by the Russian Federal Space Agency designed to study the Jovian moon system and explore Ganymede with a lander.
The Jupiter Icy Moons Explorer is an interplanetary spacecraft on its way to orbit and study three icy moons of Jupiter: Ganymede, Callisto, and Europa. These planetary-mass moons are planned to be studied because they are thought to have beneath their frozen surfaces significant bodies of liquid water, which would make them potentially habitable for extraterrestrial life.
The following outline is provided as an overview of and topical guide to Jupiter:
