History UsefulNotes / TheMoonsofJupiter

Almost as big as {{UsefulNotes/Mercury}} and the third biggest moon in the Solar System, this dead world is outside the main radiation belts of {{UsefulNotes/Jupiter}} and is the outermost Galilean moon. At larger scales, it's covered by craters on top of other craters. At smaller scales, the surface shows more variety, with plains and knobs, probably formed by the erosion of yet more craters. The general calmness of the place (0.01 rem/day average) makes it a good location to colonize, but most writers tend to ignore it. In fact, it has ''seven times less'' radiation than even our own Earth. Callisto is the only Galilean moon not part of an orbital resonance with the others.[[note]]Its expected to join up with the rest though in around 1.5 billion years or so. Creating an 8:1 Laplace resonance[[/note]] Due to the resulting lack of tidal forces from the other three Galilean moons, Callisto's surface has remained largely unchanged since it was bombarded in the early days of the Solar System, making it a valuable source of information about the conditions of that time. This lack of tidal heating also makes Callisto the largest non-differentiated body in the Solar System.

At last count, there were ''95'' of them, of which 52 are named. Many of these have only been found in the last decade. They are all named after the lovers and descendants of Jupiter/Zeus (''[[DoubleStandardRapeDivineOnMortal God of Horndogs]]!''), so you can tell he really got around. (The names of the lovers and descendants are generally the same or negligibly different between Greek and Latin, so there's not much of an issue, but in case of discrepancy both names tend to get used.)

At last count, there were ''seventy-nine'' of them, of which 52 are named. Many of these have only been found in the last decade. They are all named after the lovers and descendants of Jupiter/Zeus (''[[DoubleStandardRapeDivineOnMortal God of Horndogs]]!''), so you can tell he really got around. (The names of the lovers and descendants are generally the same or negligibly different between Greek and Latin, so there's not much of an issue, but in case of discrepancy both names tend to get used.)

Because of its close proximity to its host and the pull from its brothers, Io suffers from huge tidal forces 20,000 times stronger than the ones our moon experiences, [[LethalLavaLand resulting in constant volcanic eruptions jutting out hundreds of kilometers into space]]. The colourful surface, resembling a pizza due to massive deposits of sulfur, is constantly changing as a result and the moons leading side bulges with as much as 100m compared to its trailing. Most of the material in {{UsefulNotes/Jupiter}}'s magnetosphere comes from Io's volcanoes, including the radiation belts and a gas and plasma ring near Io's orbit; as Io's surface gravity is only 18.3% as strong as Earth's, its volcanic gases can easily reach escape velocity. The radiation levels (3600 rem/day) this close to Jupiter as a result will kill you, but you'll have a few hours to savor the unfairness of it all before your nervous system collapses.

Io was the setting for the movie ''Film/{{Outland}}'', and is also the homeworld of Arnold Judas Rimmer, from ''Series/RedDwarf''. The radiation levels may explain much about the Rimmer family, especially Arnold. Also the last world [[VideoGame/Destiny2 touched by the Traveler]]. It also hosts a series of racetracks in ''VideoGame/{{Redout}} II''. In the EdutainmentGame ''VideoGame/TheMagicSchoolBus Explores the Solar System'', Io is where you make your landing when you travel to Jupiter.

Other than Galileo making [[TheLastDance a suicide run for its final mission]][[note]]And even then the many mechanical problems experienced by Galileo meant that a lot of data was never acquired.[[/note]] Io's close proximity to Jupiter and the sheer amount of radiation at that distance means that the moon hasn't been studied very closely compared to the other Galilean moons and the below mentioned JUICE will for the most part skip it. A couple dedicated missions are proposed though, such as the NASA [[https://en.wikipedia.org/wiki/Io_Volcano_Observer Io Volcano Observer]] (IVO) and the CNSA [[https://en.wikipedia.org/wiki/Tianwen-4 Tianwen-4]]. Booth of which may launch in around 2029 if accepted.

* Inexplicably, ''Series/Babylon5'' has a major Earth Alliance space station, including Sol system's primary [[PortalNetwork jumpgate]], orbiting Io. Why this is so given the extreme radiation, debris hazards, steepness of Jupiter's gravity well, and instability of orbits around something close to a much larger celestial body is not explained.

The first four to be found (Io, Europa, Ganymede and Callisto) were discovered by Galileo himself, and thus are called the Galilean Moons. They dealt a significant blow to the notion that the Earth was the center of the universe, although [[CommonKnowledge contrary to popular belief]] they did not disprove the idea altogether; the existence Jupiter's moons did not have any bearing on the question of whether Jupiter itself orbited the Earth. Galileo [[YesMan wanted to name them after his patrons]], the Medicis, but later generations of astronomers thought otherwise. One night after Galileo discovered them, a German by the name of Simon Marius — who'd independently gotten the idea of pointing a telescope at the heavens — also saw the same four moons, and named them after four of {{UsefulNotes/Jupiter}}'s lovers from Myth/ClassicalMythology; it's these names that are still in use today.

Due to its interesting status in the solar system, Europa has been the subject of many proposed probe launches to study or even drill into its surface. Most famous was probably the joint ESA/NASA [[https://en.wikipedia.org/wiki/Europa_Jupiter_System_Mission_%E2%80%93_Laplace Europa Jupiter System Mission – Laplace]] (EJSM/Laplace) that unfortunately never got out of the planning stage. Or at least not in its original context, as both NASA and ESA has probes planned to jointly study the moon with the [[https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer Jupiter Icy Moons Explorer]] and the [[https://en.wikipedia.org/wiki/Europa_Clipper Europa Clipper]]. Both of which will make several flybys at around 2029 and 2026-31 respectively depending on launch window.

Due to its interesting status in the solar system, Europa has been the subject of many proposed probe launches to study or even drill into its surface. Most famous was probably the joint ESA/NASA [[https://en.wikipedia.org/wiki/Europa_Jupiter_System_Mission_%E2%80%93_Laplace Europa Jupiter System Mission – Laplace]] (EJSM/Laplace) that unfortunately never got of the planning stage. Or at least not in its original context, as both NASA and ESA has probes planned to jointly study the moon with the [[https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer Jupiter Icy Moons Explorer]] and the [[https://en.wikipedia.org/wiki/Europa_Clipper Europa Clipper]]. Both of which will make several flybys at around 2029 and 2026-31 respectively depending on launch window.

The discovery of the four Galilean moons was a major blow to the geocentric model of the universe. Until Isaac Newton defined his Laws of Gravity, it was widely held that Earth could not revolve around the Sun because the Moon would be left behind. Once it was observed that Jupiter was clearly dragging four objects along its orbit, it raised the question of why Earth couldn't do the same with the Moon. (That being said, it wasn't definitive proof; there was also strong evidence that while the planets revolved around the Sun, the Sun and Moon might revolve around the Earth. Solid proof of the Sun-centered Solar System would not come until the 19th century, when astronomers measured parallax in a star that showed the stars to be super-far away. By that point, most astronomers had accepted heliocentrism, but they were basing it on a sense that that solution, which was more elegant, was more likely true, rather than solid evidence.)

Because of the pull of Europa and Ganymede, Io suffers from huge tidal forces, [[LethalLavaLand resulting in constant volcanic eruptions jutting out hundreds of kilometers into space]]. The colourful surface, resembling a pizza due to massive deposits of sulfur, is constantly changing as a result. Most of the material in {{UsefulNotes/Jupiter}}'s magnetosphere comes from Io's volcanoes, including the radiation belts and a gas and plasma ring near Io's orbit; as Io's surface gravity is only 18.3% as strong as Earth's, its volcanic gases can easily reach escape velocity. The radiation levels (3600 rem/day) this close to Jupiter as a result will kill you, but you'll have a few hours to savor the unfairness of it all before your nervous system collapses.

Almost as big as {{UsefulNotes/Mercury}} and the third biggest moon in the Solar System, this dead world is outside the main radiation belts of {{UsefulNotes/Jupiter}} and is the outermost Galilean moon. At larger scales, it's covered by craters on top of other craters. At smaller scales, the surface shows more variety, with plains and knobs, probably formed by the erosion of yet more craters. The general calmness of the place (0.01 rem/day average) makes it a good location to colonize, but most writers tend to ignore it. In fact, it has ''seven times less'' radiation than even our own Earth. Callisto is the only Galilean moon not part of an orbital resonance with the others. Due to the resulting lack of tidal forces from the other three Galilean moons, Callisto's surface has remained largely unchanged since it was bombarded in the early days of the Solar System, making it a valuable source of information about the conditions of that time. This lack of tidal heating also makes Callisto the largest non-differentiated body in the Solar System.