There's something mysterious about Jupiter's many moons

Jupiter triple transitNASA, ESA, Hubble Heritage TeamThree of its largest moons cast shadows on Jupiter.

When it comes to moons, Earthlings have it easy — just one moon to keep track of. And except for cloudy days and new moons it’s usually not too hard too spot.

That’s nothing like the headache of trying to keep track of Jupiter’s moons, which seem to multiply mysteriously all the time — and occasionally even disappear.

Lately, our largest neighbour has been picking up satellites so quickly that astronomers haven’t even been sweating over giving them real names (hope S/2000 J11 rolls off your tongue!).

But let’s backtrack to the very first time someone discovered new moons around Jupiter. The year was 1610 and the someone was Galileo Galilei. Thanks to his extremely detailed note-taking, we know exactly what happened.

The original four moons

On January 7, he was admiring Jupiter through his telescope and saw a few bright dots near it. He thought they were stars, but the next evening they had moved the wrong direction compared to the planet. Later that week, he started seeing a fourth bright dot. By January 15, he identified them as moons of Jupiter, and in March he published his observations in his world-shaking book, Sidereus Nuncius. The observations helped overthrow the belief that the Earth was the center of the universe.

Jupiter moonsNASAFrom left to right: Io, Europa, Ganymede, and Callisto.

We know those first four moons now as the Galilean moons, although Galileo actually called them the Medicean Stars to suck up to his patrons, the famous Medici family. (A few years later, another astronomer actually tried to replace this term with something honouring his 
own patron, but that didn’t stick.)

In terms of individually naming the first four moons, it took a suggestion from another brilliant astronomer, Johannes Kepler — and the passage of more than 200 years — to settle on Io, Europa, Ganymede, and Callisto, named for Jupiter’s mythological mistresses. And until 1892, that was it! Just four moons, not so hard.

The Galilean moons are still the largest, and probably the coolest, of Jupiter’s moons. Io is the most volcanically active place in the solar system, with 250-mile-high lava spurts possibly fed by an ocean of molten rock. Europa, which is the smallest of these four moons at about the same size as Earth’s moon, is covered in ice and a current leading contender for finding alien life. Ganymede, the largest of Jupiter’s moons, may be hiding a saltwater ocean under its rocky crust. Callisto sports the highest concentration of impact craters than any place else in the solar system.

Io europa ganymede callisto jupiter moonsNASA/JPL/DLRFrom left to right, Io, Europa, Ganymede, and Callisto.

And we’re only going to learn more about these moons in the coming years. NASA wants to send a spacecraft to Europa in the 2020s and the European Space Agency wants to visit Europa, Ganymede, and Callisto by about 2030. If you want to get your Galileo on, you can use this calculator to check where these four moons are compared to Jupiter.

We’ve got company

Astronomers started spotting more moons around Jupiter in the early 20th century, discoveries that were then augmented by the 1979 fly-bys of the two Voyager spacecraft. By the end of 1999, we had tallied up 17 moons around Jupiter.

But that number began to explode in 2000, thanks to an astronomer named Scott Sheppard, who went looking for “irregular satellites.” Those aren’t like our moon or other larger ones, which formed by clumps of matter sticking together. Instead, they’re likely hunks of rocks the planet has pulled into its orbit — which is part of why half of Jupiter’s moons orbit in the wrong direction.

The current tally is 67 moons total, nosing out Saturn’s 62 to be the most moons claimed by a single planet. The most recent additions, number 66 and 67, were discovered in 2011.

But astronomers haven’t actually caught up to naming all those moons — only 51 have approved names. The rest are labelled with their discovery year.

A disappearing moon and a mystery

S/2000 J11 was spotted in 2000, then seemed to disappear, hidden in the glare of Jupiter itself. Astronomers thought it might have been destroyed. It took a decade to rediscover the little rock, but it’s now firmly back on the books.

There’s no telling how many more moons astronomers could discover. But there’s one intriguing tidbit tying together the moons of Jupiter, Saturn, and Uranus. For each of these planets, the mass of known moons adds up to about 1% of 1% of the mass of the planet. That’s a key observation for understanding the formation (and destruction) of moons. It even has some astronomers thinking they could be close to solving the mystery of how all these moons came to be.

NASA created an incredible visualisation of the orbits of 64 of Jupiter’s moons. First you see the four innermost small moons, then the Galilean moons. Then, zooming out from the planet, they have tracked the orbits of 56 of the smaller moons orbiting within a mind-boggling 15 million miles of Jupiter.

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