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Jupiter has a bunch of asteroids that are trapped in two specific points in its orbit!

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Reid: When we think of asteroids, we usually think of the main asteroid belt – y’know, the one between Mars’ and Jupiter’s orbits. But that’s only part of the story. Asteroids can be found all over the Solar System, including some closer to home. And there’s actually another big group of them.

Planetary scientists call them Trojan asteroids, and they’re trapped in specific parts of Jupiter’s orbit because of gravity. These space rocks are probably left over from the formation of the Solar System, and could be a window into the early history of the planets. So, how does one object trap another so precisely?

The guy who figured it out was an Italian mathematician with a very French name, Joseph-Louis Lagrange. In the late 18th century, he was exploring the so-called three-body problem, one of the great unsolvable puzzles in physics. Place any two objects in space, and you can calculate exactly how they’re gonna move, based on mutual gravitational attraction. If you add a third, the problem becomes impossible to solve – you just can’t predict each object’s movement when they’re all affecting each other.

Lagrange was looking for some approximations that could simplify the problem, and published one in 1772: If the largest object is at least 25 times more massive than the next biggest, and both are much larger than the third... the three-body problem becomes solvable! This is true, for example, when you have a satellite moving under the influence of the Earth and the Moon.

And the solution that Lagrange found based on these conditions was really neat: there are two places where the tiny object would be orbiting motionless relative to the larger two – just kinda in sync with them. And if the tiny object is nudged away, the orbit’s Coriolis force kicks in to bring it back – the same force that causes hurricanes to spin here on Earth! These regions – along with 3 others that were found separately – are called the Lagrange points.

Astronomers realized their significance right away: It turns out, the Solar System’s two largest bodies, the Sun and Jupiter, fit Lagrange’s approximation. And by the early 1900s, scientists had bigger telescopes, and could search for asteroids beyond the main belt. Just as they predicted, they found 2 populations of asteroids near Jupiter: one orbiting 60 degrees ahead, and one 60 degrees behind. Always on the lookout for clever names, astronomers used characters from the Iliad, Homer’s epic poem about the Trojan War. So that’s why they’re called Trojan asteroids.

The leading group is Greek warriors, and the following group is Trojan soldiers – with two exceptions. Both were named before the Greece vs. Troy thing was solidified, and doomed to be stuck behind enemy lines! Today, we know of nearly 6,300 Jupiter Trojans, plus one each for Earth and Uranus, four for Mars, and 17 for Neptune. But that’s probably just a tiny fraction of what’s out there.

Some astronomers estimate that there could be more than 600,000 Jupiter Trojans bigger than a kilometer across – but we haven’t detected them yet. Why? Well, asteroids are really dim and difficult to observe, because they’re far away and have dark surfaces. Since the Trojans are even farther than the main belt, they’re just that much harder to detect.

Scientists still debate how Jupiter trapped so many asteroids, but they do think these objects could be remnants from the early Solar System. One theory says that when the Sun was forming four and a half billion years ago, it was surrounded by a disk of objects called planetesimals. Some planetesimals kept growing and formed the inner planets and the cores of the outer ones, while others could’ve been trapped in Jupiter’s Lagrange points, where they’ve stayed ever since.

And some hypotheses of Solar System formation suggest that the giant planets actually moved to get to their current locations – meaning Jupiter would’ve brought its Trojans along, too! Even though we’ve talked about asteroids so far, the term “Trojan” can be used for any object caught in stable Lagrange points. Saturn, for example, hasn’t trapped any known asteroids, but it is home to four tiny Trojan moons! The planet’s large moons Tethys and Dione both have a pair of small icy satellites – the only Trojan moons that we know of in the whole Solar System.

And scientists are looking to learn more. NASA is evaluating five proposals for its next low-cost, Discovery-class mission, which could launch as early as 2020. One’s named Lucy after the fossilized skeleton that helped reveal parts of our history, and hopes to do the same for our Solar System – by traveling to the orbit of Jupiter to explore some of its Trojans for the first time. Since these objects could be some of the building blocks of our Solar System, knowing more about them could help us learn what it was like when the planets were born.

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