One of Jupiter's asteroid 'moons' is orbiting backwards

It's defying all expectations.

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QAI Publishing/UIG via Getty Images
QAI Publishing/UIG via Getty Images

As a rule, you can safely assume that moons and asteroids will all orbit in the same direction. If they didn't, interactions with their host would likely send them flying off course. However, it's now clear that there are exceptions to this rule: researchers have discovered that an asteroid is crossing Jupiter in the opposite direction of all planets in the Solar System. These backwards-orbiting rocks are rare anywhere in the system (they represent just 0.01 percent of known asteroids), but this is the very first time one has been caught doing so in tandem with a planet. How is it sticking around, then?

In a sense, it's a matter of luck. The asteroid travels on alternating sides of Jupiter, with one pass reeling it in while another pushes it outward -- it's in just the right trajectory to maintain a yin-yang balance. This is a stable orbit, too, as scientists estimate that the asteroid has been locked to this path for about 1 million years.

The finding isn't completely shocking given the scale of the cosmos (something like this was bound to crop up), but it's still important. It could help shed new light on how orbits work, and it's a friendly reminder that space doesn't always conform to expectations.

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