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  • [♪ INTRO]

  • We usually think of orbits as fairly simple shapes,

  • the planets go around the Sun in near-circles, Pluto and other far-flung rocks orbit in squashed circles,

  • and comets get really really close to our star on one end and really far away on the other.

  • But there's a small asteroid that appears to orbit Earth in, of all things, a horseshoe shape.

  • It's called Cruithne, and because of the weird path it takes,

  • it's sometimes referred to as Earth's second moon!

  • But let's get that out of the way fast.

  • Cruithne, which shares its name with an ancient Irish people, is not Earth's second Moon.

  • Because it doesn't actually orbit Earth.

  • But it does kind of look that way.

  • Over a period of nearly eight centuries, as it goes around the Sun,

  • it manages to circle the Earth in a shape that resembles a bean.

  • At least that's what it appears to do if you're actually standing here.

  • That perspective would change if you were to travel straight up

  • and get a bird's-eye view of the solar system.

  • In that case, you'd see that Cruithne's orbit goes around the Sun, but crosses Earth's orbit,

  • as well as the orbits of Venus and Mars.

  • Cruithne takes about one Earth year to complete one orbit, though not precisely.

  • The technical term for this is a co-orbital configuration,

  • where two bodies both orbit a third together.

  • So rather than incorrectly calling Cruithne a moon of Earth, you can think of it as a companion.

  • Or a travel buddy.

  • It took about eleven years after Cruithne's discovery in the 1980's

  • for astronomers to figure out its wonky orbit.

  • And it all comes down to the fact that Cruithne's orbit is really eccentric,

  • meaning far from being a perfect circle.

  • Also, the speed with which it zooms around the Sun varies a lot more than what Earth does.

  • In part, that's because the shape of its orbit influences its speed.

  • But it's also because the Earth is tugging on it, either speeding it up as it approaches us from behind,

  • or slowing it down as it passes and moves away.

  • Along with the fact that Cruithne doesn't take quite as long as Earth to orbit the Sun,

  • that makes the actual positioning of Cruithne's solar orbit relative to us

  • rotate ever so slightly every time it completes one orbit.

  • Add in how close it is to Earth, and a little bit of gravitational pull, and boom.

  • Bean shape.

  • And over time, that bean traces out a horseshoe as we see it from Earth.

  • Now, you might see those overlapping orbits and worry about Cruithne hitting us one day.

  • It's about 5 kilometers across, about half the size of the rock that hit Earth 66 million years ago,

  • so it would definitely do a lot of damage.

  • But lucky for us, the closest it ever appears to get is about 15 million kilometers.

  • Even that won't happen again for centuries.

  • Not only do our complicated gravitational interactions keep the Earth and Cruithne at least that far apart,

  • but Cruithne's orbit is also slanted at an angle relative to Earth's.

  • So even though it looks like the orbits intersect,

  • that's actually an illusion caused by looking at the system from a 2D perspective.

  • Now, horseshoe orbits aren't totally unheard of,

  • and Cruithne isn't the only body in the solar system that has one.

  • A couple of Saturn's moons do, too.

  • But they actually are orbiting Saturn, not the Sun.

  • And astronomers have found other small bodies that pretend to orbit the Earth along with Cruithne.

  • Cruithne's orbit isn't stable, though.

  • Simulations predict it'll only last about five thousand years.

  • After that it might actually fall into orbit around the Earth and become a proper moon.

  • But only a temporary one.

  • Another three millennia and it'll escape the Earth's clutches and go back to orbiting the Sun.

  • 'Cause the gravitational influence of multiple bodies is wild like that.

  • If Cruithne does become a temporary satellite,

  • it'll be dubbed a minimoon.

  • Earth actually had one back in 2006, called 2006 RH120.

  • But it's back in orbit around the Sun, now.

  • Cruithne's weird orbit isn't just a novelty.

  • Studying it could help us understand the gravitational situation of our solar system billions of years ago.

  • Like, what could have allowed Cruithne to fall into its current orbit,

  • and what other bodies were up to at the time.

  • And in the further-flung future, it could serve as a way station for longer missions,

  • or even a source of mining.

  • A few proposals have been made to visit Cruithne, but there's nothing official planned just yet.

  • So here's to our itty bitty travel buddy,

  • who make the vastness of space feel a tiny bit less lonely.

  • Cruithne may be an awesome little space buddy, and it's no threat to us any time soon.

  • A lot more likely than asteroid impacts are threats to our Internet security.

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  • They have their own lightweight Chrome browser that's super fast and goes to work in seconds.

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  • [♪ OUTRO]

This episode of SciShow Space is supported by NordVPN.

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B2 中高級

克魯斯恩,馬蹄形軌道的小行星。 (Cruithne, the Asteroid With a Horseshoe Orbit)

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    林宜悉 發佈於 2021 年 01 月 14 日
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