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Uploaded:2019-08-13
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There’s a small asteroid that appears to orbit Earth in a horseshoe shape. Sometimes referred to as Earth’s second moon, but it's orbit is much weirder than that.

Host: Reid Reimers

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Sources:
https://www.nature.com/articles/42662
http://theconversation.com/earths-other-moon-and-its-crazy-orbit-could-reveal-mysteries-of-the-solar-system-38010
https://www.businessinsider.com.au/cruithne-is-not-earths-second-moon-2015-3?r=US&IR=T
https://futurism.com/a-day-on-cruithne-earths-second-moon
https://earthsky.org/space/does-earth-have-a-second-moon
http://www.astro.uwo.ca/~wiegert/3753/3753.html
https://physicsworld.com/a/earths-second-moon-target-of-proposed-mission/
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Images:
https://svs.gsfc.nasa.gov/11935
https://en.wikipedia.org/wiki/File:Horseshoe_orbit_of_Cruithne_from_the_perspective_of_Earth.gif
https://en.wikipedia.org/wiki/File:Orbits_of_Cruithne_and_Earth.gif
https://www.spacetelescope.org/videos/hubblecast52f/
https://en.wikipedia.org/wiki/File:Cruithne.jpg
https://commons.wikimedia.org/wiki/File:Animation_of_3753_Cruithne_orbit.gif
https://en.wikipedia.org/wiki/File:Animation_of_Epimetheus_orbit_-_Rotating_reference_frame.gif
https://www.eso.org/public/videos/esocast82b/
https://www.istockphoto.com/vector/cute-cartoon-earth-with-moon-gm517518868-89525783
This episode of SciShow Space is supported by NordVPN.

Head over to nordVPN.com/Space to learn more about virtual private networks and internet security. [♪ 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.

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