Earth's other 'moon' and its crazy orbit could reveal mysteries of the solar system

February 25, 2015 by Duncan Forgan, The Conversation
Cruithne’s wacky orbit around the sun. Credit: YouTube, CC BY-SA

We all know and love the moon. We're so assured that we only have one that we don't even give it a specific name. It is the brightest object in the night sky, and amateur astronomers take great delight in mapping its craters and seas. To date, it is the only other heavenly body with human footprints.

What you might not know is that the moon is not the Earth's only natural satellite. As recently as 1997, we discovered that another body, 3753 Cruithne, is what's called a quasi-orbital satellite of Earth. This simply means that Cruithne doesn't loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner in what's called a "horseshoe" orbit.

Cruithne's orbit

To help understand why it's called a horseshoe orbit, let's imagine we're looking down at the solar system, rotating at the same rate as the Earth goes round the sun. From our viewpoint, the Earth looks stationary. A body on a simple horseshoe orbit around the Earth moves toward it, then turns round and moves away. Once it's moved so far away it's approaching Earth from the other side, it turns around and moves away again.

Cruithne from a stationary Earth position

Horseshoe orbits are actually quite common for moons in the solar system. Saturn has a couple of moons in this configuration, for instance.

What's unique about Cruithne is how it wobbles and sways along its horseshoe. If you look at Cruithne's motion in the solar system, it makes a messy ring around Earth's orbit, swinging so wide that it comes into the neighbourhood of both Venus and Mars. Cruithne orbits the sun about once a year, but it takes nearly 800 years to complete this messy ring shape around the Earth's orbit.

Cruithne close up

So Cruithne is our second moon. What's it like there? Well, we don't really know. It's only about five kilometres across, which is not dissimilar to the dimensions of the comet 67P/Churyumov-Gerasimenko, which is currently playing host to the Rosetta orbiter and the Philae lander.

The surface gravity of 67P is very weak – walking at a spirited pace is probably enough to send you strolling into the wider cosmos. This is why it was so crucial that Philae was able to use its harpoons to tether itself to the surface, and why their failure meant that the lander bounced so far away from its landing site.

Given that Cruithne isn't much more to us at this point than a few blurry pixels on an image, it's safe to say that it sits firmly in the middling size range for non-planetary bodies in the solar system, and any human or machine explorers would face similar challenges as Rosetta and Philae did on 67P.

If Cruithne struck the Earth, though, that would be an extinction-level event, similar to what is believed to have occurred at the end of the Cretaceous period. Luckily it's not going to hit us anytime soon – its orbit is tilted out of the plane of the solar system, and astrophysicists have shown using simulations that while it can come quite close, it is extremely unlikely to hit us. The point where it is predicted to get closest is about 2,750 years away.

Cruithne is expected to undergo a rather close encounter with Venus in about 8,000 years, however. There's a good chance that that will put paid to our erstwhile spare moon, flinging it out of harm's way, and out of the Terran family.

It's not just Cruithne

The story doesn't end there. Like a good foster home, the Earth plays host to many wayward lumps of rock looking for a gravitational well to hang around near. Astronomers have actually detected several other quasi-orbital satellites that belong to the Earth, all here for a little while before caroming on to pastures new.

So what can we learn about the solar system from Cruithne? Quite a lot. Like the many other asteroids and comets, it contains forensic evidence about how the planets were assembled. Its kooky orbit is an ideal testingground for our understanding of how the solar system evolves under gravity.

Possible clash: Venus. Credit: J.Gabás Esteban, CC BY-SA

As I said before, it wasn't until the end of the 20th century that we even realised that bodies would enter such weird horseshoe orbits and stay there for such a long time. The fact they do shows us that such interactions will have occurred while the solar system was forming. Because we think terrestrial planets grow via collisions of bodies of Cruithne-size and above, this is a big new variable.

One day, Cruithne could be a practice site for landing humans on asteroids, and perhaps even mining them for the rare- metals our new technologies desperately crave. Most importantly of all, Cruithne teaches us that the solar system isn't eternal – and by extension, neither are we.

Secrets: solar system. Credit: Tashal

Explore further: How many moons does Earth have?

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25 comments

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Denovalin
4.9 / 5 (14) Feb 25, 2015
Calling it a second moon is not really correct. But I will say the pic above makes me remember how much fun I had with a Spirograph as a kid.
Uncle Ira
4.1 / 5 (9) Feb 25, 2015
Calling it a second moon is not really correct. But I will say the pic above makes me remember how much fun I had with a Spirograph as a kid.


@ Denovalin-Skippy. You are right about that. I just looked him up on the Google place and that is what they said too. He is not a truly moon according to the requirements of being the moon. They said he was more like a wandering asteroid more than a moon because it does not get it's orbit decided by the earth's gravity. It just happens by coincident and chance to be doing what it does sometimes close to earth and sometimes not close to the earth. So the Skippy who wrote the article lied about him being a moon.
tadchem
4 / 5 (4) Feb 25, 2015
It is also quite likely that the Moon itself was formed, according to the Giant Impact Hypothesis, by the impact of a Mars-sized object (Theia) that somehow hit the proto-earth *gently* enough that the planet was not completely shattered and scattered.
This could have easily happened if Theia had been in a horsehoe orbit with the proto-earth.
dumpsta101
5 / 5 (1) Feb 25, 2015
wow this is a great story! I cant wait to see what the ramifications of this and what other discoveries will follow this about our own solar system! Still cant wait to see what are in the oceans of Europa as well. :)
Coyoty
5 / 5 (1) Feb 25, 2015
Doesn't Toro count as a moon as much as Cruithne?
skills4u
5 / 5 (2) Feb 25, 2015
What a neat orbit. Could Cruithne have been the "Star of Bethlehem "? it would explain "it" lingering over Bethlehem for a while (as it approaches the Earth) then "changing direction".

Duncan Forgan,
Do you know where Cruithne was 2014 years ago ?
Uncle Ira
3.7 / 5 (6) Feb 25, 2015
What a neat orbit. Could Cruithne have been the "Star of Bethlehem "? it would explain "it" lingering over Bethlehem for a while (as it approaches the Earth) then "changing direction".


Non Cher. He is too little because they didn't have any telescopes to see him with and no satellites to see him with either. That's why it took so long for them to find him.
skills4u
3.7 / 5 (3) Feb 25, 2015
Yes,now I find it's closet approach is .1 AU, 40 x the moons distance from Earth, or 10 million miles .
PhotonX
5 / 5 (6) Feb 26, 2015
Denovalin:
Calling it a second moon is not really correct.
Indeed, but at least they had the good grace to put 'moon' in quotes in the heading, though not in the article itself.
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Could Cruithne have been the "Star of Bethlehem"? it would explain "it" lingering over Bethlehem for a while...Do you know where Cruithne was 2014 years ago? Hmmmm... Even if it *were* close, it wouldn't 'linger over' anywhere, as the celestial sphere rotates once a day. And over which location on Earth any astronomical object appears to linger depends on which direction you're facing. The far away kings could never have associated it with Bethlehem. I suspect that even a 5 km asteroid would probably not be visible at all to the naked eye even at closest approach, and not a remarkably prominent object.
.
Also, for what it's worth, apparently the calendar makers miscalculated Jesus' birth by around 4 years, it actually being 4 BCE.
.
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ThomasQuinn
3.7 / 5 (3) Feb 26, 2015
"We're so assured that we only have one that we don't even give it a specific name."

I'm pretty sure it's called Luna, and yes, that's a name, not a description. http://en.wikiped...ddess%29
alextheaboveaverage
not rated yet Feb 26, 2015
Why does Uncle Ira call everyone Cher?
Uncle Ira
4.4 / 5 (7) Feb 26, 2015
Why does Uncle Ira call everyone Cher?


That's just the way he talks Cher. He doesn't mean anything by it. Down where he is from a lot of peoples say that when they are talking to you. Do you guys up there where you are have things you say in your neighborhood that other peoples don't say?

Uncle Ira
4.5 / 5 (8) Feb 26, 2015
P.S. for you alex-something-Skippy. Ol Ira-Skippy also calls peoples Skippy and podna and doesn't mean anything by that either. When he calls you couyon or p'tit boug then he is calling you something bad.

Oh yeah, I almost forget. Next time why you don't ask him why he calls everybody that instead of just flinging it out for everybody to answer? He probably knows why better than everybody else does.
mbee1
1 / 5 (3) Feb 26, 2015
You folks are assuming it is not a partly captured comet. If it was a comet it could have looked a lot brighter than it currently does especially if it came pretty close to the earth say withing the moons orbit.
animah
5 / 5 (2) Feb 27, 2015
When he calls you p'tit boug


Considering boug is an archaic form of bougre, I dare say you are calling them something bad indeed...

:-)
Uncle Ira
4 / 5 (4) Feb 27, 2015
When he calls you p'tit boug


Considering boug is an archaic form of bougre, I dare say you are calling them something bad indeed...

:-)


In the French that the Paris-Skippys speak it means something like dude, or fellow, or guy. In the regular French, like we use down here it usually means boy, or little man, or little guy, the p'tit boug means "little boy" down here in Louisiana where we speak the proper French.

Oh yeah, I almost forget. Don't tell one of those couyons from France I said that because you will get some trouble going with that.
Uncle Ira
4.2 / 5 (5) Feb 27, 2015
You folks are assuming it is not a partly captured comet. If it was a comet it could have looked a lot brighter than it currently does especially if it came pretty close to the earth say withing the moons orbit.


Skippy did you do like I did and check him out with the Google-Skippy? I guess not, Returnering-Skippy never does that either, that's why he says so many silly things that he could have checked out before he made the fool of himself.
skills4u
not rated yet Feb 27, 2015
PhotonX... If you watch the video of its orbit, it at one point is heading directly toward the Earth, it is this approach which would appear to "stand still", then as it gets to its closest point to Earth it starts turning, moving away from the Earth which gives it the appearance of "turning.

With that said, I had earlier mentioned it was too small and far away to be seen with the naked eye,but it does match the description of the star of Bethlehem's movement.
EarthlingX
5 / 5 (1) Feb 27, 2015
This satellite asteroid is quite a nice target for visit. It's an asteroid and (almost) a moon.
That could help with space advocates, more than it's low delta V from LEO.
ThomasQuinn
3.7 / 5 (3) Mar 01, 2015
You folks are assuming it is not a partly captured comet. If it was a comet it could have looked a lot brighter than it currently does especially if it came pretty close to the earth say withing the moons orbit.


The geology of the moon suggests, with a likelihood bordering on certainty, that the material the moon is made of is of an earthly origin. Even in the unlikely event that it isn't, a comet is one of the least likely (or even possible) alternatives - its composition is unlike that of ANY known comet.
animah
5 / 5 (3) Mar 01, 2015
In the French that the Paris-Skippys speak it means something like dude

Yes but what I meant is that bougre is where the word bugger (as in buggery!) comes from. Not a nice connotation at all...
Captain Stumpy
3.7 / 5 (6) Mar 01, 2015
In the French that the Paris-Skippys speak it means something like dude

Yes but what I meant is that bougre is where the word bugger (as in buggery!) comes from. Not a nice connotation at all...

Huh...
i thought it was the word for booger LOL
http://www.etymon...m=booger

the French translation is literally "little Chap" or "petite chap" and can be said to mean "small" or "young" chap

Considering the French history of LA and Ira's typical Cajun type, i would say that the actual definition is as said: Dude

Also, considering the historical use of it in LA (it is still heavily used there - anecdotal based upon my experience, not science) and that it is also used to mean little/small/young/petite chap or Dude, then i would stick to the meaning denoted by Ira

the etymological origins of Bugger, Booger and all that only typically assign the negative connotation in the Queen's English, not American or French

just sayin
Caliban
5 / 5 (4) Mar 01, 2015
One further note of clarification: even in the King's English, "bugger" as a noun applied to denote a person is generally interpreted to mean(although still considered vulgar or colloquial) virtually the same as in the French usage --ie, guy, dude, fellah-- whereas the verb/adverb/adjective from "to bugger" retains the meaning for/from which it originally derives.

Or, at least, that's my understanding of it.
t_d_lowe
not rated yet Mar 01, 2015
The moon does have a name, it is called Lune, and the sun is called Sol.
TechnoCreed
5 / 5 (2) Mar 02, 2015
@animah
In the French that the Paris-Skippys speak it means something like dude

Yes but what I meant is that bougre is where the word bugger (as in buggery!) comes from. Not a nice connotation at all...
In French 'bougre' is rarely used alone; usually in a phrase a saying. It can mean a lot of things both friendly and unfriendly depending of context. In this context written in Cajun 'ptit boug' is little boy. I would say 'ptit gars' in Quebec and in France they would say 'le môme'. Now if he tells you that, you should answer: Don't you boy me papere. ;-)

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