How many moons does Earth have?

Jul 05, 2013 by Fraser Cain, Universe Today
Formation of the Moon

Look up into the night sky and count the moons. You can see only one moon, "the" Moon. But does the Earth have any other moons? Around the Solar System, multiple moons are the rule. Jupiter has 67 natural satellites, even Mars has two asteroid-like moons.

Could Earth have more than one?

Officially, the answer is no. The Earth has a single moon.

Today.

It's possible Earth had more than one moon in the past, millions or even billions of years ago. Strange terrain on the far side of the Moon could be explained by a second moon crashing into it, depositing a layer of material tens of kilometers deep.

Moons could come and go over the billions of years of the Earth's history.

For example, Mars has two Moons, but not for long. Phobos, the larger moon, is spiraling inward and expected to crash into the planet within the next 10 million years. And so, in the future, Mars will only have a single Moon, Deimos.

It's also possible that the Earth might capture a Moon in the future. Neptune's largest moon, Triton, orbits in the opposite direction from the rest of the moons around the planet. This suggests that Triton was actually a captured Kuiper Belt Object which strayed too close to the planet.

In fact, we did capture a 5-metre asteroid called 2006 RH120. It orbited the Earth four times during 2006/2007 before getting ejected again.

So we can assume events like this have happened in the past.

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Additionally, we might have more moons, but they haven't been discovered yet because they're just too small. Researchers have calculated that there could be meter-sized asteroids in orbit around the Earth, remaining in orbit for hundreds of years before push them out again.

And there are other objects that interact with Earth's orbit in strange ways. Scientists don't consider them moons, but they do stick around in our neighbourhood:

2006 RH120

Asteroid 3753 Cruithne is in an orbital resonance with the Earth. It has a highly eccentric orbit, but takes exactly one year to orbit the Sun. From our perspective, it follows a slow, horse-shoe shaped path across the sky. Since the discovery of Cruithne in 1986, several other resonant near-Earth objects have been discovered.

There's 2010 TK7, the Earth's only known Trojan asteroid. It leads the Earth in the exact same around the Sun, in a gravitationally stable point in space.

2007 TK7

So, the answer… Earth only has a single Moon. Today. We might have had more moons in the past, and we might capture more in the future, but for right now… enjoy the one we've got.

Explore further: NASA issues 'remastered' view of Jupiter's moon Europa

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User comments : 7

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axemaster
3.2 / 5 (5) Jul 05, 2013
For example, Mars has two Moons, but not for long. Phobos, the larger moon, is spiraling inward and expected to crash into the planet within the next 10 million years. And so, in the future, Mars will only have a single Moon, Deimos.

Yeah, that won't happen though. We'll stabilize its orbit or dispose of it somehow long before it has a chance to smack into anything.
GSwift7
3.7 / 5 (3) Jul 05, 2013
Yeah, that won't happen though. We'll stabilize its orbit or dispose of it somehow long before it has a chance to smack into anything.


Good point. If we are still around by then, there'll be people there for sure. It wouldn't take much to give it enough of a boost to extend its life by another million years. We could probably do that with existing technology, if we had any motivation to do it.
antialias_physorg
4.2 / 5 (5) Jul 05, 2013
Last I checked Phobos wasn't expected to hit Mars. It was expected to eventually break up and form a dust ring.

Anyhow: 10 million years is still a ways to go. I don't think we'll be bothered by something like that by then (one way or another).
Neinsense99
3.3 / 5 (12) Jul 06, 2013
Normally, one. No doubt on Friday night, where drunken rednecks congregate, there may be more 'moons' than usual.
antigoracle
2.6 / 5 (5) Jul 10, 2013
Korea says, several million.
TheGhostofOtto1923
1 / 5 (4) Jul 10, 2013
Yeah, that won't happen though. We'll stabilize its orbit or dispose of it somehow long before it has a chance to smack into anything
Valuable real estate. Good place for a cop station.

We will be capturing asteroids and placing them in orbit around the moon and the earth soon enough, for habitation and raw materials.
rkolter
not rated yet Jul 16, 2013
As much as I would like to see us capture asteroids and mine them, I don't think the people of Earth would be too pleased at having asteroids bumped into orbit around the Earth. Consider this scenario: While pushing the asteroid into orbit, your computer blue-screens. You call technical support, but it's a 30-second delay between you, and they disconnect figuring you will call back when you have a better connection. You slowly float in until you're down to a 15 second delay, at which time they inform you the issue is with the software and that they can send you a new CD. You protest and a ticket is generated. When the delay is down to a second and a half, they get back to you with a fix. The fix is applied, but does not resolve the issue. Your last act is to steer the asteroid in the general vicinity of the helpdesk.

This is why asteroids will never be parked in orbits around the Earth.

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