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View Full Version : Could the Earth have 2 moons in the future.



Darrrius
2006-Mar-30, 01:17 PM
Just a quick question....

Is it a possibility that some time in the future, near or far, Earth could capture a Comet or Asteroid into ist orbit? If so, would this object then be considered a moon of Earth? Would this be possible, or would anything coming close enough have to smash into Earth or hit the moon?

Many Thanks

antoniseb
2006-Mar-30, 01:38 PM
It would be possible for an object to follow a trajectory where it interacts with the Earth and Moon's gravities in just the right ay that it could be captured. Please not that capture in a two body system isn't possible, but the moon's presence in the system changes it from impossible to extremely unlikely.

This would have to be an object that is already in a near Earth orbit.

Darrrius
2006-Mar-30, 02:54 PM
Ok thanks for the answer Anton. So possible but unlikely. When you say 2 body system - is the Earth and the moon not a 2 body system?

antoniseb
2006-Mar-30, 03:02 PM
is the Earth and the moon not a 2 body system?
The proposed second moon is a third body.

aurora
2006-Mar-30, 05:27 PM
It does raise an interesting question about how Mars captured its moons.

As well as possible moons in the past that have since crashed into Mars.

Romanus
2006-Mar-30, 05:41 PM
^
I think the general consensus among the "capture" crowd is that Mars captured its moons when it was very young, and surrounded by an extended atmosphere that made it more easy than it seems in theory.

antoniseb
2006-Mar-30, 05:42 PM
It does raise an interesting question about how Mars captured its moons.
It's not clear that Mars captured the moons so much as collected them. It is possible that these moons are composed of material ejected from Mars, probably from a collision, or perhaps through some super volcanism (or both).

tony873004
2006-Mar-30, 07:39 PM
The Earth actually could capture objects from interplanetary space without the help of the Moon. The solar gravitational tide could provide enough delta v to slow an object on an already very slow hyperbolic trajectory below capture velocity.

But it works both ways. The solar gravitational tide can just as easily pump a little energy into an object giving it escape velocity. Any object captured in this manner would most likely enter Earth orbit through the Earth/Sun Lagrange 1 or Lagrange 2 point. It would only make a few orbits before being cast back into interplanetary space.

Additionally, an object just barely hyperbolic could pass through the upper part of Earth's atmosphere, and get captured. This too would be temporary as it would have no means to boost its perigee out of the atmosphere. It would either get destroyed from multiple passes through the atmosphere, or spiral down until it hits the Earth.

Both these scenerios are extremely unlikely. As antoniseb pointed out, capture scenerios involving help from the Moon are also possible, and more probable than the other two methods. But all three of these scenerios require that the asteroid be in an orbit almost identical to Earth's, without being trapped in a resonance. Earth has likely cleared its neighborhood of all such possible candidates. So a renewing mechanism to deliver more objects into orbits with the potential of Earth capture would be required.

Kaptain K
2006-Mar-30, 10:48 PM
There is one other "capture mechanism" that seems to have been missed so far. Binary asteroids, on just the right path. One could be captured and the other flung out. Granted, the chances are slim, but it is possible.

antoniseb
2006-Mar-30, 11:04 PM
Good point. Again, this is a three body situation.

Irishman
2006-Mar-30, 11:07 PM
Doesn't size play a role in the overall difference between the Earth/Moon case and Mars/Phobos/Deimos? The Moon is a lot closer in size to the Earth, so the instability for a third body is much larger. Essentially, the Moon's effect on that third object will be substantial.

Just like Jupiter and Saturn can have lots of moons, because there is so much more mass.

tony873004
2006-Mar-30, 11:43 PM
And one more 3-body method... Two asteroids, not part of a binary system coincidentally pass close to each other while simultaneously passing close to Earth. One gets an extra boost on the way out, while one remains in orbit.

Although the odds are extremely thin, this method could deliver to Earth a satellite in a stable circular orbit, although it would require a rather massive asteroid.

Jupiter and Saturn are also much further from the Sun than Earth. Hence their Hill spheres are larger, and they can accomodate more moons.

aurora
2006-Mar-31, 07:10 PM
It's not clear that Mars captured the moons so much as collected them. It is possible that these moons are composed of material ejected from Mars, probably from a collision, or perhaps through some super volcanism (or both).

I don't recall hearing that before.

Have there been studies of Phobos and Deimos that show that their material is more Mars-like than main-asteroid-belt-like (to coin a phrase)?