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hhEb09'1
2008-May-09, 02:08 PM
NASA: Earth May Once Have Had Three Moons (http://www.foxnews.com/story/0,2933,354598,00.html)
However, researchers Jack J. Lissauer of NASA's Ames Research Center near Mountain View, Calif., and John E. Chambers of the Carnegie Institution of Washington figure quite a bit of that ejected matter would have recollected into two other small moons at the so-called "Lagrangian points" or "Trojan points."

eburacum45
2008-May-09, 02:50 PM
Yes; those moons might have lasted as much as a billion years. I have been wondering if the impact of those moonlets might have caused the Imbrium or Crisium mare or some other major feature on the Moon's surface, like Tsiolkovskiy crater.

Another piece of idle speculatiion is wondering what might have happened if those moonlets had been larger, and collided with the Moon with enough force to split chunks off. This might have led to a permanent multiple moon system, as discussed in a couple of threads elsewhere
(here, for instance (http://www.bautforum.com/questions-answers/71168-planet-two-moons-double-eclipse-possible-2.html))

tony873004
2008-May-13, 06:15 PM
I'm skeptical of the Lagrangian moons (although I haven't read his research paper, only the linked Fox news story).

You can't just toss stuff at a Lagrangian point and have it stick. Getting it to stay there is much like getting something to remain in orbit: You approach hyperbolically, then you have to brake into orbit.


Lissauer and Chambers figure the two small moons would have orbited Earth for about 100 million years before the slight gravitational tugs from the solar system's other planets sent them spiraling off course, either to crash into the Moon or Earth or to drift off into space.
And drifting off into space means orbiting the Sun in an Earth-crossing orbit for millions of years before striking the Earth or Moon.
But its not the solar system's other planets that would have perturbed Lagrangian moons out of orbit. It's the solar tide, which is magnitudes greater than perturbations from the other planets.


...I have been wondering if the impact of those moonlets might have caused the Imbrium or Crisium mare or some other major feature on the Moon's surface, like Tsiolkovskiy crater.
I've wondered the same thing. But not from Lagrangian moons. Its very likely that Earth had multiple moons in the beginning, but for months, not millions of years. As the debris from the Mars-sized impactor coalaced, it likely formed several moons. The largest one (which became the current moon) dominated, and most of the smaller moons collided with it within the first few months. After it was almost its present size, it had enough gravity to eject some of the remaining moonlets into interplanetary space, where they would orbit the Sun in Earth-crossing orbits for millions of years, perhaps to return and punch large holes into Luna's the newly-formed crust.

It's also possible that there were some additional stable moons that lasted for millions of years. But as the Moon migrated outward, it would have destabalized their orbits.

hhEb09'1
2008-May-13, 06:39 PM
I'm skeptical of the Lagrangian moons (although I haven't read his research paper, only the linked Fox news story).

You can't just toss stuff at a Lagrangian point and have it stick. Getting it to stay there is much like getting something to remain in orbit: You approach hyperbolically, then you have to brake into orbit.Yahbut...

As the debris from the Mars-sized impactor coalaced, it likely formed several moons. The debris might not approach the lagrange points as a mass--in other words, the collisions would come from all directions. Opposing debris is the braking mechanism. All other things being equal, the most stable collection points are the lagrange points. There's your moons. In theory.

tony873004
2008-May-13, 07:48 PM
I would guess that there is a bias towards the direction the debris was orbiting. But thinking about it a little more, initially, there are no Lagrange points as the debris is spread out in a ring, then the Lagrange points develop as the moonlets become massive, so they could get trapped that way, rather than needing a braking mechanism.

I've got an article describing a simulation I ran ( http://www.orbitsimulator.com/gravity/articles/moonbuilder.html ), where 100 moonlets combined to form the Moon. Although in this simulation I didn't get any Lagrangian moons, other simulations I've ran did produce Lagrangian trios. These simulations are over-simplified, but they can serve to show what's possible. So maybe we once did have Lagrangian moons.

Simulations of the current Earth / Moon system show that objects placed on the L4 and L5 points are perturbed away by the Sun, almost immediately. I think my record for trapping one is about 20 years, but usually they last only a few months to low years. So as a trio of Moons spiraled out, at some point their Lagrangian configuration would be destroyed, leaving us with only 1 moon.

agingjb
2008-May-13, 08:12 PM
I'm surprised that Earth-Moon L4 and L5 points are so unstable, given the existence of Saturn-Dione, and Saturn-Tethys Trojans in a much more complex system, but I suppose the size of Saturn and the relative distance of the Sun makes a difference.

tony873004
2008-May-13, 08:18 PM
Earth's moon orbits about 1/3 of the way to the edge of its Hill Sphere. Saturn's resonant moons orbit much deeper within Saturn's Hill Sphere, making the solar perturbations negligable.

tony873004
2008-May-13, 08:30 PM
If you click through the links and get to the Icarus Abstract:

Using numerical integrations, we show that orbits near the Earth–Moon L4 and L5 points can survive for over a billion years even when solar perturbations are included
Which is exactly the opposite of what I'm saying, unless by Earth-Moon L4 and L5 points they mean (Earth-Moon) - Sun system, where these L points are 1 AU from Earth, rather than 1 Lunar distance. Now I need to run down to the library and read the whole article :)

tony873004
2008-Oct-09, 05:10 PM
...But its not the solar system's other planets that would have perturbed Lagrangian moons out of orbit. It's the solar tide, which is magnitudes greater than perturbations from the other planets...

No, you're wrong! (tells myself)

Sorry for digging up a 5 month-old thread. But I've read another paper (I don't remember the name or author at the moment) that claimed that there are L4 and L5 points in the Earth/Moon system that remain stable indefinately despite the solar tide, if the simulations ignore the planets. But include the planets, and they're only stable for a few million years.

I e-mailed the author of the paper, and he sent me some data that I could use in Gravity Simulator. Although I didn't run the simulation for millions of years, my L4 & L5 particles stayed put for thousands of years with the planets included.