Won't the Moon's orbit decay and it will crash into the Earth? If so, when?

[Leo_Bloom]

@scr4

…

And of course, atmospheric drag isn’t a factor for geostationary satellites. Though they do need thrusters to stay in the designated spot, otherwise they may drift slowly along the orbit.

What makes them drift?

[Andy_L]

@Leo_Bloom

What makes them drift?

Gravity from the Moon and the Sun, and pertubations due to the non-sphericality of the Earth (Geostationary orbit - Wikipedia)

[Chronos]

What about Nemesis!

If Nemesis exists, it’s much too far away for the Sun to accrete onto it, or vice-versa.

[TriPolar]

Didn’t the moon get knocked out of orbit in 1999?

[Leo_Bloom]

Thank you all. So far I have found helpful:
As cited to me above–

Geostationary orbit

As well as:

Orbital station-keeping

Orbital decay

[Leo_Bloom]

And a satellite at the Lagrangian points of Earth/Moon?
The point itself drifts …?

[Chronos]

And a satellite at the Lagrangian points of Earth/Moon?

If you put a satellite at L4 or L5, it would stay there. But the question is, why would you? I can’t think of any reason you’d want to keep a satellite there. There are plenty of things that could work just fine there, but they’re all things that it also wouldn’t matter if they wandered around some. Those points would also have the disadvantage of being a very high orbit, and so needing more energy to launch to them, and needing more powerful antennas to communicate, and because they’re stable points, they tend to accumulate dust and other junk that would probably be bad for your device.

[Leo_Bloom]

Lagrangian points: just checking my knowledge in this orbit thread. Yay! Leo makes a non-nonsensical statement!

(I await corrections…:))

But since we’re thread-drifting (heh), given the stuff up-thread about precession and tides, how are there distinct moon/Earth points? Shouldn’t they shift around?

[Andy_L]

@Chronos

If you put a satellite at L4 or L5, it would stay there. But the question is, why would you? I can’t think of any reason you’d want to keep a satellite there. There are plenty of things that could work just fine there, but they’re all things that it also wouldn’t matter if they wandered around some. Those points would also have the disadvantage of being a very high orbit, and so needing more energy to launch to them, and needing more powerful antennas to communicate, and because they’re stable points, they tend to accumulate dust and other junk that would probably be bad for your device.

I think perturbations from the Sun, the other planets, and the non-sphericities in the Earth would cause objects at the L4/L5 points to move a bit, circling the ideal locations of the points.

[dtilque]

@Andy_L

I think perturbations from the Sun, the other planets, and the non-sphericities in the Earth would cause objects at the L4/L5 points to move a bit, circling the ideal locations of the points.

That’s expected behavior for objects at L4/5 points. Check out the positions of Trojan asteroids, which are at the Sun-Jupiter L4/5 points. None are actually at the Lagrange points. They all oscillate around the points, usually in long kidney-bean-shaped figures. By long, I mean that some can get as close as 30degrees from Jupiter (the Lagrange points are 60degrees away).

[Dahu]

@OldGuy

Actually there is some decay in the moon’s orbit. The earth moon pair are emitting gravitational waves since they are accelerating around their common center of mass. Eventually this loss of energy would make the orbit decay. Though as pointed out currently they are receding from each other as angular momentum from the Earth’s rotation is transferred to the Moon’s revolution by tidal forces. And this is many times as important as the gravitational waves.

The decay in the orbit won’t start until the day and month (that is the moon’s orbital period) are equal in length so that the moon is apparently stationary overhead one spot on the earth

When the moon is apparently stationary over one spot on earth, then isn’t that a geostationary orbit? But its current orbit is much further out than that, and it’s receding, so… help me out here… how can it begin to decay?

[Fish_Cheer]

@Dahu

When the moon is apparently stationary over one spot on earth, then isn’t that a geostationary orbit? But its current orbit is much further out than that

If the Earth’s rotation is slowing down, the geostationary orbit will be higher, won’t it?

[Dahu]

@Fish_Cheer

If the Earth’s rotation is slowing down, the geostationary orbit will be higher, won’t it?

Ah I see. Sorry, didn’t take that into account.

[Leo_Bloom]

@Chronos

If you put a satellite at L4 or L5, it would stay there. But the question is, why would you? I can’t think of any reason you’d want to keep a satellite there. There are plenty of things that could work just fine there, but they’re all things that it also wouldn’t matter if they wandered around some. Those points would also have the disadvantage of being a very high orbit, and so needing more energy to launch to them, and needing more powerful antennas to communicate, and because they’re stable points, they tend to accumulate dust and other junk that would probably be bad for your device.

The Webb telescope (Hubble replacement) will be parked at L2 (itself, sun, Earth).

[Leo_Bloom]

Some reasons why NASA chose that Lagrange point. Note comments on heat (better for infrared observation), good quality of communications, and other satellites already lurking about.

[Chronos]

Yeah, L1 and L2 have practical uses, but they’re also unstable, so any satellite sitting on one of them needs station-keeping thrusters to prevent it drifting off. L3 (the one on the far side of the Sun) is also unstable, but I can’t think of any practical use for it off the top of my head (there may be one, but I can’t think of what it would be).