# Won't the Moon's orbit decay and it will crash into the Earth? If so, when?
**URL:** https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035
**Category:** Factual Questions
**Created:** [January 29, 2012, 12:12am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035 "2012-01-29T00:12:40Z")
**Posts on this page:** 16
**Page:** 2
### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 29, 2012, 5:29pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/21 "2012-01-29T17:29:28Z")
> [@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?
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### Author:  [@Andy\_L](https://boards.straightdope.com/u/Andy_L)
#### Post date: [January 29, 2012, 6:12pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/22 "2012-01-29T18:12:41Z")
> [@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](http://en.wikipedia.org/wiki/Geostationary_orbit#Orbital_stability))
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [January 29, 2012, 9:23pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/23 "2012-01-29T21:23:54Z")
> [@](#):
>
> What about Nemesis!
If Nemesis exists, it’s much too far away for the Sun to accrete onto it, or vice-versa.
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [January 29, 2012, 9:48pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/24 "2012-01-29T21:48:02Z")
Didn’t the moon get knocked out of orbit in 1999?
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 29, 2012, 9:56pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/25 "2012-01-29T21:56:02Z")
Thank you all. So far I have found helpful:
As cited to me above–
> **[Geostationary orbit](https://en.wikipedia.org/wiki/Geostationary_orbit)**
>
> A geostationary orbit, also referred to as a geosynchronous equatorial orbit\[a\] (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) in altitude above Earth's equator, 42,164 km (26,199 mi) in radius from Earth's center, and following the direction of Earth's rotation.
> An object in such an orbit has an orbital period equal to Earth's rotational period, one sidereal day, and so to ground observers it appears motionless, in a fixed position in the sky. The concept of a geostationary ...
As well as:
> **[Orbital station-keeping](https://en.wikipedia.org/wiki/Orbital_stationkeeping)**
>
> In astrodynamics, orbital station-keeping is keeping a spacecraft at a fixed distance from another spacecraft or celestial body. It requires a series of orbital maneuvers made with thruster burns to keep the active craft in the same orbit as its target. For many low Earth orbit satellites, the effects of non-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, gravitational forces from Sun/Moon, solar radiation pressure and air drag, mus...
> **[Orbital decay](https://en.wikipedia.org/wiki/Orbital_decay)**
>
> Orbital decay is a gradual decrease of the distance between two orbiting bodies at their closest approach (the periapsis) over many orbital periods. These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or components of any binary system. If left unchecked, the decay eventually results in termination of the orbit when the smaller object strikes the surface of the primary; or for objects where the primary has an atmosphere, the smaller object burns, explodes, ...
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 29, 2012, 10:00pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/26 "2012-01-29T22:00:41Z")
And a satellite at the Lagrangian points of Earth/Moon?
The point itself drifts …?
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [January 29, 2012, 11:37pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/27 "2012-01-29T23:37:05Z")
> [@](#):
>
> 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.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 30, 2012, 12:28am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/28 "2012-01-30T00:28:54Z")
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?
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### Author:  [@Andy\_L](https://boards.straightdope.com/u/Andy_L)
#### Post date: [January 30, 2012, 12:39am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/29 "2012-01-30T00:39:37Z")
> [@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.
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### Author:  [@dtilque](https://boards.straightdope.com/u/dtilque)
#### Post date: [January 30, 2012, 9:51am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/30 "2012-01-30T09:51:37Z")
> [@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).
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### Author:  [@Dahu](https://boards.straightdope.com/u/Dahu)
#### Post date: [January 30, 2012, 9:00pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/31 "2012-01-30T21:00:41Z")
> [@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?
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### Author:  [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)
#### Post date: [January 30, 2012, 9:16pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/32 "2012-01-30T21:16:30Z")
> [@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?
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### Author:  [@Dahu](https://boards.straightdope.com/u/Dahu)
#### Post date: [January 30, 2012, 9:19pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/33 "2012-01-30T21:19:58Z")
> [@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.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 31, 2012, 4:12am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/34 "2012-01-31T04:12:47Z")
> [@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](http://www.jwst.nasa.gov/orbit.html) (itself, sun, Earth).
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 31, 2012, 4:20am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/35 "2012-01-31T04:20:59Z")
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](http://asterisk.apod.com/viewtopic.php?f=8&t=19226) about.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [January 31, 2012, 5:05am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/36 "2012-01-31T05:05:49Z")
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).
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