# 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:** 20
**Page:** 1
### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [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/611035/1 "2012-01-29T00:12:40Z")
See subject. So I can get my worries over the Sun’s supernova into perspective.
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### Author:  [@Gagundathar](https://boards.straightdope.com/u/Gagundathar)
#### Post date: [January 29, 2012, 12:14am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/2 "2012-01-29T00:14:00Z")
Luna is receding ever so slowly away from Terra.
So, no.
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### Author:  [@Lemur866](https://boards.straightdope.com/u/Lemur866)
#### Post date: [January 29, 2012, 12:20am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/3 "2012-01-29T00:20:35Z")
No it won’t decay. In fact, the Moon is getting farther and farther away from the Earth.
What do you think makes an orbit 'decay"? The reason an orbit decays is if the body is low enough that friction from the tenuous atmosphere slows it, which means it falls lower, which means more atmosphere, which means more frictional slowing, and eventually the object falls to earth.
Even if the Moon wasn’t getting farther away, its orbit wouldn’t decay, because it is very high above the Earth’s atmosphere, such that atmospheric drag isn’t just essentially nil, it is nil. And also, the Moon is very very massive compared to a satelite, and so even if it were being slowed by atmospheric friction it would take a lot of friction to slow it down.
I guess people have a hard time accepting that a body in motion will stay in motion unless acted upon by another force. We’re used to the notion that things in motion eventually stop moving and fall to the Earth. It took until Newton for scientists to figure out that it takes energy to make things stop or start moving, but it takes no energy for them to keep moving.
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [January 29, 2012, 12:52am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/4 "2012-01-29T00:52:18Z")
The sun can’t go supernova either. It’s too small.
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### Author:  [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)
#### Post date: [January 29, 2012, 1:02am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/5 "2012-01-29T01:02:58Z")
IIRC, in theory given enough time (many, many billions of years) the Moon will continue to slowly move away from the Earth as it is now, then eventually the direction of drift will reverse, its orbit will decay and and the Moon crash into the Earth. The Moon radiates a small amount of energy in the form of gravity waves as it orbits; this serves as a form of “drag” that slowly drains energy from its orbit.
But it’s quite likely it will instead be destroyed by the Sun expanding into a red giant, which will happen much sooner.
> [@Lemur866](#):
>
> It took until Newton for scientists to figure out that it takes energy to make things stop or start moving, but it takes no energy for them to keep moving.
As I understand it, it does in fact cost energy to keep moving in a curve; objects doing so radiate energy.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 29, 2012, 1:04am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/6 "2012-01-29T01:04:42Z")
Thanks.
So, (of course I’m not arguing, just asking)
1. Why is the moon “drifting” away?
2. Why did Skylab fall?
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [January 29, 2012, 1:11am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/7 "2012-01-29T01:11:17Z")
Sorry, strike question 2 above. **Lemur** told me the answer. It never occurred to me that the atmosphere at Skylab or Space Station height was still significant enough.
Given that, doesn’t the Space Station have to add little force nudges to avoid Skylab’s fiery fate?
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### Author:  [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)
#### Post date: [January 29, 2012, 1:11am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/8 "2012-01-29T01:11:46Z")
> [@Leo\_Bloom](#):
>
> 1. Why did Skylab fall?
It was way closer to the Earth than the moon, and atmospheric drag did it in.
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### Author:  [@OldGuy](https://boards.straightdope.com/u/OldGuy)
#### Post date: [January 29, 2012, 1:15am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/9 "2012-01-29T01:15:42Z")
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
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### Author:  [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)
#### Post date: [January 29, 2012, 1:17am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/10 "2012-01-29T01:17:00Z")
> [@Leo\_Bloom](#):
>
> Given that, doesn’t the Space Station have to add little force nudges to avoid Skylab’s fiery fate?
Yes. Its orbital decay amounts to 2 km per month, and money is being to spent to counteract that.
[Wikipedia article](http://en.wikipedia.org/wiki/International_Space_Station):
> [@](#):
>
> Zvezda is at the rear of the station according to its normal direction of travel and orientation, its engines are used to boost the station’s orbit. Alternatively Russian and European spacecraft can dock to Zvezda’s aft (rear) port and use their engines to boost the station.
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### Author:  [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)
#### Post date: [January 29, 2012, 1:17am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/11 "2012-01-29T01:17:59Z")
> [@Leo\_Bloom](#):
>
> 1. Why is the moon “drifting” away?
> **[Moon | Tidal effects](https://en.wikipedia.org/wiki/Moon#Tidal_effects)**
>
> The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides closest to each other, resulting in tidal forces. Ocean tides are the most widely experienced result of this, but tidal forces also considerably affect other mechanics of Earth, as well as the Moon and their system.
> The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth...
> [@Leo\_Bloom](#):
>
> 1. Why did Skylab fall?
Atmospheric drag, as **Lemur866** described.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [January 29, 2012, 1:20am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/12 "2012-01-29T01:20:48Z")
Gravitational waves for the Earth-Moon system are beyond negligible. I’d actually bet on aerodynamic drag from the interplanetary medium having a larger effect.
And yes, the Space Station does need occasional boosts to its orbit to keep it up. All satellites you want to keep around for an extended time do-- The Hubble is another good example. And the Russians didn’t want the expense of continuing to do this for Mir, which is why they deliberately brought it down.
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### Author:  [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)
#### Post date: [January 29, 2012, 1:23am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/13 "2012-01-29T01:23:19Z")
> [@Leo\_Bloom](#):
>
> 1. Why is the moon “drifting” away?
Basically the tides. The earth bulges because of the water. That bulge and the moon are gravitationally tied, of course, which results in some of the rotational energy being transferred to the moon.
Energy and momentum must be conserved. In the moon’s case, it results in a higher orbit. In the earth’s case, the length of the day is slightly lengthening (i.e. the earth’s spin is slowing slightly) as rotational energy is transferred.
If the earth were a bit more rigid, this effect wouldn’t be observed.
I should add that, if it weren’t for that pesky sun going nova some day, eventually the earth would become tidally locked with the moon. So, the moon would be locked over the same bit of earth all the time.
As for the ISS, yes, its orbit is maintained with periodic boosts. The [wiki](http://en.wikipedia.org/wiki/International_Space_Station#Orbit_and_mission_control) article has a good description of the process.
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### Author:  [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)
#### Post date: [January 29, 2012, 1:29am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/14 "2012-01-29T01:29:14Z")
> [@Great\_Antibob](#):
>
> if it weren’t for that pesky sun going nova some day
As stated by **Exapno Mapcase** above, it won’t.
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### Author:  [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)
#### Post date: [January 29, 2012, 2:33am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/15 "2012-01-29T02:33:40Z")
Yeah, but it will bloat out to, and probably beyond, Earth’s orbit, so I’m pretty sure that it will still be pretty miserable.
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### Author:  [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)
#### Post date: [January 29, 2012, 4:09am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/16 "2012-01-29T04:09:24Z")
Just to be clear, there is a big difference between nova and supernova. The OP said supernova, and our sun is much too small for that, buy at least nine times. A nova on the other hand needs two components. A white dwarf to go nova, and a companion star to donate matter to initiate the nova. White dwarf is a main sequence stage for stars that include stars of the mass of our sun, and our sun will probably eventually become a white dwarf. But we don’t have a companion star, so still no nova.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [January 29, 2012, 4:37am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/17 "2012-01-29T04:37:03Z")
> [@Chronos](#):
>
> All satellites you want to keep around for an extended time do–
There are many satellites in low earth orbit that don’t have thrusters for raising the orbit, and are expected to last many years. Of course their lifetime depends on how high the original orbit is. For example, [ROSAT](http://en.wikipedia.org/wiki/ROSAT), launched into a 580km high orbit in 1990, reentered last year.
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.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [January 29, 2012, 6:10am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/18 "2012-01-29T06:10:49Z")
Yeah, my statement there was a bit too sweeping. It’s still mostly true for low Earth orbit, depending on what you mean by “an extended time”, but there are higher orbits that don’t need boosting for any human-relevant timescale.
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### Author:  [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)
#### Post date: [January 29, 2012, 8:43am UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/19 "2012-01-29T08:43:31Z")
> [@Francis\_Vaughan](#):
>
> Just to be clear, there is a big difference between nova and supernova.
I wasn’t aware of that. Thank you.
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### Author:  [@The\_Man\_In\_Black](https://boards.straightdope.com/u/The_Man_In_Black)
#### Post date: [January 29, 2012, 5:14pm UTC](https://boards.straightdope.com/t/wont-the-moons-orbit-decay-and-it-will-crash-into-the-earth-if-so-when/611035/20 "2012-01-29T17:14:25Z")
> [@Francis\_Vaughan](#):
>
> But we don’t have a companion star, so still no nova.
What about Nemesis! [Nemesis (hypothetical star) - Wikipedia](http://en.wikipedia.org/wiki/Nemesis_(hypothetical_star))
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