<p>We need to reboost the ISS because it is in low Earth orbit. At an average altitude of 400 km above the surface the Earth, there is still air there. While it is very rarified air, it is there. Atmospheric drag is the sole reason the ISS needs to regularly perform attitude maintenance maneuvers.</p>
<p>The roughly exponential drop-off in atmospheric density with increased altitude means that atmospheric drag is not a factor for satellites orbiting higher than a few thousand kilometers above the surface of the Earth. For example, nobody accounts for atmospheric drag when modeling the orbits of geosynchronous satellites, which orbit at 35786 km above the surface of the Earth. The Moon orbits ten times higher still, at about 380000 km above the surface of the Earth. Atmospheric drag is simply not a factor at that altitude.</p>
<p>What is a factor at that altitude are the interactions between the Moon and the Earth's oceans. Most scientists think the Moon formed much closer to the Earth than it is now. Tidal interactions have gradually made the Moon move away from the Earth. This recession is measurable thanks to retroreflectors left on the surface of the Moon by the US and the USSR in the 1960s and 1970s.</p>

CC BY-SA 4.0
2021-08-11T10:23:58.320Z
2021-08-11T10:30:15.720Z
2021-08-11T10:30:15.720Z
David Hammen
<p>Three reasons (at least!):</p>
<ol>
<li>The Moon is 400,000 km away, the ISS is only 400 km, so the Earth's atmosphere is a lot thicker there, increasing atmospheric drag enormously. Actually there's no longer an atmosphere <em>per se</em> at 400,000 km due to the solar wind.</li>
<li>The Moon's area to mass ratio is much, much smaller than that of the ISS. The drag force depends on area, but the resulting acceleration (F/m) will be much lower than that of the ISS, which is basically an empty tin can. Think of throwing a tennis ball and a rock of the same size, which slows down faster?</li>
<li>There are much stronger forces currently <a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Tidal_evolution" rel="nofollow noreferrer">pushing the Moon away from us</a> than any atmospheric drag could be at 400,000 km from Earth if there was any atmosphere left, which there isn't.</li>
</ol>

CC BY-SA 4.0
2021-08-11T02:07:13.120Z
@BenCrowell than atmospheric drag on the moon at its current distance, i.e. points 1 and 2. I've added "than atmospheric drag 400,000 km from Earth" though I think it was pretty self-evident.
uhoh
CC BY-SA 4.0
2021-08-11T07:39:28.150Z
There is no atmosphere at 400,000 km away.
Peter Erwin
CC BY-SA 4.0
2021-08-11T23:20:17.917Z
@PeterErwin ya you are right, there's no more atmosphere *per se* at that distance. I must have been thinking of [Earth is sending oxygen to the moon](https://www.sciencemag.org/news/2017/01/earth-sending-oxygen-moon) I'll make an adjustment...
uhoh
CC BY-SA 4.0
2021-08-11T23:24:04.867Z
@BenCrowell ditto. Does the edit help?
uhoh
CC BY-SA 4.0
2021-08-11T01:20:58.160Z
2021-08-11T23:23:33.557Z
2021-08-11T23:23:33.557Z
uhoh
<p>The Moon velocity Earth-Moon gravity attraction make the system balanced.</p>
<p>From Earth, it might look like the Moon is stationary, meaning it is not moving, but in reality, each year the Moon gets some centimetres away from Earth. Without having the force of Gravity from earth-moon would have just floated away from us. The moon's velocity and distance from Earth allow it to make a perfect balance between fall and escape.</p>
<p>Calculations of the evolution of the Earth/Moon system tell us that with this rate of separation that in about 15 billion years the Moon will stop moving away from the Earth.</p>
<p>In case the velocity of rotation of the moon was a little bit faster, it would have escaped the Earth's Gravity. On the other hand, if it's a little bit slower, it would have fallen on Earth. That's why the moon doesn't fall on Earth.</p>

CC BY-SA 4.0
2021-08-11T17:20:18.163Z
Everything you mentioned about the Earth-Moon system is also true for the Earth-ISS system. The are both orbiting at the "correct" velocity. Your description does not explain why the ISS gets lower but the Moon does not.
JohnHoltz
CC BY-SA 4.0
2021-08-11T17:21:16.193Z
Orbits aren't knife-point balances of velocity and distance.  At the distance of the Moon, almost _any_ velocity less than about 1.4 km/s, in almost any direction, results in a stable elliptical orbit around Earth.
notovny
CC BY-SA 4.0
2021-08-11T15:58:30.410Z
2021-08-11T15:58:30.410Z
Lariliss
<p>If you need to boost ISS to stay in orbit why moon doesn't fall down to earth and KABOOM?</p>

CC BY-SA 4.0
2023-02-13T20:09:37.853Z
Because it's actually falling IN orbit, maybe?
Oscar Lanzi
<orbit><the-moon><orbital-mechanics>
Why doesn't the Moon fall out of orbit?
user42450
# Why doesn't the Moon fall out of orbit?

(Asked by: username_0 on 2021-08-11)

If you need to boost ISS to stay in orbit why moon doesn't fall down to earth and KABOOM?

**Tags:** orbit, the-moon, orbital-mechanics

> **Comments:**
> **username_1**: Because it's actually falling IN orbit, maybe?  

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## 3 answers

### Answer by username_2 on 2021-08-11. Score: 6

We need to reboost the ISS because it is in low Earth orbit. At an average altitude of 400 km above the surface the Earth, there is still air there. While it is very rarified air, it is there. Atmospheric drag is the sole reason the ISS needs to regularly perform attitude maintenance maneuvers.

The roughly exponential drop-off in atmospheric density with increased altitude means that atmospheric drag is not a factor for satellites orbiting higher than a few thousand kilometers above the surface of the Earth. For example, nobody accounts for atmospheric drag when modeling the orbits of geosynchronous satellites, which orbit at 35786 km above the surface of the Earth. The Moon orbits ten times higher still, at about 380000 km above the surface of the Earth. Atmospheric drag is simply not a factor at that altitude.

What is a factor at that altitude are the interactions between the Moon and the Earth's oceans. Most scientists think the Moon formed much closer to the Earth than it is now. Tidal interactions have gradually made the Moon move away from the Earth. This recession is measurable thanks to retroreflectors left on the surface of the Moon by the US and the USSR in the 1960s and 1970s.


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### Answer by username_3 on 2021-08-11. Score: 2

Three reasons (at least!):

1. The Moon is 400,000 km away, the ISS is only 400 km, so the Earth's atmosphere is a lot thicker there, increasing atmospheric drag enormously. Actually there's no longer an atmosphere *per se* at 400,000 km due to the solar wind.
2. The Moon's area to mass ratio is much, much smaller than that of the ISS. The drag force depends on area, but the resulting acceleration (F/m) will be much lower than that of the ISS, which is basically an empty tin can. Think of throwing a tennis ball and a rock of the same size, which slows down faster?
3. There are much stronger forces currently [pushing the Moon away from us](https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Tidal_evolution) than any atmospheric drag could be at 400,000 km from Earth if there was any atmosphere left, which there isn't.

> **Comments:**
> **username_3**: @BenCrowell than atmospheric drag on the moon at its current distance, i.e. points 1 and 2. I've added "than atmospheric drag 400,000 km from Earth" though I think it was pretty self-evident.  
> **username_4**: There is no atmosphere at 400,000 km away.  
> **username_3**: @PeterErwin ya you are right, there's no more atmosphere *per se* at that distance. I must have been thinking of [Earth is sending oxygen to the moon](https://www.sciencemag.org/news/2017/01/earth-sending-oxygen-moon) I'll make an adjustment...  
> **username_3**: @BenCrowell ditto. Does the edit help?  

---

### Answer by username_5 on 2021-08-11. Score: -3

The Moon velocity Earth-Moon gravity attraction make the system balanced.

From Earth, it might look like the Moon is stationary, meaning it is not moving, but in reality, each year the Moon gets some centimetres away from Earth. Without having the force of Gravity from earth-moon would have just floated away from us. The moon's velocity and distance from Earth allow it to make a perfect balance between fall and escape.

Calculations of the evolution of the Earth/Moon system tell us that with this rate of separation that in about 15 billion years the Moon will stop moving away from the Earth.

In case the velocity of rotation of the moon was a little bit faster, it would have escaped the Earth's Gravity. On the other hand, if it's a little bit slower, it would have fallen on Earth. That's why the moon doesn't fall on Earth.

> **Comments:**
> **username_6**: Everything you mentioned about the Earth-Moon system is also true for the Earth-ISS system. The are both orbiting at the "correct" velocity. Your description does not explain why the ISS gets lower but the Moon does not.  
> **username_7**: Orbits aren't knife-point balances of velocity and distance.  At the distance of the Moon, almost _any_ velocity less than about 1.4 km/s, in almost any direction, results in a stable elliptical orbit around Earth.  

---

