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

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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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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 than any atmospheric drag could be at 400,000 km from Earth if there was any atmosphere left, which there isn't.
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    $\begingroup$ There are much stronger forces currently pushing the Moon away from us! This doesn't make sense. Stronger than what? $\endgroup$
    – user15381
    Aug 11 at 1:46
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    $\begingroup$ @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. $\endgroup$
    – uhoh
    Aug 11 at 2:07
  • $\begingroup$ There is no atmosphere at 400,000 km away. $\endgroup$ Aug 11 at 7:39
  • $\begingroup$ @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 I'll make an adjustment... $\endgroup$
    – uhoh
    Aug 11 at 23:20
  • $\begingroup$ @BenCrowell ditto. Does the edit help? $\endgroup$
    – uhoh
    Aug 11 at 23:24
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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.

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  • $\begingroup$ 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. $\endgroup$
    – JohnHoltz
    Aug 11 at 17:20
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    $\begingroup$ 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. $\endgroup$
    – notovny
    Aug 11 at 17:21

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