Why don't man-made satellites drift away from the Earth?

[wywong]

Main Question or Discussion Point

What is the difference between man-made satellites and the moon that causes the former to drift towards the earth but the latter drift away?

 

[Janus]

The difference is that the majority of man made satellites orbit close enough to the Earth that their periods are shorter than the rotational period of the Earth. The small tidal acceleration acting on them acts opposite to that on the Moon which takes much longer than a day to orbit the Earth. They lose orbital angular momentum by transferring it to the Earth, while the the case of the Moon, the Earth transfers angular momentum to it.
This is not just a man-made vs. natural satellite phenomenon either. Phobos, one of the moons of Mars, orbits faster than Mars rotates and is slowly being drawn into the planet.

 

[wywong]

Thanks a lot!

Do geostationary satellites neither gain nor lose orbital angular momentum due to tidal acceleration?

 

[jtbell]

I'd expect the tidal acceleration of artificial Earth satellites to be extremely small. Has it actually been observed?

 

[snorkack]

Thanks a lot!

Do geostationary satellites neither gain nor lose orbital angular momentum due to tidal acceleration?

They do not. But if they are low mass, they are fundamentally unstable.

 

[LURCH]

No, geostationary satellites do not experience positive or negative acceleration from tidal forces (to any appreciable degree, at least). They are not orbiting faster than the Earth is revolving, so they don’t get slowed down by trying to drag the Earth along and speed it up. They are also not orbiting more slowly than the planet revolves, so they are not flung up by the Earth trying to speed them up.

 

[wywong]

Thanks folks!

 

[rbelli1]

I'd expect the tidal acceleration of artificial Earth satellites to be extremely small. Has it actually been observed?

Good point. What portion of the orbital decay is tidal and what portion is atmospheric drag?

BoB

 

[Nik_2213]

IIRC, it varies with altitude, orbital alignment, season, insolation and solar activity.
Didn't Skylab famously come down 'early' because that solar max puffed up the atmosphere just enough to increase drag ? Upside of such is that a solar max may 'knock down' a lot of orbital trash...

Um, I seem to remember there's a solar tide, too, complicated by multiple orbital planes...

 

[russ_watters]

Good point. What portion of the orbital decay is tidal and what portion is atmospheric drag?

BoB

Essentially none of it is tidal. Artificial satellites are way too small to raise significant tides on Earth.

 

[sophiecentaur]

No, geostationary satellites do not experience positive or negative acceleration from tidal forces (to any appreciable degree, at least). They are not orbiting faster than the Earth is revolving, so they don’t get slowed down by trying to drag the Earth along and speed it up. They are also not orbiting more slowly than the planet revolves, so they are not flung up by the Earth trying to speed them up.

i.e. they are already 'locked'.
All the others are going round faster so any tidal effect would be speeding the Earth up and slowing them down. But until the total mass of artificial satellites becomes comparable with the mass of the Moon, it won't be measurable.

 

[snorkack]

i.e. they are already 'locked'.
All the others are going round faster so any tidal effect would be speeding the Earth up and slowing them down. But until the total mass of artificial satellites becomes comparable with the mass of the Moon, it won't be measurable.

No.
Deimos orbits near areostationary orbit. And its mass is tiny compared to that of Moon. Is the tidal effect measurable?
Because for Phobos, specific inspiral times are offered. (And Mars has no sea.)
Also, the total mass of satellites is not relevant. An axisymmetric ring raises no tides and meets no friction whatever its total mass.

 

[consuli]

Thus, geostationary satellites can operate siginificantly longer with same amount of energy than non-stationary ones, as the first ones do have to compensate positive ore negative tidal acceleration effects during their life time?

Whatever "signficantly longer" means for satellites.

Consuli

 

[russ_watters]

Thus, geostationary satellites can operate significantly longer with same amount of energy than non-stationary ones, as the first ones do have to compensate positive ore negative tidal acceleration effects during their life time?

No; again, man-made satellites are far too small to cause noticeable tidal effects.

 

[Nik_2213]

Please, with disclaimer that the Earth is not uniform, it is a lumpy oblate spheroid at best, and the resulting gravity effects produce wibbles in orbits, compounded by Lunar and Solar tides. Remember the difficulties Apollo etc had with Lunar mascons affecting, even destabilising low orbits ??

https://en.wikipedia.org/wiki/Gravity_of_Earth
https://en.wikipedia.org/wiki/Gravitation_of_the_Moon
https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)

 

[russ_watters]

Please, with disclaimer that the Earth is not uniform, it is a lumpy oblate spheroid at best, and the resulting gravity effects produce wibbles in orbits, compounded by Lunar and Solar tides.

That is not the same thing as causing decay. It just means the path taken isn't smooth. The mechanism by which the tides cause the moon to drift away requires that it be pulling against it's own tidal bulge.

Remember the difficulties Apollo etc had with Lunar mascons affecting, even destabilising low orbits ??

No...

 

[consuli]

Errata corrected
Thus, geostationary satellites can operate siginificantly longer with same amount of energy than non-stationary ones, as the first ones do not have to compensate positive ore negative tidal acceleration effects during their life time?

Although it might be of much importance, here ...

 

[jbriggs444]

Errata corrected
Thus, geostationary satellites can operate siginificantly longer with same amount of energy than non-stationary ones, as the first ones do not have to compensate positive ore negative tidal acceleration effects during their life time?

Geostationary communications satellites do need to expend station keeping fuel to maintain their assigned position. My understanding is that the primary influence is from the moon which tends to cause orbits to precess out of the equatorial plane.

TDRS-1 was forced to burn station keeping fuel to boost from a highly eccentric transfer orbit to its final geostationary orbit when the second burn of the Inertial Upper Stage failed.

https://en.wikipedia.org/wiki/TDRS-1#Lifespan