Libration Points In A Geostationary Orbit

[mto]

This story (link) (http://news.yahoo.com/s/space/20100503/sc_space/outofcontrolsatellitethreatensothernearbyspacecraf t) about an adrift Intelsat satellite states the following:

Depending on their position at the time of failure, these satellites tend to migrate toward one of two libration points, at 105 degrees west and 75 degrees east. Figures compiled by XL Insurance of New York, an underwriter of space risks, say that more than 160 satellites are gathered at these two points, which Bednarek described as the orbital equivalent of valleys. What would cause drifting satellites to migrate towards 105 degrees west and 75 degrees east?

[Hornblower]

This story (link) (http://news.yahoo.com/s/space/20100503/sc_space/outofcontrolsatellitethreatensothernearbyspacecraf t) about an adrift Intelsat satellite states the following:
What would cause drifting satellites to migrate towards 105 degrees west and 75 degrees east?

The Earth is lumpy, and if I am not mistaken the resulting irregularities in the gravitational field give us these sweet spots where the satellites are more resistant to perturbations from the Sun and the Moon. Those perturbations cause the satellites to drift away from their original longitudes in the first place when they run out of station-keeping fuel.

[grant hutchison]

Yes, these points are directly opposite each other on Earth's equator, and reflect the slight elliptical nature of the mass distribution around the equator. Geostationary satellites positioned elsewhere are on the slope of a very slight potential well, and will trickle "downhill" without regular station-keeping.

Grant Hutchison

[mto]

Thanks for the answers. I had thought it might be something to do with varying gravity but the term 'libration' was unfamiliar to me.

[grant hutchison]

Thanks for the answers. I had thought it might be something to do with varying gravity but the term 'libration' was unfamiliar to me.It comes from the Latin libra, the balance scales: like the constellation. A libration point is a point where opposite forces balance, and objects displaced away from the balance point will tend to swing back and forth through it, like a balance scale rocking.

Grant Hutchison