[J'Dona]

I was wondering, does anyone here know what would happen to a satellite if it were placed at the Earth-moon barycenter, between the two but nearer to the moon, where the gravity of the Eatrh and moon cancel each other out? I'm wondering as I've heard of proposals to build space telescopes and such there, but I could never work out how the satellite would stay there without going out of orbit. As far as I know, the pull of gravity from both the Earth and moon would cancel out and the telescope would carry on in a straight line, but the Earth-moon system would continue to rotate. As soon as the moon was nearer, the telescope would start to be attracted by the moon and then it woldn't be fixed at the barycenter. Does anyone know how it works out?

Obviously this is different from the center of mass of the Earth-moon system, which is under the Earth's surface, in case anyone thought I was referring to that.

[swansont]

The point between the earth and the moon where the force is zero is a point of unstable equilibrium. Sooner or later an object there will be perturbed, and start feeling a force.

It is one of the five Lagrange points, called L1. There are three points along that line, and all are unstable. But there are two more that are stable - check the link for more details, and Google on 'Lagrange points'

edit to add: Note that there are Lagrange points for both the earth-moon and earth-sun systems. The original question was about the earth-moon system; this link is to an explanation that shows the earth-sun points. But the concept is the same.

[Martin]

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It is one of the five Lagrange points,
http://www.physics.m...h/lagrange.html
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it's a nice link
it mentions the fact that the WMAP satellite, which is currently one of the most scientifically productive observatories in orbit, is at L2

which, I guess, is about a million km further out from the sun than we are----maybe in the earth's shadow at least sometimes?

do you happen to know why WMAP was put out at L2
I can guess for a couple of reasons (one being not to have the Earth blocking it's view of the sky)

if being at L2 means it spends a significant amount of time in the earth's shadow does that mean it needs an alternative power supply like isotope?

[J'Dona]

Ah, I just did some extra looking up on this and found this nice link:
http://www.space.com...ion_040621.html

There's a short video that explains it quite well. Thanks for the information though swansont. :)