Looking for stuff at the L4 and L5 points....

[n2doc]

THEY are the places gravity forgot. Vast regions of space, millions of kilometres across, in which celestial forces conspire to cancel out gravity and so trap anything that falls into them. They sit in the Earth's orbit, one marching ahead of our planet, the other trailing along behind. Astronomers call them Lagrangian points, or L4 and L5 for short. The best way to think of them, though, is as celestial flypaper.

In the 4.5 billion years since the formation of the solar system, everything from dust clouds to asteroids and hidden planets may have accumulated there. Some have even speculated that alien spacecraft are watching us from the Lagrangian points, looking for signs of intelligence.

Putting little green men to one side for the moment, even the presence of plain old space rocks would be enough to keep most people happy. "I think you certainly might find a whole population of objects at L4 and L5," says astrophysicist Richard Gott of Princeton University.

After nearly a century of speculation, we are on the verge of finding out what they are hiding once and for all. Later this year, two spacecraft that spend their lives studying the sun will begin their slow journeys through L4 and L5.

Space scientists plan to use instruments on board NASA's STEREO probes A and B to search for celestial objects becalmed at the Lagrangian points. What they find could hugely enhance our view of how the solar system formed, tell us more about the colossal impact that formed the moon, and warn us if another major collision is on the cards.

more:

http://www.newscientist.com/article/mg20126962.000-do-gravity-holes-harbour-planetary-assassins.html

[Thor_MN]

1. OK, I've always understood L1

But the others have never made sense to me. I can see that at L1, the gravitaional forces would be equal and opposite (cancel each other). I've never been able to see how the forces could cancel at the others. Or are the others just places where the line where the gravitational forces of the Earth and Sun are equal (but not opposite in direction) intersects a stable orbit around the sun?

[Posteritatis]

2. Here's a good explanation:

http://www.esa.int/esaSC/SEMM17XJD1E_index_0.html

[muriel_volestrangler]

3. Here's an explanation with some maths

(following the maths, especially for L4 and L5, the stable points for which STEREO A and B are headed, is beyond me at the moment, though I might once have been able to follow it, at university).

http://www.physics.montana.edu/faculty/cornish/lagrange.pdf

But who can resist a maths paper with this in it:

"While there can be no Planet X, the long e-folding time makes L3 a good place to park your invasion force while final preparations are made ..." :D

[On the Road]

4. I Remember an Old SciFi Movie

featuring a planet at L3 which went undetected because remained directly behind the sun.