STEREO Spacecraft To Explore Earth's L4 and L5

Hugh Pickens writes "Launched on October 25, 2006, NASA's twin Solar Terrestrial Relations Observatory (STEREO) spacecraft are about to enter the L4 and L5 Lagrangian points, special points in our orbit around which spacecraft and other objects can loiter because the gravitational pull of earth and the sun balances the forces from the object's orbital motion. (The spacecraft won't linger at the Lagrangian points; they are just passing through.) 'These places may hold small asteroids, which could be leftovers from a Mars-sized planet that formed billions of years ago,' said NASA Project Scientist Michael Kaiser. STEREO will look for asteroids with a wide-field-of-view telescope. 'If we discover the asteroids have the same composition as the Earth and moon, it will support Belbruno and Gott's version of the giant impact theory. The asteroids themselves could well be left-over from the formation of the solar system.' L4 and L5 are also good places to observe space weather. 'With both the sun and Earth in view, we could track solar storms and watch them evolve as they move toward Earth. Also, since we could see sides of the sun not visible from Earth, we would have a few days warning before stormy regions on the solar surface rotate to become directed at Earth,' says Kaiser."

Comments

Re:I wonder, how.

[hcpxvi]

That (the lagrange points being unstable equilibria) is true of L1, L2 and L3 (all on the Earth-Sun line, L1 between Earth and Sun, L2 outside the Earth's orbit and L3 round the other side of the Sun). L4 and L5, OTOH, are stable equilibria and junk can collect there. The equivalent points for Jupiter have observable collections of asteroids in them.

Re:May hold?

[Bakkster]

Since the L4 and L5 points don't move relative to our perspective, any objects we would see there would move very little compared to the background of stars. Movement across a series of telescope images is the usual method for detecting small objects in our solar system, and it can't be used for these locations.

To detect objects here, you would need to look at images taken over a series of months and centered on the points to find objects that didn't move with the rest of our perspective. This would probably need to be done by a space telescope, since by the time a ground based telescope could see the points, the sun is already rising or still setting. Even then, the objects are only half lit by the sun, due to our angle of viewing, so they would be especially dim. In addition, sending a spacecraft to the area would allow the sattelites to determine the composition of the asteroids to see if they came from an Earth collision or are leftover from the solar system's birth.