The Earth is surrounded, it's said, by cosmic flypaper areas of space that cancel gravity and trap unwilling objects inside. Quite what those objects are have remained a mystery... but we're about to find out.

The cosmic flypaper is actually known as "Lagrangian points" after mathematician Joseph-Louis Lagrange, who discovered it in 1772, and there are five such points in Earth's orbit (known by scientists as L1, L2, L3, L4 and L5). For billions of years, these enormous (millions of kilometers wide in the cases of Ls 4 and 5) areas of space that lack any gravity - due to Earth's gravitational pull canceling out that of the Sun - have been trapping all manner of space debris inside, and now scientists are aiming to use two NASA probes to find out just what lies in there, and what it can tell us about the origins of our galaxy... and more specifically, of our moon:

Most astronomers believe that the moon formed from the debris generated when a Mars-sized object struck the Earth a glancing blow about 4 billion years ago. Their problem is in understanding where the object came from.

Computer models show that incoming objects from elsewhere in the solar system would tend to strike the Earth with too much energy. Instead of creating the moon, they obliterate the Earth. So the impactor must have originated close by, the theory goes, where it could not accelerate too much before hitting.

Another clue is that the moon contains the same abundance of oxygen isotopes as the Earth, hinting that whatever hit us must also have had the same isotope abundance. When astronomers look out into the solar system, to Mars for example, the isotope abundances are different. So this, too, hints that the impactor formed close by. But where?

What is puzzling is how an object could grow so close to the Earth and reach the size of Mars before a collision took place. Their mutual gravity should have pulled them together long before. Unless, says [Princeton University astrophysicist Richard] Gott, it formed at a Lagrangian point. "An object could sit at one of these stable points and just grow," he says.

Once it grew sufficiently large, gravitational interactions with other objects, such as Venus, could nudge it out of the Lagrangian point and onto a collision course with Earth.

NASA's probes - STEREO 1 and 2 - will begin journeying through L4 and L5 later this year.

Do gravity holes harbour planetary assassins? [New Scientist]