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A pair of NASA "space weather" monitoring spacecraft are having something of a change of pace, as they search for evidence of a long-lost world which may once have crashed into the Earth, and so formed the Moon.

The two probes in question are those of the space agency's Solar Terrestrial Relations Observatory (STEREO) programme, intended to gain a side view of the sun's interaction with Earth.

Jupiter's L4 and L5 points are absolutely chocka.

As such, the STEREO space brace are primarily intended for spying out solar storms, sunspots and the like. But they can also do other things, and just at the moment the two craft are passing through the Earth/Sun system's L4 and L5 Lagrange points.

Lagrange points are locations where drifting space objects tend to accumulate, as orbital and gravitational forces combine to hold them there. You get an L4 and L5 point sixty degrees ahead and behind on the orbit of any body orbiting a larger one, in this case with the Earth orbiting the Sun. The animated illustration shows the L4 and L5 points of Jupiter, which stays almost stationary at the top of the picture with two large asteroid clusters (the "Trojans" and the "Greeks") to either side - with so-called "Hilda" asteroids circulating between them, mostly just outside the main asteroid belt. The faster-orbiting Earth is seen near the centre.

In the case of the previously unvisited Earth/Sun L4 and L5 "gravitational parking lots," as NASA calls them, there could be some major stuff to be found.

"These places may hold small asteroids, which could be leftovers from a Mars-sized planet that formed billions of years ago," says NASA's Michael Kaiser. "About 4.5 billion years ago when the planets were still growing, this hypothetical world, called Theia, may have been nudged out of L4 or L5 by the increasing gravity of the other developing planets like Venus and sent on a collision course with Earth. The resulting impact blasted the outer layers of Theia and Earth into orbit, which eventually coalesced under their own gravity to form the moon."

Boffins think the planet-smash theory of lunar formation could explain why the Moon has such a relatively unimpressive iron core, being made up mostly of melted crusty bits smashed off in the possible Earth/Theia pileup and then blobbed together.

"Taking the time to observe L4 and L5 is kind of cool because it's free," adds Kaiser. "We're going through there anyway and we're moving too fast to get stuck. In fact, after we pass through these regions, we will see them all the time because our instruments will be looking back through them to observe the sun."

The STEREOcraft will look for asteroids with wide-field-of-view telescopes that come as part of their solar-corona probing kit. Any asteroid will probably appear as just a point of light, orbiting the L4 or L5 point. It'll be possible to tell if a dot is an asteroid because it will shift its position against stars in the background as it moves in its orbit. Boffinry chiefs plan to put all the imagery online as soon as they get it, so that amateur enthusiasts can aid them in discovering any objects of interest: you can find full details here.

"If we discover the asteroids have the same composition as the Earth and moon, it will support [the 'Theia'] version of the giant impact theory," says Kaiser. "Also, the L4/L5 regions might be the home of future Earth-impacting asteroids." ®

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