Twin satellites buzz around man in the moon

Two new satellites are now in orbit around the moon, and they could reveal whether our moon ate a sibling many moons ago.

The GRAIL probes, which launched together in September, separately went into orbit on 31 December and 1 January.

They are designed to produce the most detailed map ever made of the lunar gravitational field, which is lumpy thanks to mountains, craters, lava flows, and larger irregularities ? the moon's far side is much more mountainous than its near side, for example.

"We don't actually know why the near side and far side are different," says mission principal scientist Maria Zuber of the Massachusetts Institute of Technology in Cambridge.

One theory is that Earth once had two moons, and the second one wrapped itself around the lunar far side in a low-velocity collision that created the highlands. GRAIL will look for signs of such a crash.

Surviving eclipse

Orbiting at a height of 55 kilometres, the twin satellites will use microwave signals to measure the distance between them, which varies according to the pull of the underlying terrain, to within the width of a human hair.

The resulting gravity map is expected to be 100 times more accurate than our existing knowledge of the lunar near side, and 1000 times more accurate on the far side. "When we can improve by a factor of two, we can learn a lot, and improving by a factor of 1000 is transformative," says Zuber.

Both the probes' batteries and solar panels are generating more power than expected. Zuber thinks they will have enough power to survive a lunar eclipse in June ? when the Earth blocks sunlight from falling on them. That should allow the probes to keep operating for six more months.

If so, the team will lower the spacecraft to an adventurous "treetop-skimming" orbit only 25?km above the surface. This will enable them to study the structure of craters spanning just 15?km ? "the most common landform on the surfaces of terrestrial planets", Zuber says.

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