Two new satellites are now in orbit around the moon and they could give us some valuable info about whether our moon merged with another many moons ago. The GRAIL probes (short for Gravity Recovery and Interior Laboratory) launched together in September and entered orbit on December 31st and January 1st. The probes are orbiting at a height of 55 kilometers (34 miles), where they will use microwave signals to measure the distance between them. This distance varies depending on the pull of the underlying terrain.
The satellites will give us the most detailed map yet of the lunar gravitational field; and there is plenty of detail to gather as the moon is covered in mountains, craters, and plains of volcanic rock. But that isn’t all. Of particular interest is the moon’s far side, which is much more mountainous than the side facing us.
No one knows why the far side differs so vastly from the near side, but they will sift through the data to have a look at one recent theory that believes the Earth may have had two moons. This theory states that the second moon wrapped itself around the lunar far side of the moon in a low-velocity collision that created the mountains.
The probes’ batteries and solar panels are performing better than expected, so they hope both will survive a lunar eclipse in June. There’s no reason why the probes can’t be operational for six more months.