GRAIL, which stands for Gravity Recovery And Interior Laboratory, is a set of two orbiters that launched on September 10. GRAIL-A will get to its destination of circling the moon on the afternoon of December 31, with GRAIL-B following the next day at 5:05 p.m. ET.

They will arrive about 25 hours apart, giving breathing room in between these important milestones, says GRAIL project manager David Lehman.

The timing was set by celestial mechanics, in order to miss eclipses in December and June, scientists say. As of Wednesday, GRAIL-A was 65,860 miles from the moon, with GRAIL-B following behind at 79,540 miles from the moon.

The orbiters are each 3.5 feet high and 2.5 feet deep. They have been traveling since the launch on September 10.

A familiar sight for earthlings as long as humans have roamed the planet, the moon is still mysterious in many ways. We know more about Mars than we do about our own moon, says Maria Zuber, principal investigator for GRAIL. Scientists theorize that the moon formed when a Mars-sized object crashed into Earth, but we know little about this process of the moon being formed.

It remains unknown why the near side of the moon is so different from the far side. The basins are flooded with volcanic material on the near side (which faces us), while the far side has mountainous highlands. Why did these sides evolve differently? "We think the answer is locked in the interior," Zuber said at a NASA news briefing Wednesday.

A study published in the journal Nature over the summer suggested that there may have been a second moon moving at about the same speed as our moon, and in the same orbit. The second moon bumped into the original moon, the theory goes, and caused the formation of a mountain instead of a crater. This model predicts the whole exterior of the moon was once molten and cooled from the outside in such a way that when the second moon hit our moon, it pushed all of that molten material to the near side.

GRAIL can help test this hypothesis. The spacecraft are designed to help scientists explore the interior of the moon indirectly by measuring the pull of gravity as they fly over the surface. With data about these changes in gravity and topography, scientists will be able to construct a map of what's inside our celestial neighbor.

Scientists wanted the spacecraft to be as small as possible so they could launch in one rocket. But they weren't sure if the orbiters would make it through eclipses that happen at the moon every six months, since their technology depends on lithium-ion batteries and solar panels. A lunar eclipse occurred in early December, and the next one is in June. Based on the current performance, scientists think GRAIL can continue through the June eclipse.

Should one of the orbiters completely fail, scientists could not get the high precision they would get from the two spacecraft; the mission depends on having both, Zuber said. If the lunar orbit insertion doesn't happen for one of the spacecraft, there would be another opportunity to try again in late April.

cnn.com