UPDATED: GRAIL probes impact on Moon

The planned path of GRAIL's final orbit.

NASA

The twin GRAIL probes, Ebb and Flow, are hot off the completion of their mission to map the Moon's interior. But to protect historic sites on the lunar surface, NASA is planning to make them a part of the Moon's exterior, smashing them into a mountain near the lunar north pole. The event will be broadcast live on NASA TV at a bit after 5pm EST today.

In the past, NASA got good scientific value out of smashing something into the Moon, as it was able to detect a variety of chemicals, including water, in the debris. This time around, however, there's not much science planned. Instead, the goal is to protect the Moon landing sites, which have significant historic value. "NASA wanted to rule out any possibility of our twins hitting the surface anywhere near any of the historic lunar exploration sites like the Apollo landing sites or where the Russian Luna probes touched down," said David Lehman, GRAIL project manager.

To fulfill their mission, the GRAIL spacecraft orbited less than 100km above the Moon's surface, and they just don't have enough fuel to lift themselves out of that gravity hole. As a result, NASA has chosen to crash them into a mountain now, while it still has control of them, in order to make sure they don't wreck any human artifacts.