After nearly a year in orbit, two NASA -owned moon crafts called "Ebb" and "Flow" will crash into a dark lunar mountainside later this evening. The two probes are part of the Gravity Recovery and Interior Laboratory (GRAIL) and were launched in September 2011 to map the gravity field of the moon. That in turn would give scientists clues about how the moon formed. In fact, the picture above is a moon shot that astronomers put together  based on gravity data that Ebb and Flow collected between March and May this year. The twins have been orbiting the moon since January 1 2012.
But now the probes are out of juice. If they're left to drift and crash by themselves, they might land on historically- or scientifically-important sites, NASA has explained . Their course has been shifted this morning and they will make contact with a mountain a little before 5:30pm ET. NASA is livestreaming commentary  of the crash, but "being the size of washer driers," the dual impacts are unlikely to be visible as a flash, BBC's Jonathan Amos explains .
[Image: NASA ]