Ebb and Flow, twin spacecraft in NASA's Gravity Recovery And Interior Laboratory (GRAIL) program, are set to crash into the surface of the moon at nearly 4,000 miles per hour today at about 5:28PM EST. The probes have been orbiting the moon since the beginning of this year, creating a high-resolution gravity field map that will be used to help understand how Earth and other planets formed. Because Ebb and Flow are low on fuel and so close to the moon's surface — just 14 miles away — NASA will crash the probes near the Goldschmidt crater, thus avoiding important historical sites like the Apollo landing.

Ebb is expected to hit the surface first at 5:28PM, and Flow will follow about 20 minutes later. "It is going to be difficult to say goodbye," said GRAIL principal investigator Maria Zuber, but the small probes are undergoing one last experiment before they run aground. On Friday, Ebb and Flow started to burn the remainder of their fuel to inform NASA exactly how much was left in their tanks. Using this information, the space agency hopes to improve fuel predictions for its future spacecraft.

Update: NASA will provide live commentary of the event starting at 5PM, but because the probes are landing on the dark side of the moon, there will be no video coverage of the impact.