NASA‘s Gravity Recovery and Interior Laboratory (GRAIL) probes today completed a burn that irreversibly altered their orbit and have begun skimming the surface of the moon. Over the weekend, the probes, named Ebb and Flow, will orbit ever closer to the surface of the moon. On Monday, December 17, at around 5:28 pm EST the probes will slam into the side of a lunar mountain while traveling at approximately 3,760 miles per hour.

“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 at NASA’s Jet Propulsion Laboratory (JPL). “Our navigators calculated the odds before this maneuver as about seven in a million. Now, after these two successful rocket firings, there is zero chance.”

The crash landing is a planned event, necessary because the probes’ low orbit and fuel levels make them useless for any further study. The impact will take place near the moon’s north pole, close to a crater named Goldschmidt. The area will be in shadow at the time, and no photos or video of the event are expected.

NASA will, however, be live-streaming commentary of the event starting at 5 pm EST on Monday. The commentary will come from the control room at the JPL and will include interviews with the GRAIL team. It can be viewed on NASA TV or on the JPL Ustream channel.

The probes began orbiting the moon on January 1, 2012. In their year of orbit, Ebb and Flow collected data that helped scientists create the highest-resolution gravity field map of any celestial body to date.