The slope of an unnamed mountain near the moon’s north pole will be the final destination for NASA’s twin GRAIL spacecraft, which are scheduled to crash into the lunar surface at high speed today. The impacts are planned for 5:28 P.M. Eastern Standard Time.
The twin probes, nicknamed Ebb and Flow, have orbited the moon for almost a year to map the lunar gravity field. The GRAIL probes detect local differences in the moon’s gravitational pull by flying in formation and tracking the distance between the two spacecraft. Any gravitational anomalies—an extra pull, say, from a massive landform here or there—will impart a nudge on the spacecraft overhead, expanding or shrinking the distance between the two probes.
Earlier this month GRAIL scientists published a series of papers in Science with new results from the mission, including an unprecedentedly high-resolution map of the moon’s gravitational field. The researchers also found that the lunar crust had been highly fractured during the barrage of impactors that struck the inner solar system early billions of years ago.
Their collection of data complete, Ebb and Flow are now low in orbit and low on fuel. So NASA plans to plow the spacecraft into the side of a northern slope at 1.7 kilometers per second. The intentional crash-landing will take place far from any lunar heritage sites, including the areas visited by the six Apollo missions that successfully reached the surface of the moon between 1969 and 1972.
The crash site will be shadowed, so the impacts will probably not be visible to any telescopes or orbiting spacecraft, but NASA will provide live commentary of the event from the control room of the Jet Propulsion Laboratory in Pasadena, Calif. The commentary begins at 5:00 P.M. (EST), about a half-hour before impact, on NASA TV.