NASA’s two Gravity Recovery and Interior Laboratory (GRAIL) spacecraft have struck the Moon in a controlled impact. At 5:28:46 EST (222846 GMT) Ebb, the first spacecraft, struck a mountain near the lunar North Pole. The second, Flow, hit about 20 seconds later. Because the impact occurred during a new moon, no images were available of the impact, though NASA was able to determine the time of the event by monitoring the moment that telemetry ended. The Jet Propulsion Laboratory in Pasadena, California provided live television and online commentary.

The GRAIL spacecraft were launched in September 2011 and had completed their mission to map lunar gravitational anomalies as part of an effort to complete the most detailed gravity map of any body in the Solar System. On December 14, the spacecraft fired their engines to set them on their fatal trajectory. On December 15, the twin probes’ experiments were switched off. Earlier today, the spacecraft fired their engines for the last time to empty their fuel tanks so that engineers could determine how much was left.

Artist's concept of the GRAIL spacecraft (Image: NASA)

The purpose of the controlled impact was to ensure that the probes didn’t crash near any historic Russian or American landing sites on the Moon. Because of their low orbit, the GRAIL spacecrafts’ orbits would have soon decayed, bringing them down naturally.

This is not the first time such controlled impacts have been made on the Moon. The first unmanned spacecraft to reach the satellite in 1959, the USSR’s Luna 2, was sent on a collision course because the technique of going into orbit around the Moon hadn’t been perfected yet. Since then, other unmanned or abandoned manned spacecraft have been sent to hit the Moon either as part of a photo reconnaissance mission, to learn more about the surface, to cause shock waves for seismic studies or to reduce the amount of space debris in lunar orbit.

The impact site of the GRAIL spacecraft is latitude 75.62° N, longitude 26.63° E in the vicinity of Goldschmidt crater. When day returns to the site, NASA’s Lunar Reconnaissance Orbiter (LRO) will survey the area. After the impact, NASA announced approval for naming the impact site “Sally Ride” after the late U.S. astronaut.

The animation below shows the final seconds of the GRAIL spacecraft’s orbit.

Source: NASA

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