At a press conference today, NASA confirmed that its two Gravity Recovery and Interior Laboratory (GRAIL) Spacecraft will crash into a lunar mountain next week. The controlled impact will occur on Monday, December 17 at approximately 5:28 p.m. EST (22:28 GMT). The impact area is at latitude 75.62° N, longitude 26.63° E near the lunar North Pole in the vicinity of Goldschmidt crater.
Launched in September 2011, the GRAIL spacecraft have been in polar orbit around the Moon, flying as low as 11 km (6.8 mi) to map lunar gravitational anomalies. The data from these measurements allowed NASA to create the highest resolution gravity map yet made of any body in the Solar System. The extended science mission will end on December 15 when the spacecrafts’ experiments are switched off.
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The nature of the GRAIL mapping mission required a great deal of orbital correction by the two craft, with engines firing three times a week. Now the two orbiters are running out of fuel, and NASA will send commands tonight to target the polar mountain for impact. The first craft, Ebb, will make a controlled impact on Monday at 5:28:40. The second, Flow, will hit 20 seconds later. They will be traveling at 3,760 mph (6,051 km/h).
The impact area was chosen to avoid affecting heritage sites where American and Russian spacecraft have landed, and as an opportunity to learn about the composition of the impact area with an eye on detecting the presence of volatiles such as water. Before the impact, NASA will order the two craft to start their engines one last time to burn off any remaining fuel so that NASA engineers can determine how much was left in the tanks.
Because the impact is occurring during a new moon, no images of the impact will be available.
Source: NASAView gallery - 7 images