Watch NASA's twin GRAIL spacecraft crash into the moon live

CALIFORNIA, USA: Twin lunar-orbiting NASA spacecraft that have allowed scientists to learn more about the internal structure and composition of the moon are being prepared for their controlled descent and impact on a mountain near the moon's north pole at about 2:28 p.m. PST (4:00 a.m. IST) today, according to a NASA press release.

Ebb and Flow, the Gravity Recovery and Interior Laboratory (GRAIL) mission probes, are being sent purposely into the lunar surface because their low orbit and low fuel levels preclude further scientific operations.

The duo's successful prime and extended science missions generated the highest resolution gravity field map of any celestial body. The map will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved, said the press release.

"It is going to be difficult to say goodbye," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge. The mountain where the two spacecraft will make contact is located near a crater named Goldschmidt.

Both spacecraft have been flying in formation around the moon since January 1. They were named by elementary school students in Bozeman, who won a contest. The first probe to reach the moon, Ebb, also will be the first to go down, at 2:28:40 p.m. PST. Flow will follow Ebb about 20 seconds later.

NASA will provide live commentary of the scheduled lunar surface impacts of its twin Gravity Recovery and Interior Laboratory (GRAIL) spacecraft beginning at 2 p.m. PST (3:30 a.m. IST) today. The event will be broadcast on NASA Television and streamed on the agency's website.

Commentary will originate from the control room at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. Coverage will last about 35 minutes and include live interviews with GRAIL team members, said the NASA press release.

For NASA TV streaming video, schedule and downlink information, visit: http://www.nasa.gov/ntv. The coverage will also be streamed live on Ustream at: http://www.ustream.tv/nasajpl2. Viewers can also join the conversation on Twitter by following the hashtag #GRAIL.