Ultima missione per le sonde gemelle della missione Grail: EBB e FLOW, della Nasa, che dopo mesi trascorsi nell’orbita lunare per permettere ai ricercatori di conoscere meglio la sua struttura, sono arrivate al termine della loro attività e si preparano all’impatto sul suolo della Luna, previsto alle 23:28 di lunedì 17 dicembre su una montagna vicino al Polo Nord lunare, nei pressi del cratere Goldschimdt.
Le due sonde hanno permesso di realizzare la mappa del campo gravitazionale di un corpo celeste con la più alta risoluzione mai ottenuta. La mappa fornirà una migliore comprensione su come si sono formati ed evoluti la Terra e gli altri pianeti rocciosi del Sistema Solare. 

"La NASA ha escluso qualsiasi possibilità che le sonde colpiscano la superficie vicino ad uno dei siti storici di esplorazione lunare, come siti di atterraggio delle missioni Apollo o dove le sonde russe hanno toccato terra" ha detto David Lehman, GRAIL project manager della NASA Jet Propulsion Laboratory a Pasadena, in California "I nostri navigatori hanno calcolato le probabilità prima di questa manovra".
FONTE: NASA
The maneuvers began at 7:07 a.m. PST (10:07 a.m. EST) today when the Ebb spacecraft fired its main engines for 55.8 seconds, changing its orbital velocity by 10.3 mph (4.6 meters per second). Sixteen seconds later, still at 7:07 a.m. PST, the Flow spacecraft began its maneuver, executing a burn 55.4 seconds in duration with a resulting change in orbital velocity of 10.3 mph (4.6 meters per second). The spacecraft were named Ebb and Flow by elementary school students in Bozeman, Mont., who won a nationwide contest.
Ebb and Flow are being sent purposely into the lunar surface because their low orbit and low fuel levels preclude further scientific operations.
"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
The unnamed mountain where the two spacecraft will make contact is on the moon’s nearside, near its north pole, in the vicinity of a crater named Goldschmidt. Both spacecraft will hit the surface at3,760 mph (1.7 kilometers per second). No imagery of the impact is expected, because the region will be in shadow at the time.
Both spacecraft have been orbiting the moon since Jan. 1, 2012. The duo’s successful primary mission yielded the highest-resolution gravity field map of any celestial body. Future gravity field models developed from data collected during the extended mission will be of even higher resolution. The map will provide a better understanding of how the moon, Earth and other rocky planets in the solar system formed and evolved.
JPL manages the GRAIL mission for NASA’s Science Mission Directorate in Washington. The mission is part of the Discovery Program managed at NASA’s Marshall Space Flight Center in Huntsville, Ala. Lockheed Martin Space Systems in Denver built the spacecraft.
For more information about GRAIL, visit: http://www.nasa.gov/grail