Tonight NASA will be pushing twin lunar-orbiting spacecraft down towards our moon in order to crash them out with one final mission after nearly a full Earth-year's work. This final mission will have Gravity Recovery and Interior Laboratory (GRAIL) mission probes "Ebb" and "Flow" rammed purposefully into an unnamed moon mountain using up the remaining fuel reserves on both units. This final mission will help NASA engineers validate models outlining fuel consumption so as to improve future mission fuel needs.
The two craft still flying above the heavenly body as this article is published have reached a point at which their usefulness is no longer greater than their potential for giving us information on remaining fuel supplies. At the moment its also true that the exact amount of fuel that either craft has inside it is not known precisely, this knowledge being part of the endgame of the crash.
The image you see above is of the moon (surprise!) showing "Lunar Heritage Sites" as well as the final mile the GRAIL team will be flying along - starting down there at the South of the moon and crashing right up near the top, circled in red. Below you'll see just about as detailed a look as you're going to get of the impact site - the actual crash won't be visible as it'll be in relative dark as it happens. Images from NASA/GSFC.
The crashes will occur in order with Ebb reaching solid moon surface at approximately 2:28:40 p.m. PST. The craft known as Flow will reach the surface right around 20 seconds after Ebb has landed. Both craft have been flying "in formation" according to NASA since January 1st, 2012.
"Our lunar twins may be in the twilight of their operational lives, but one thing is for sure, they are going down swinging. Even during the last half of their last orbit, we are going to do an engineering experiment that could help future missions operate more efficiently." - GRAIL project manager David Lehman of NASA's Jet Propulsion Laboratory in Pasadena, California
The final descent that these craft will be making will have depletion burn of their fuel reserves in a way that makes them skim the surface of the moon until "the elevated terrain of the target mountain gets in their way." In other words, yes, they will literally be crashing into a moon mountain. Lehman continued, "We've had our share of challenges during this mission and always come through in flying colors, but nobody I know around here has ever flown into a moon mountain before. It'll be a first for us, that's for sure."
Above: Ebb and Flow points of impact on both of their friendly moon mountain resting places. Image from NASA/JPL-Caltech/MIT/GSFC.
The original set of tasks set forth for Ebb and Flow included capturing gravity field maps of the moon, they having generated the highest-resolution gravity field map of any celestial body to date. Congratulations, Ebb and Flow! You did well! Now it's time for you to crash into a moon mountain at 3,760 mph (1.7 kilometers per second) - have fun!