On September 10, 2011 two small spacecraft, GRAIL A and GRAIL B, were launched aboard a single Delta II rocket. The two spacecraft were on a mission to map the gravitational field of our moon.

Mapping the gravitational field of the moon is no small feat. Both crafts had to approach the moon at speeds that were only a fraction of that used in the Apollo missions. Grail A and B cruised above the lunar surface at a distance of 50 km while maintaining a tightly controlled 175-225 km spacing between one another. Over the course of several passes the two craft were able to detect gravitational variations on the lunar surface as they drifted incrementally closer to or further from one another.

Although mapping the gravitational variance across the lunar surface was one of the GRAIL mission’s chief responsibilities, it also had other goals.  According to Wikipedia these other goals were:

·         Map the structure of the lunar crust and lithosphere

·         Understand the asymmetric thermal evolution of the Moon

·         Determine the subsurface structure of impact basins and the origin of lunar mascons

·         Ascertain the temporal evolution of crustal brecciation and magmatism

·         Place limits on the size of the Moon's inner core

While the two spacecraft have performed admirably, today will mark the end of their mission… But oh what an end it will be.

Sometime today the first of the two craft will make its final decent toward the lunar surface where it will crash into the side of a unnamed lunar mountain at 2:28:40 pm PT. Twenty seconds later, it’s partner craft will meet the same fate.

But why would NASA want to crash these twin spacecraft?

Aside from the fact that they were running out of fuel, NASA has a vested interest in preserving lunar heritage sites (Apollo landers, etc.) that dot the moon’s surface.  Scientists also think that the impact could tell them a little bit more about what lies just beneath the lunar crust.  However, due to the crafts size, scientists are doubtful they’ll discover much.

Creating a gravitational map may seem to be an extravagant goal, but NASA scientists believe their mission will prove critical to further lunar exploration. A gravity map of the moon can act as a hi-res navigational tool for future missions.  According to GRAIL’s principle investigator Maria Zuber, “If there was a particular boulder you wanted to target, you could land at it now. This actually decreases the cost of future missions to the moon… Although this is not why we did the mission, it’s an additional benefit.”

Watch a Video of the Grail Mission’s Crash Trajectory



Images and Video Courtesy of NASA and Wikipedia

 

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