NASA’s GRAIL mission to map the gravity field of the Moon has enjoyed success beyond any expectations, but all good things must come to an end.
“In my wildest dreams I could not have imagined that this mission would have gone any better than it has,” Dr. Maria Zuber, the Principal Investigator for GRAIL, reported during a teleconference about the end of the GRAIL mission, held on December 12.
But even the most successful missions must come to an end, and GRAIL is no different. NASA reports that the two washing machine-sized craft are running low on fuel, a situation that was expected. The GRAIL spacecraft, during their extended mission, flew over the Moon at an average altitude of only 11 km. Dr. Zuber explained that any time a spacecraft assumes a low orbit of a planetary body with a “bumpy” gravity field it will use a lot of fuel to maintain that orbit.
But now that the GRAIL spacecraft, named Ebb and Flow, are running out of fuel and cannot maintain their orbit, they will inevitably fall and impact the lunar surface. The GRAIL team has decided to do this in a controlled fashion, by using the spacecrafts’ engines to direct their course to impact a mountain near the Moon’s north pole.
But even in death, Ebb and Flow will serve the cause of science. The reason the GRAIL team targeted an elevated feature was so that the Lunar Reconnaissance Orbiter could examine the crash site. Observations of that site could reveal important information about the mechanical properties of the mountain, which is really just a part of the rim of an ancient crater near the Moon’s north pole. The GRAIL team also noted the possibility, though only very slight, that the impact may liberate some volatile material from inside the lunar regolith. This would be an extraordinary find, because current thinking about the Moon says that volatiles, like water, could only be found in permanently shadowed regions of certain craters.
The data gathered by Ebb and Flow during their primary mission has already been released, and the GRAIL team has submitted three papers to Science Magazine’s Science Express publications. In these papers, the team reports that the lunar crust is far thinner than the previous estimate of about 50 km average thickness. GRAIL’s estimate for the average thickness ranges between 34 and 43 km. Even more intriguing, the team reports that some of the larger impact features, like the Mare Orientalis, might have penetrated through the entire crust and exposed mantle material.
And Dr. Zubin notes that the science returns from GRAIL aren’t applicable only to the Moon. Dr. Zubin suggested that the constant rain of impactors has broken up the lunar crust, leaving fractures that extend for tens of kilometers below the surface. If a similar process occurred on Mars, the water that, perhaps, once persisted on the surface might have sunk down into the deep cracks, concealing it from our past and present searches.
Even more analysis will be possible when the data from the extended mission is released in May of next year.
As a side note, David Lehman, the GRAIL project manager at JPL, notes that GRAIL underran its budget by $8-9 million.
The final science return by GRAIL will occur at about 5:28 a.m. ET on December 17, when Ebb crashes into the crater rim, followed 30 seconds later by Flow. The two spacecraft will impact the crater rim at about 1.7 km/s. Since the spacecraft are very small, they will not leave large craters. That area of the Moon will be in shadow at the time, but the LRO spacecraft will overfly the area later and attempt to pinpoint the impact sites.