Twin spacecraft that mapped the gravity field of the Moon with unprecedented precision have succumbed to the very force they were made to measure.
Ebb and Flow, the two probes that make up NASA’s GRAIL, the Gravity Recovery and Interior Laboratory, crashed one after the other into the Moon at 5:28 p.m. Eastern Standard Time on 17 December — an end that mission planners always had in mind — after a mind-blowingly successful flight that has included producing the first ultra-high-resolution picture of the Moon’s gravitational field (right).
The probes, which flew in tandem exchanging radio signals in lunar orbits that were minutely but measurably perturbed by variations in the Moon’s density, were conceived of by Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology in Cambridge. Their scientific findings so far include the result that the Moon’s crust is thinner than thought during the Apollo era, and that some impact craters thought to exist from lower-resolution maps are not there. The GRAIL researchers still needs to carry out the more detailed data analysis needed to map out the Moon’s more mysterious core.