After dancing around the moon for months, the twin satellites
nicknamed Ebb and Flow performed their final number, crashing
into the same peak near the lunar north pole and bringing the
GRAIL mission to a close. A cheer went up in the control room at
the Jet Propulsion Laboratory in La Cañada Flintridge as Ebb
slammed into the mountain about 2:28 p.m. Monday, followed about
half a minute later by Flow. The probes were traveling at nearly
3,800 mph and landed an estimated 25 miles apart. Even as the
team clapped and shook hands in the control room, GRAIL project
manager David Lehman acknowledged that the moment was
bittersweet. "Today I brought my handkerchief, just in case," he
said. The GRAIL mission spacecraft, each the size of a compact
washing machine, were launched in September 2011 to study every
bump and pothole in the moon's gravitational field. "We have
essentially collected more than 99.99%" of the data the team had
set out to collect, said Massachusetts Institute of Technology
geophysicist Maria Zu