NASA’s fuel-depleted Gravity Recovery and Interior Laboratory (Grail) A and B lunar mission probes slammed into a mountain near Goldschmidt crater at the Moon’s North Pole late Dec. 17, ending a one-year mission.

The carefully targeted impact of the two washing machine-sized spacecraft, renamed Ebb and Flow, occurred on schedule, with Ebb striking first at 5:28 p.m. EST, and Flow striking 32 sec. later. The spacecraft impacted the Moon at 3,800 mph.

NASA’s Jet Propulsion Laboratory choreographed the mission’s finale to keep the two spacecraft circling in low polar orbits for as long as possible to both stretch out high-resolution gravity field data gathering and to prevent even the slightest possibility of impacts at one of the Moon’s historic U.S. Apollo and Soviet-era Luna mission landing sites. In all, there were 23 heritage sites to avoid.

Ebb and Flow approached the North Pole massif from the south, skimming just above the lunar terrain along trajectories of 1 to 1 1/2 deg. to avoid overshooting their unnamed 2 km-high (1.25 mi.-high) target, said David Lehman, the Jet Propulsion Laboratory’s Grail project manager.

NASA’s Lunar Reconnaissance Orbiter, which surveyed the target mountain ridge ahead of the impact, will make a follow-up verification pass in about two weeks to search for evidence that water or other volatiles were tossed up by the two impacts.

Launched Sept. 10, 2011, Ebb and Flow maneuvered into orbit on Dec. 31, 2011 and Jan. 1, 2012 respectively, flying in formation to generate high-resolution maps of the Moon’s lumpy gravity field. The mission cost was $500 million.

“The Grail team has done a great job,” Lehman said. “We’ve done it on schedule and under budget.”

Data from the prime mission between March and May has confirmed theories suggesting the Moon’s origins are from a collision between a Mars-sized object and the Earth, and that rocky planets of the inner Solar System were heavily bombarded by celestial debris during the formative era.

“It’s improved our knowledge of the Moon by orders of magnitude,” JPL Director Charles Elachi said of Grail.