Twin moon-mapping probes have ended their year-long mission by smashing themselves into a lunar mountain.
Named Ebb and Flow, the Gravity Recovery And Interior Laboratory (GRAIL) spacecraft - each the size of a washing machine - had been orbiting the moon at about the altitude of a commercial airliner, making detailed maps of the surface gravity.
The probes sped up slightly as they encountered stronger gravity from denser regions and slowed down as they flew over less-dense areas.
By precisely measuring the distance between the two probes, scientists discovered that the moon's crust was thinner than expected and that the impacts which created the familiar texture of craters pock-marking the surface did even more damage underground.
"Ebb and Flow have removed a veil from the moon," said lead researcher Maria Zuber, of the Massachusetts Institute of Technology. "Removing that veil will help us understand the formation of the moon."
Robert Massey of Britain's Royal Astronomical Society told Al Jazeera that the probes allowed for more detailed study of the moon than ever before.
"They found evidence of ancient lava flows forming dykes under the surface," he said. "This kind of thing that it just is not possible to see just by taking photographs."
Running out of fuel and edging closer to the lunar surface, the probes were commanded to smash themselves - at a speed of more than 6,000kph - into a mountain near the moon's north pole, avoiding a chance encounter with any spacecraft or other relics left during previous expeditions.
"We do feel the angst about the end of the mission," said Charles Elachi, director of NASA's Jet Propulsion Laboratory in Pasadena, California, which oversaw the probes' mission and ultimate demise on Monday night. "On the other hand, it is a celebration because this mission has accomplished tremendous science."
The study has provided crucial data on the moon's thermal history and surface, which will help astrophysicists better understand the formation of early atmospheres and the deposit of volatile compounds on astral bodies.
It has also brought advanced technical prowess into many US classrooms - with students able to operate some of the spacecraft's cameras.
The probes' final resting place, which was shrouded in shadow at the time of impact, meaning no pictures of the crash could be taken, was named after the first US woman in space, Sally Ride. She also orchestrated GRAIL's educational outreach programme. Ride died of pancreatic cancer in July, two months after the GRAIL twins completed their primary mission.
The GRAIL satellites join the remnants of nearly 80 spacecraft which have either landed or crashed on the moon since 1959.