Spacecraft plan to crash into Moon
After nearly a year circling the Moon, Nasa's Ebb and Flow will meet their demise when they crash - on purpose - into the lunar surface.
But do not expect to see celestial fireworks - next week's impact near the Moon's north pole by the washing machine-sized spacecraft will not carve a gaping crater or kick up a lot of debris. And it will be dark when it happens.
"We are not expecting a big flash or a big explosion" that will be visible from Earth, said mission chief scientist Maria Zuber, of the Massachusetts Institute of Technology.
Still, it will mark a violent end to a successful mission that has produced the most high-resolution gravity maps of Earth's closest neighbour. On Friday, engineers will turn off the science instruments in preparation for Monday's finale.
Previous unmanned trips to the Moon have studied its lumpy gravitational field, but Ebb and Flow are the first ones dedicated to this goal. Since entering orbit over new year's weekend, the formation-flying spacecraft have peered past the craggy surface into the interior.
Initially, the spacecraft flew about 35 miles above the surface and later dropped down to 14 miles. About an hour before Monday's impact, they will fire their engines until they run out of fuel and slam at 3,800 mph into a predetermined target - a mountain near the north pole that is far away from the Apollo landing sites.
Ebb will hit first followed by Flow 20 seconds later. Though the drama will not be visible from Earth, the Lunar Reconnaissance Orbiter will fly over the crash site afterwards and attempt to spot them.
The last time Nasa aimed at the Moon was in 2009. The world watched through telescopes and over the internet as a spacecraft and its booster rocket smashed into a permanently shadowed crater - a one-two punch that fizzled when spectators saw little more than a fuzzy white flash.
The mission's end will also mark the close of a student campaign that used cameras aboard the spacecraft to image lunar targets including on the Moon's far side. The MoonKAM project was spearheaded by a science education company founded by Sally Ride, the first American woman in space. Ms Ride died of pancreatic cancer in July at age 61.
Even after Ebb and Flow complete their mission, scientists will continue to pore over the bounty of data they collected. Among their findings so far: The Moon is more beat up than previously imagined. The crust is much thinner than thought. And there is no evidence that Earth once had two moons that collided to form the one we see in the night sky.