Ebb And Flow
An artist's depiction of Ebb and Flow, the twin spacecraft involved in NASA's Grail mission NASA/JPL-Caltech/MIT

Two NASA probes are about to crash into the moon, but don't worry – that's just part of their mission.

Ebb and Flow are two spacecraft sent as part of the Gravity Recovery and Interior Laboratory mission, known as Grail. This scientific expedition aims to figure out the interior structure of the moon by mapping its gravitational field in precise detail. The pair have been orbiting the moon in tandem since January but are low on fuel and fated to fall.

"It is going to be difficult to say goodbye," Grail investigator and Massachusetts Institute of Technology researcher Maria Zuber said in a statement. "Our little robotic twins have been exemplary members of the Grail family, and planetary science has advanced in a major way because of their contributions."

Over the course of their mission, Ebb and Flow have helped scientists create the highest-resolution gravity field map of any celestial object. This map will help researchers understand the formation and evolution of rocky planets throughout the solar system, including Earth.

Ebb will be the first of the pair to hit the moon, at about 5:28 p.m. EST Monday, followed 20 seconds later by flow. Scientists have calculated that the probes will crash into an unnamed mountain, near a lunar crater dubbed Goldschmidt. It's sure to be a spectacular crash landing, since Ebb and Flow will be falling at more than 3,700 mph. Unfortunately, since the crash site will be in shadow when the two probes land, we won't be able to see it.

On their last day of operation, Ebb and Flow are performing one last scientific maneuver. The two probes will rev up their main engines and empty out their fuel tanks, which will help NASA check its predictions for their fuel consumption rates. This final engineering experiment will help NASA scientists better predict how much fuel future space probes will need.

The deliberate crash landing, in addition to helping NASA scientists calculate fuel expenditures, is also a kind of insurance. The space agency didn't want to entertain even a small risk that Ebb or Flow could hit any of the historic landmarks on the moon: the landing sites for Apollo missions and Russia's Luna probes, which are among the first man-made objects to reach the moon.

On Friday morning, Ebb and Flow briefly gunned their engines to modify their orbits, sealing their fate. Since then, each pass over the lunar surface has brought them closer to their final destination.

"Our lunar twins may be in the twilight of their operational lives, but one thing is for sure: They are going down swinging," Grail project manager David Lehman of NASA's Jet Propulsion Laboratory said in a statement.