Two NASA moon probes will end their gravity-mapping mission in spectacular fashion Monday (Dec. 17), crashing intentionally into a cliff near the lunar north pole.

The twin Grail spacecraft, known as Ebb and Flow, will slam into the raised rim of a moon crater at 5:28 p.m. EST (2258 GMT) Monday, mission team members said today (Dec. 13). The probes will impact about 20 seconds apart, with each traveling at 3,760 mph (6,050 kph) or so.

The crash zone is far from any area where previous moon missions such as NASA's Apollo efforts touched down, so Grail's final moments won't endanger sites of historical importance, officials said.

A successful mission comes to an end

The $496 million Grail mission short for Gravity Recovery and Interior Laboratory launched in September 2011, and Ebb and Flow arrived in lunar orbit about three months later. The two probes have been zipping around the moon ever since, mapping its gravity field in unprecedented detail.

"Grail has produced the highest-resolution, highest-quality gravity field for any planet in the solar system, including Earth," Grail principal investigator Maria Zuber, of MIT, told reporters today. [Video: Ebb and Flow's Impending Crash]

That map has revealed an incredibly pulverized lunar crust, she added, suggesting that the moon, Earth, Mars, Mercury and Venus were pounded by long-ago impacts far more violently than previously thought.

Grail's primary science mission ran from March to May, during which the spacecraft zipped around the moon at an average altitude of 34 miles (55 kilometers). Ebb and Flow dropped down to about 14 miles (23 km) for an extended phase, which wraps up in the next few days and should make the gravity map even better.

There will be no more extended missions, because Ebb and Flow are almost out of fuel. The spacecraft will crash into the lunar surface eventually, so the Grail team is bringing them down in a controlled fashion. (An uncontrolled crash would pose an eight-in-a-million risk of hitting a heritage site, researchers said.)

"This is all according to plan," Zuber said.

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Twin NASA Probes Readying for Monday Moon Crash