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Cape Canaveral, FL (Florida Today) -- Twin NASA spacecraft will crashdeliberately into a mountain on the moon today, ending a successfulmission that is shedding light on the evolution of the solar system'sterrestrial planets.

Each about the sizeof compact washing machines, the Ebb and Flow spacecraft are low onfuel, and they've finished a $496 million mission aimed at making themost accurate gravitational map of the moon to date.

Sothe spacecraft will be directed on a shallow dive into a mile-highmountain near the moon's north pole - a controlled crash-landing thatwill occur in the lunar night.

Expected velocity at impact is 3,760 mph. But don't expect anything dramatic here on Earth.

"Weare not expecting a big splash or a big explosion, OK?" said MariaZuber, a professor of geophysics at Massachusetts Institute ofTechnology and the mission's principal scientific investigator.

"These are two small spacecraft ... with empty fuel tanks. So we are not expecting a flash that is visible from Earth."

NASA'sLunar Reconnaissance Orbiter will zoom over the crash site not longafter impact, but no imagery is expected because the mountainous regionwill be in shadow at the time.

Still, it will be a destructive end to NASA's Gravity Recovery and Interior Laboratory, or GRAIL, mission.

"It'sgoing to make a crater on the moon, and they are going to be completelyblown apart at the time," said GRAIL project manager David Lehman ofNASA's Jet Propulsion Laboratory in Pasadena, Calif.

You wouldn't want to be standing at the crash site.

"It would be like a washing machine coming in and landing on you, and it would be a very bad day for you," Lehman said.

TheEbb and Flow spacecraft blasted off on Sept. 10, 2011, aboard a UnitedLaunch Alliance Delta II rocket at Cape Canaveral Air Force Station'sComplex 17.

It wasan era-ending finale for the venerable Delta II, a workhorse rocketdesigned and developed after the 1986 Challenger accident to fly NavstarGlobal Positioning System (GPS) spacecraft into orbit. The GRAIL twinsarrived in lunar orbit three months later - on Dec. 31, 2011, and Jan.1, 2012.

Flying in formation, they zoomed around the moon at an altitude of34 miles and trained scientific instruments on its surface, making thehighest-fidelity gravitational maps of any rocky body in the solarsystem - including Earth.

Data from the spacecraft show the lunar crust is much thinner than scientists had believed before.

What'smore, the lunar surface - which has been bombarded by asteroids andcomets for billions of years - is much more pulverized than previouslythought.

"What wefound is just how broken up and fractured the crust of the moon is,"Zuber said. "There is evidence that fracturing extends maybe severaltens of kilometers into the upper mantle."

Andthat finding could go a long way toward explaining one of the greatmysteries of the solar system - where all the water that once was awashon the surface of Mars might have gone.

"Allplanetary crusts have been bombarded in a similar way. Earth has ...Marshas. With Mars, there's a lot of questions about where did the waterthat we think was on the surface of Mars go," Zuber said.

"Well,if a planetary crust is that fractured, these fractures provide apathway deep inside the planet, and it's very easy to envision now how apossible ocean at its surface could have found its way deep into thecrust of a planet."

One other key finding relates to the origin of the moon - where it came from; how it was created.

Zuber said instruments on GRAIL determined that the amount of aluminum in the crusts of the moon and Earth are nearly equal.

"Andthat observation provides additional support for the theory that themoon formed by a giant impact of a Mars-sized body into Earth," shesaid.

Bottom line: the moon is a chip off the old block.

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