Twin GRAIL probes Ebb and Flow finish their lunar mission by crashing into the moon

Cape Canaveral, FL (WTSP/Florida Today) -- Twin NASA spacecraft deliberately crashed into a mountain on the moon Monday afternoon, ending a successful mission that is shedding light on the evolution of the solar system's terrestrial planets.

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

So the spacecraft were directed on a shallow dive into a mile-high mountain near the moon's north pole - a controlled crash-landing that occurred in the lunar night.

NASA made sure the the spacecraft would be brought down far away from any historical site on the moon.

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

"We are not expecting a big splash or a big explosion, OK?" said Maria Zuber, a professor of geophysics at Massachusetts Institute of Technology 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."

The last time the space agency intentionally fired manmade objects at the moon was in 2009, but it was for the sake of science. The crash was a public relations dud - spectators barely saw a faint flash - but the experiment proved that the moon contained water.

NASA's Lunar Reconnaissance Orbiter will zoom over the crash site not long after Monday's impact, but no imagery is expected because the mountainous region will be in shadow at the time.

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

"It's going to make a crater on the moon, and they are going to be completely blown apart at the time," said GRAIL project manager David Lehman of NASA'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.

The Ebb and Flow spacecraft blasted off on Sept. 10, 2011, aboard a United Launch Alliance Delta II rocket at Cape Canaveral Air Force Station's Complex 17.

It was an era-ending finale for the venerable Delta II, a workhorse rocket designed and developed after the 1986 Challenger accident to fly Navstar Global Positioning System (GPS) spacecraft into orbit. The GRAIL twins arrived 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 of 34 miles and trained scientific instruments on its surface, making the highest-fidelity gravitational maps of any rocky body in the solar system - including Earth.

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

What's more, the lunar surface - which has been bombarded by asteroids and comets for billions of years - is much more pulverized than previously thought.

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

And that finding could go a long way toward explaining one of the great mysteries of the solar system - where all the water that once was awash on the surface of Mars might have gone.

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

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

"And that observation provides additional support for the theory that the moon formed by a giant impact of a Mars-sized body into Earth," she said.

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