Streaking through vacuum at a mile per second just above the cratered surface of the moon, two washing machine-size science probes that have completed their mission to map the lunar gravity field will slam into a mile-high mountainside Monday, bringing a successful $500 million mission to a kamikaze conclusion.
The twin probes, named Ebb and Flow in a student naming contest, have been flying in formation at extremely low altitude since January 1, 2012, mapping subtle changes in the moon's gravitational pull to gain insights into its internal structure.
With all of the mission's scientific objectives accomplished, the trajectories of both 440-pound satellites were fine tuned Friday to set up twin impacts on a rugged cliff near the moon's north pole that is part of the rim of a buried crater.
Ebb is expected to hit the mountain at 5:28 p.m. EST (GMT-5) Monday. Flow will follow suit about 30 seconds later, crashing some 25 miles away from its twin. And with that, the Gravity Recovery and Interior Laboratory -- GRAIL -- mission will come to an abrupt end.
"We are not expecting a big flash or a big explosion" Maria Zuber, GRAIL principal investigator at the Massachusetts Institute of Technology, told reporters last week. "These are two small spacecraft, we use the term apartment-size washer/dryer-size spacecraft with empty fuel tanks. So we are not expecting a flash visible from Earth."
But NASA's Lunar Reconnaissance Orbiter will be on the lookout for any signs of the crashes during subsequent passes over the region.
"We've had our share of challenges during this mission and always come through in flying colors, but nobody I know around here has ever flown into a moon mountain before," David Lehman, GRAIL project manager at the Jet Propulsion Laboratory, said in a statement. "It'll be a first for us, that's for sure."
Each spacecraft executed a rocket firing Friday using up most of the probes' dwindling propellant to ensure the twin spacecraft hit their target, well away from any U.S. or Russian "historic heritage" landing sites.
While the odds of accidentally hitting one of the legacy landers were extremely remote, mission managers ordered the targeted impact to make absolutely sure.
"In terms of the scientific measurements, we have achieved everything we could have possibly hoped for," Zuber said. "Frankly, in my wildest dreams, I could not have imagined that this mission would have gone any better than it has.
"But when you orbit at very low altitudes above a planetary body that has a very bumpy gravity field, you use a lot of fuel. And so the mission is going to come to an end."
Launched September 10, 2011, Ebb and Flow reached the moon at the end of the year with the second spacecraft slipping into orbit on New Year's Day. After maneuvers to put both spacecraft into exactly the same orbit, the probes flew in close formation, constantly sending timed radio signals back and forth to precisely measure the distance between them.
The initial phases of the mission were carried out at an average altitude of 40 miles above the lunar surface. After a break over the summer due to solar power constraints, mapping resumed in August at an average altitude of just 14 miles. On December 6, a final set of observations was carried out at an altitude of just 6.8 miles above the surface.
Sailing over buried mass concentrations, craters, mountain ranges, basins and other geologic features, the satellites' velocity changed ever so slightly, one after the other, due to subtle gravitational differences. The ranging system was accurate enough to detect differences of as little as one micron, or the width of a red blood cell.
By carefully analyzing those changes, scientists have been able to map out the gravity field in unprecedented detail, shedding new light on the moon's evolution and, by extension, the evolution of Earth and other terrestrial worlds.
"GRAIL has produced the highest resolution, highest quality gravity field for any planet in the solar system, including Earth," Zuber said. "One of the major results that we found is evidence that the lunar crust is much thinner than we had believed before."
She said the data indicated "a couple of the large impact basins probably excavated the moon's mantle, which is very useful in terms of trying to understand the composition of the moon as well as the Earth. We actually think the Earth's mantle has a similar composition."
Another perhaps not-so-surprising result: the heavily cratered surface of the moon is extremely fractured and broken up by countless impacts.
"We found evidence that the shallow subsurface of the moon is largely pulverized, the crust of the moon has a very high average porosity indicative of the fact that it's been broken up by impacts," Zuber said. "And there is evidence that fracturing extends maybe several tens of kilometers possibly into the upper mantle."
The findings illustrate the role of "impact bombardment" on the evolution of early planetary crusts, Zuber said, including those of Earth and Mars.
"With Mars, there (are) a lot of questions about where did the water that we think was on the surface of Mars go? 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 the surface could have found its way deep into the crust of a planet."
Details about the moon's deep interior are expected to be announced after additional data analysis.