Twin GRAIL satellites aim for crash on moon mountain
BY STEPHEN CLARK
Posted: December 13, 2012
NASA will send two decommissioned probes to a crash landing on the moon Monday, punctuating a violent end for a mission which mapped the lunar gravity field with more precision than any other object in the solar system.
Because of its low impact angle, the satellites will probably leave features more resembling skid marks than a crater, said Maria Zuber, GRAIL's principal investigator from the Massachusetts Institute of Technology.
One probe will strike the moon at 5:28:40 p.m. EST (2248:40 GMT) Monday, followed about 20 seconds later by impact of the second satellite, according to NASA.
"We have achieved everything we could have possibly hoped for in terms of observations," Zuber said. "Frankly, in my wildest dreams, I could not have imagined this mission would go as well as it has."
The twin GRAIL spacecraft, launched in September 2011, entered orbit around the moon nearly one year ago. The probes started measuring the lunar gravity field in March.
After a mapping the moon from a 34-mile-high orbit in the first half of 2012, the GRAIL probes are now concluding an extended mission to obtain higher-resolution gravity data from an average altitude of 14 miles.
Since Dec. 6, the GRAIL satellites, nicknamed Ebb and Flow, have moved even closer to the moon, flying at a mean altitude of less than 10 miles and buzzing just a few miles above the moon's highest peaks, according to Zuber.
On Friday, thrusters on each satellite will fire to guide the spacecraft toward the unnamed mountain. The maneuver will also ensure the satellites avoid striking landing sites from the Apollo, Surveyor and Soviet space programs. Engineers calculated there was a 1-in-125,000 chance the satellites would hit one of the heritage landing sites, according to David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory.
The orbits of the GRAIL satellites are too unstable to keep the spacecraft flying without regular help from rocket thrusters. With the satellites low on propellant, the probes would impact the moon naturally, officials said.
Before impacting the moon Monday, the spacecraft will burn their rocket engines to empty their fuel tanks, an engineering experiment designed to refine techniques to estimate propellant quantities on satellites.
Most satellites have no way of directly measuring how much fuel is left in on-board tanks, and engineers can estimate how much propellant is inside GRAIL's reservoirs by the length of the depletion burn.
The mission worked by constantly measuring the distance between the two satellites as they orbited the moon. The tug of gravity from a surface or underground mass of rock registers in the range measurements between the spacecraft.
"Conceptually, it's exceedingly simple because we're just measuring the distance between two points," Zuber said.
The satellites measure range by bouncing radio signals between each other. An on-board processor transmits the radiometric data to the ground, where scientists use the information to create gravity maps.
Because variations in the moon's gravity are so small, the GRAIL satellites can detect changes in their range as little as the width of a human hair, according to Sami Asmar, GRAIL project scientist at JPL.
Scientists released the first results from GRAIL's primary mission in early December. The gravity maps show lava-filled dike features buried beneath the lunar soil, evidence the moon expanded in size in its early history.
"There is no record whatsoever of these features at the moon's surface," Zuber said. "If any record existed, it would have been wiped out by these early impacts. These dikes actually provide evidence for early expansion of the moon just after it formed."
GRAIL data also show the lunar crust is thinner than predicted, and its composition supports the theory the moon was created when a Mars-sized object collided with Earth long ago, expelling material which coalesced into the moon.
"We think that the moon formed in a giant impact with the earth, sending debris out into space that collected to form the moon," said Jeff Andrews-Hanna, GRAIL guest scientist from the Colorado School of Mines.
The gravity model also indicates some asteroid and comet impacts pierced the moon's crust and exposed the lunar mantle.
"One of the major results we found is evidence that the lunar crust is much thinner than we had believed before, and a couple of the large impact basins probably excavated the moon's mantle, which is very useful when trying to understand the composition of the moon's mantle," Zuber said.