[meteorite-list] Twin GRAIL Probes Prepare for Dec. 17 Mission-Ending Moon Impact

[Ron Baalke]

Dec. 13, 2012

Dwayne Brown 
Headquarters, Washington 
202-358-1726 
dwayne.c.brown at nasa.gov 

D.C. Agle 
Jet Propulsion Laboratory, Pasadena, Calif. 
818-393-9011 
agle at jpl.nasa.gov 

Sarah McDonnell 
Massachusetts Institute of Technology, Cambridge, Mass. 
617-253-8923 
s_mcd at mit.edu 

RELEASE: 12-434

TWIN NASA PROBES PREPARE FOR DEC. 17 MISSION-ENDING MOON IMPACT

PASADENA, Calif. -- Twin lunar-orbiting NASA spacecraft that have 
allowed scientists to learn more about the internal structure and 
composition of the moon are being prepared for their controlled 
descent and impact on a mountain near the moon's north pole at about 
2:28 p.m. PST (5:28 p.m. EST) Monday, Dec. 17. 

Ebb and Flow, the Gravity Recovery and Interior Laboratory (GRAIL) 
mission probes, are being sent purposely into the lunar surface 
because their low orbit and low fuel levels preclude further 
scientific operations. The duo's successful prime and extended 
science missions generated the highest resolution gravity field map 
of any celestial body. The map will provide a better understanding of 
how Earth and other rocky planets in the solar system formed and 
evolved. 

"It is going to be difficult to say goodbye," said GRAIL principal 
investigator Maria Zuber of the Massachusetts Institute of Technology 
in Cambridge. "Our little robotic twins have been exemplary members 
of the GRAIL family, and planetary science has advanced in a major 
way because of their contributions." 

The mountain where the two spacecraft will make contact is located 
near a crater named Goldschmidt. Both spacecraft have been flying in 
formation around the moon since Jan. 1, 2012. They were named by 
elementary school students in Bozeman, Mont., who won a contest. The 
first probe to reach the moon, Ebb, also will be the first to go 
down, at 2:28:40 p.m. Flow will follow Ebb about 20 seconds later. 

Both spacecraft will hit the surface at 3,760 mph (1.7 kilometers per 
second). No imagery of the impact is expected because the region will 
be in shadow at the time. 

Ebb and Flow will conduct one final experiment before their mission 
ends. They will fire their main engines until their propellant tanks 
are empty to determine precisely the amount of fuel remaining in 
their tanks. This will help NASA engineers validate fuel consumption 
computer models to improve predictions of fuel needs for future 
missions. 

"Our lunar twins may be in the twilight of their operational lives, 
but one thing is for sure, they are going down swinging," said GRAIL 
project manager David Lehman of NASA's Jet Propulsion Laboratory in 
Pasadena, Calif. "Even during the last half of their last orbit, we 
are going to do an engineering experiment that could help future 
missions operate more efficiently." 

Because the exact amount of fuel remaining aboard each spacecraft is 
unknown, mission navigators and engineers designed the depletion burn 
to allow the probes to descend gradually for several hours and skim 
the surface of the moon until the elevated terrain of the target 
mountain gets in their way. 

The burn that will change the spacecrafts' orbit and ensure the impact 
is scheduled to take place Friday morning. 

"Such a unique end-of-mission scenario requires extensive and detailed 
mission planning and navigation," said Lehman. "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. It'll be a first for us, that's for sure." 

During their prime mission, from March through May, Ebb and Flow 
collected data while orbiting at an average altitude of 34 miles (55 
kilometers). Their altitude was lowered to 14 miles (23 kilometers) 
for their extended mission, which began Aug. 30 and sometimes placed 
them within a few miles of the moon's tallest surface features. 

JPL manages the GRAIL mission for NASA's Science Mission Directorate 
in Washington. The mission is part of the Discovery Program managed 
at NASA's Marshall Space Flight Center in Huntsville, Ala. Lockheed 
Martin Space Systems in Denver built the spacecraft. 

For more information about GRAIL, visit: 

http://www.nasa.gov/grail 

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