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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 13 Dec 2012 11:31:22 -0800 (PST)
Message-ID: <201212131931.qBDJVMi9014879_at_zagami.jpl.nasa.gov>

Dec. 13, 2012

Dwayne Brown
Headquarters, Washington
dwayne.c.brown at nasa.gov

D.C. Agle
Jet Propulsion Laboratory, Pasadena, Calif.
agle at jpl.nasa.gov

Sarah McDonnell
Massachusetts Institute of Technology, Cambridge, Mass.
s_mcd at mit.edu

RELEASE: 12-434


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

"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

"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:


Received on Thu 13 Dec 2012 02:31:22 PM PST

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