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| This still image shows the final flight path for NASA's twin spacecraft, which will impact the moon on Dec. 17, 2012, around 2:28 p.m. PST [Image credit: NASA/JPL-Caltech/GSFC/ASU] |
Pasadena, California: 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.