[meteorite-list] Lunar Reconnaissance Orbiter Sees GRAIL's Explosive Farewell

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 19 Mar 2013 11:41:35 -0700 (PDT)
Message-ID: <201303191841.r2JIfZCK025365_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?release=2013-103

Lunar Reconnaissance Orbiter Sees GRAIL's Explosive Farewell
Jet Propulsion Laboratory
March 19, 2013

Many spacecraft just fade away, drifting silently through space after
their mission is over, but not GRAIL. NASA's twin GRAIL (Gravity
Recovery and Interior Laboratory) spacecraft went out in a blaze of
glory on Dec. 17, 2012, when they were intentionally crashed into a
mountain near the moon's north pole.

The successful mission to study the moon's interior took the plunge to
get one last bit of science: with the spacecraft kicking up a cloud of
dust and gas with each impact, researchers hoped to discover more about
the moon's composition. However, with the moon about 380,000 kilometers
(over 236,000 miles) away from Earth, the impact plumes would be
difficult to observe from here. Fortunately, GRAIL had company. NASA's
Lunar Reconnaissance Orbiter (LRO) is orbiting the moon as well, busily
making high-resolution maps of the lunar surface. With just three weeks
notice, the LRO team scrambled to get their orbiter in the right place
at the right time to witness GRAIL's fiery finale.

"We were informed by the GRAIL team about three weeks prior to the
impact exactly where the impact site would be," said LRO Project
Scientist John Keller of NASA's Goddard Space Flight Center in
Greenbelt, Md. "The GRAIL team's focus was on obtaining the
highest-resolution gravity measurements possible from the last few
orbits of the GRAIL spacecraft, which led to uncertainty in the ultimate
impact site until relatively late."

LRO was only about 100 miles (160 kilometers) from the lunar surface at
the time of the impact, and variations in gravity from massive features
like lunar mountains tugged on the spacecraft, altering its orbit.

The site was in shadow at the time of the impact, so the LRO team had to
wait until the plumes rose high enough to be in sunlight before making
the observation. The Lyman Alpha Mapping Project (LAMP), an ultraviolet
imaging spectrograph on board the spacecraft, saw mercury and
enhancements of atomic hydrogen in the plume.

"The mercury observation is consistent with what the LRO team saw from
the LCROSS impact in October 2009," said Keller. "LCROSS (Lunar CRater
Observation and Sensing Satellite) saw significant amounts of mercury,
but the LCROSS site was at the bottom of the moon's Cabeus crater, which
hasn't seen sunlight for more than a billion years and is therefore
extremely cold."

LRO's Lunar Reconnaissance Orbiter Camera was able to make an image of
the craters from the GRAIL impacts despite their relatively small size.

The two spacecraft were relatively small -- cubes about the size of a
washing machine with a mass of about 200 kilograms (440 pounds) each at
the time of impact. The spacecraft were traveling about 3,800 mph (6,100
kilometers per hour) when they hit the surface.

"Both craters are relatively small, perhaps 4 to 6 meters (about 13 to
20 feet) in diameter and both have faint, dark, ejecta patterns, which
is unusual," said Mark Robinson, LROC principal investigator at Arizona
State University's School of Earth and Space Sciences, Tempe, Ariz.
"Fresh impact craters on the moon are typically bright, but these may be
dark due to spacecraft material being mixed with the ejecta."

"Both impact sites lie on the southern slope of an unnamed massif
[mountain] that lies south of the crater Mouchez and northeast of the
crater Philolaus," said Robinson. "The massif stands as much as 2,500
meters [about 8,202 feet] above the surrounding plains. The impact sites
are at an elevation of about 700 meters [around 2,296 feet] and 1,000
meters [3,281 feet], respectively, about 500 to 800 meters
[approximately 1,640 to 2,625 feet] below the summit. The two impact
craters are about 2,200 meters [roughly 7,218 feet] apart. GRAIL B
[renamed Flow] impacted about 30 seconds after GRAIL A [Ebb] at a site
to the west and north of GRAIL A."

Lunar Reconnaissance Orbiter complemented the GRAIL mission in other
ways as well. LRO's Diviner lunar radiometer observed the impact site
and confirmed that the amount of heating of the surface there by the
relatively small GRAIL spacecraft was within expectations. LRO's Lunar
Orbiter Laser Altimeter (LOLA) instrument bounced laser pulses off the
surface to build up a precise map of the lunar terrain, including the
three-dimensional structure of features like mountains and craters.

"Combining the LRO LOLA topography map with GRAIL's gravity map yields
some very interesting results," said Keller. "You expect that areas with
mountains will have a little stronger gravity, while features like
craters will have a little less. However, when you subtract out the
topography, you get another map that reveals gravity differences that
are not tied to the surface. It gives insight into structures deeper in
the moon's interior."

JPL manages the GRAIL mission for NASA's Science Mission Directorate in
Washington. GRAIL 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 the mission's press kit and other information about GRAIL, visit:
http://www.nasa.gov/grail . You can follow JPL News on Facebook at:
http://www.facebook.com/nasajpl and on Twitter at:
http://www.twitter.com/nasajpl .

The research was funded by the LRO mission, currently under NASA's
Science Mission Directorate at NASA Headquarters in Washington. LRO is
managed by NASA's Goddard Space Flight Center in Greenbelt, Md.

Images are posted at:
http://www.nasa.gov/mission_pages/LRO/news/grail-results.html

DC Agle (818) 393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle at jpl.nasa.gov

2013-103
Received on Tue 19 Mar 2013 02:41:35 PM PDT