NASA Twin Spacecraft On Final Approach For Moon Orbit

PASADENA, Calif. -- NASA's twin spacecraft to study the moon from
crust to core are nearing their New Year's Eve and New Year's Day
main-engine burns to place the duo in lunar orbit.

Named Gravity Recovery And Interior Laboratory (GRAIL), the spacecraft
are scheduled to be placed in orbit beginning at 1:21 p.m. PST (4:21
p.m. EST) for GRAIL-A on Dec. 31, and 2:05 p.m. PST (5:05 p.m. EST)
on Jan. 1 for GRAIL-B.

"Our team may not get to partake in a traditional New Year's
celebration, but I expect seeing our two spacecraft safely in lunar
orbit should give us all the excitement and feeling of euphoria
anyone in this line of work would ever need," said David Lehman,
project manager for GRAIL at NASA's Jet Propulsion Laboratory (JPL)
in Pasadena, Calif.

The distance from Earth to the moon is approximately 250,000 miles
(402,336 kilometers). NASA's Apollo crews took about three days to
travel to the moon. Launched from Cape Canaveral Air Force Station
Sept. 10, 2011, the GRAIL spacecraft are taking about 30 times that
long and covering more than 2.5 million miles (4 million kilometers)
to get there.

This low-energy, long-duration trajectory has given mission planners
and controllers more time to assess the spacecraft's health. The path
also allowed a vital component of the spacecraft's single science
instrument, the Ultra Stable Oscillator, to be continuously powered
for several months. This will allow it to reach a stable operating
temperature long before it begins making science measurements in
lunar orbit.

"This mission will rewrite the textbooks on the evolution of the
moon," said Maria Zuber, GRAIL principal investigator from the
Massachusetts Institute of Technology (MIT) in Cambridge. "Our two
spacecraft are operating so well during their journey that we have
performed a full test of our science instrument and confirmed the
performance required to meet our science objectives."

As of Dec. 28, GRAIL-A is 65,860 miles (106,000 kilometers) from the
moon and closing at a speed of 745 mph (1,200 kph). GRAIL-B is 79,540
miles (128,000 kilometers) from the moon and closing at a speed of
763 mph (1,228 kph).

During their final approaches to the moon, both orbiters move toward
it from the south, flying nearly over the lunar south pole. The lunar
orbit insertion burn for GRAIL-A will take approximately 40 minutes
and change the spacecraft's velocity by about 427 mph (688 kph).
GRAIL-B's insertion burn 25 hours later will last about 39 minutes
and is expected to change the probe's velocity by 430 mph (691 kph).

The insertion maneuvers will place each orbiter into a near-polar,
elliptical orbit with a period of 11.5 hours. Over the following
weeks, the GRAIL team will execute a series of burns with each
spacecraft to reduce their orbital period from 11.5 hours down to
just under two hours. At the start of the science phase in March
2012, the two GRAILs will be in a near-polar, near-circular orbit
with an altitude of about 34 miles (55 kilometers).

When science collection begins, the spacecraft will transmit radio
signals precisely defining the distance between them as they orbit
the moon. As they fly over areas of greater and lesser gravity,
caused both by visible features such as mountains and craters and by
masses hidden beneath the lunar surface. they will move slightly
toward and away from each other.

An instrument aboard each spacecraft
will measure the changes in their relative velocity very precisely,
and scientists will translate this information into a high-resolution
map of the Moon's gravitational field. The data will allow mission
scientists to understand what goes on below the surface. This
information will increase our knowledge of how Earth and its rocky
neighbors in the inner solar system developed into the diverse worlds
we see today.

JPL manages the GRAIL mission. MIT is home to the mission's principal
investigator, Maria Zuber. The GRAIL 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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