NASA's GRAIL-A Spacecraft 24 Hours Away From Moon

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

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

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

Caroline McCall 617-253-1682
Massachusetts Institute of Technology, Cambridge
cmcall5@mit.edu

News release: 2011-397 December 30, 2011

NASA's GRAIL-A Spacecraft 24 Hours Away From Moon

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-397&cid=release_2011-397

PASADENA, Calif. -- NASA's Gravity Recovery And Interior Laboratory (GRAIL)-A spacecraft is within
24 hours of its insertion burn that will place it into lunar orbit. At the time the spacecraft crossed the
milestone at 1:21 p.m. PST today (4:21 p.m. EST), the spacecraft was 30,758 miles (49,500
kilometers) from the moon.

Launched aboard the same rocket on Sept. 10, 2011, GRAIL-A's mirror twin, GRAIL-B, is also
closing the gap between itself and the moon. GRAIL-B is scheduled to perform its lunar orbit insertion
burn on New Year's Day (Jan. 1) at 2:05 p.m. PST (5:05 p.m. EST).

As they close in on the moon, both orbiters move toward the moon from the south, flying nearly directly
over the lunar south pole. The lunar orbit insertion burn for GRAIL-A will take approximately 40
minutes to complete and change the spacecraft's velocity by about 427 mph (687 kph). GRAIL-B's
insertion burn – occurring 25 hours later -- will last about 39 minutes and is expected to change its
velocity by 430 mph (692 kph).

The insertion maneuvers will place each orbiter into a near-polar, elliptical orbit with an orbital period of
11.5 hours. Over the following weeks, the GRAIL team will execute a series of burns with each
spacecraft to reduce their period 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).

During the science phase, the moon will rotate three times underneath the GRAIL orbit. The collection
of gravity data over one complete rotation (27.3 days) is referred to as a Mapping Cycle. When science
collection begins, the spacecraft will transmit radio signals precisely defining the distance between them
as they orbit the moon in formation. Regional gravitational differences on the moon are expected to
expand and contract that distance. GRAIL scientists will use these accurate measurements to define the
moon's gravity field. The data will allow mission scientists to understand what goes on below the surface
of our natural satellite. This information will help us learn more about how the moon, Earth and other
terrestrial planets formed.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the GRAIL mission. The Massachusetts
Institute of Technology, Cambridge, 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, Denver, built the spacecraft. JPL is a division of the
California Institute of Technology in Pasadena.

More information about GRAIL is online at: http://www.nasa.gov/grail and http://grail.nasa.gov .

The GRAIL press kit can be found online at:
http://www.jpl.nasa.gov/news/press_kits/graiLaunch.pdf .

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