NASA's Moon Twins Going Their Own Way

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
NASA Headquarters, Washington
Dwayne.c.brown@nasa.gov

Caroline McCall 617-253-1682
Massachusetts Institute of Technology, Cambridge
Cmcall5@mit.edu

News release: 2011-314 Oct. 6, 2011

NASA's Moon Twins Going Their Own Way

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-314&cid=release_2011-314

PASADENA, Calif. – NASA's Gravity Recovery And Interior Laboratory (GRAIL)-B spacecraft
successfully executed its first flight path correction maneuver Wednesday, Oct. 5. The rocket
burn helped refine the spacecraft's trajectory as it travels from Earth to the moon and provides
separation between itself and its mirror twin, GRAIL-A. The first burn for GRAIL-A occurred
on Sept. 30.

"Both spacecraft are alive and with these burns, prove that they're kicking too, as expected," said
David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory in Pasadena,
Calif. "There is a lot of time and space between now and lunar orbit insertion, but everything is
looking good."

GRAIL-B's rocket burn took place on Oct. 5 at 11 a.m. PDT (2 p.m. EDT). The spacecraft's main
engine burned for 234 seconds and imparted a velocity change of 56.1 mph (25.1 meters per
second) while expending 8.2 pounds (3.7 kilograms) of propellant. GRAIL-A's burn on Sept. 30
also took place at 11 a.m. PDT. It lasted 127 seconds and imparted a 31.3 mph (14 meters per
second) velocity change on the spacecraft while expending 4 pounds (1.87 kilograms) of
propellant.

These burns are designed to begin distancing GRAIL-A and GRAIL-B's arrival times at the
moon by approximately one day and to insert them onto the desired lunar approach paths.

The straight-line distance from Earth to the moon is about 250,000 miles (402,336 kilometers). It
took NASA's Apollo moon crews about three days to cover that distance. Each of the GRAIL
twins is taking about 30 times that long and covering more than 2.5 million miles (4 million
kilometers) to get there. This low-energy, high-cruise time trajectory is beneficial for mission
planners and controllers, as it allows more time for spacecraft checkout. The path also provides a
vital component of the spacecraft's single science instrument, the Ultra Stable Oscillator, to be
continuously powered for several months, allowing it to reach a stable operating temperature
long before beginning the collection of science measurements in lunar orbit.

GRAIL-A will enter lunar orbit on New Year's Eve, and GRAIL-B will follow the next day.
When science collection begins, the spacecraft will transmit radio signals precisely defining the
distance between them as they orbit the moon. 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.

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. Launch management for the mission is the responsibility of NASA's
Launch Services Program at the Kennedy Space Center in Florida. 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 .

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