Appearance of the Moon during the GRAIL impacts

GRAIL's Final Resting Spot.  These maps of Earth's moon highlight the region where the twin spacecraft of NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission will impact on Dec. 17, marking the end of its successful endeavor to map the moon's gravity. The two washing-machine-sized spacecraft, named Ebb and Flow, will impact an unnamed mountain above of 75°N. [NASA/GSFC/Arizona State University].

Pasadena (JPL) -- 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 2028 UT (5:28 p.m. U.S. EST) Monday, December 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, providing a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

Perspective on the Moon at the estimated time of the GRAIL impacts, projected at 2229 UT, 17 December 2012. Waxing between New and First Quarter (4.58 days 26.8% illumination), the distance between Earth and Moon will be increasing at roughly 10 km per minute from 372,628 kilometers. In eastern North America, the Moon will have transited, riding low and west from overhead Only the best equipped observers can hope to observe the actual release of kinetic energy, an extremely fast flash, near the horn of the north-northwest limb [Virtual Moon Atlas].

"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."

Lunar Heritage Sites and GRAIL's Final Mile. This graphic highlights locations on the moon NASA considers "lunar heritage sites" and the path NASA's Gravity Recovery and Interior Laboratory spacecraft will take on their final flight. Navigators on the GRAIL team have designed an end of mission plan that rules out the extremely remote possibility of either of the two GRAIL spacecraft impacting near any of these historic locations. The Apollo 11, 12, 14, 16 and 17 landing sites are indicated with green circles. The Surveyor sites are indicated with yellow squares. The Soviet Union's Luna and Lunakhod landing sites are indicated with red diamonds and red squares, respectively.  The ground track for the Ebb and Flow spacecraft during their final half-orbits is shown in black. The maps are color-coded by topography. Red and white indicate the high areas. Blue and violet indicate low areas [NASA/JPL-Caltech].

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. PST. 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.

Ebb and Flow's Final Moments. These side-by-side, 3-D comparisons depict the unnamed lunar mountain targeted by the NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission for controlled impact of the Ebb and Flow spacecraft. They also include the ground tracks the spacecraft are expected to follow into the lunar terrain. These graphics were generated using data from the Lunar Orbiter Laser Altimeter instrument aboard NASA's Lunar Reconnaissance Orbiter spacecraft. On the left is the mountain with the ground track and mission termination point for the Ebb spacecraft. On the right is the mountain, ground track and mission termination point for the Flow spacecraft [NASA/JPL-Caltech/MIT/GSFC].

The burn that will change the spacecrafts' orbit and ensure the impact is scheduled to take place Friday morning, Dec. 14.

"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.

The published impact coordinates for the GRAIL twins has been well-surveyed by the Lunar Reconnaissance Orbiter Camera (LROC), at least nine times at high-resolution. LROC QuickMap 125 meter resolution [NASA/GSFC/Arizona State University].

JPL manages the GRAIL mission for NASA's Science Mission Directorate in Washington. The 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. JPL is a division of the California Institute of Technology in Pasadena.

For more information about GRAIL, visit: http://grail.nasa.gov and http://www.nasa.gov/grail.