On Sept. 10, 2011, NASA'sGRAIL spacecraftlaunched on a mission to the moon!
GRAIL, which stands for Gravity Recovery and Interior Laboratory, was sent to the moonto map its gravitational fields. Scientists could study the moon's interior structure by looking at maps of the moon's gravity.
Asteroid impacts from billions of years ago left dense pockets of material under the lunar surface, which can exert extra gravitational pull on spacecraft orbitingthe moon. So by measuring that gravitational pull, the GRAIL mission could reveal information about what lies beneath the surface.
The mission actually consisted of twotwin spacecraft named Ebb and Flow, which would fly in tandem around the moon to map variations in its gravitation field. Both lifted off on the sameDelta II rocketfrom Cape Canaveral, Florida and arrived in lunar orbit almost four months later.
They mapped the moon for about nine months before NASA decided it was finished with the mission, and the two orbiterscrashed into the moon.
Why it mattered
The moon is a weird place. It may look like a cold, gray ball of solid rock, but its density varies from location to location due to odd "mass concentrations" (or mascons for short) buried under the lunar surface. That leads to a strange,lumpy gravity that's not uniformacross the lunar surface.
That's where GRAIL's Ebb and Flow probes came in. Each of the twin spacecraft carried three instruments tomap the interior of the moon. Those included a Lunar Gravity Ranging System, a Radio Science Beacon and a camera called MoonKam Lunar-Imaging system.
Together, the instruments mapped the mascons on the moon in extreme detail to monitor where lunar gravity increased from the norm. Astronomers had long suspected thatmascons on the moonand other planets formed during violent asteroid impacts.
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"GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient Moon when its interior was much hotter than it is now," Jay Melosh, a GRAIL co-investigator at Purdue University, said ina NASA overview.
The MoonKam toolwas actually a student experiment (MoonKam is short for Moon Knowledge Acquired by Middle School Students). It was built by undergradutate students at the University of California, San Diego in partnership with the Sally Ride Science foundation to encourage younger students to study science and space. (The namesEbb and Flow as GRAIL probe nameswere also suggested by 4th grade students at Emily Dickinson Elementary School in Bozeman, Montana, as part of a national competition.)
The $496 million GRAIL probes orbited the moon in an extremely low orbit, coming within 30 miles (50 km) of the lunar surface at their closest approach. They were separated from each other at distances that ranged from 109 miles to 140 miles (175 km to 225 km).
NASAcrashed the GRAIL probes into the moonon Dec. 17, 2012 once its fuel reserves reached the point that it could not maintain its low lunar orbit. They crashed into a crater rim near the moon's north pole to prevent them from randomly crashing uncontrolled into someplace like anhistoric Apollo moon landing site.
Want more space history? Check out ourfull On This Day In Space Story archiveand watch ourOn This Day In Space videos on YouTube.
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Contributing expert
Hanneke Weitering is a multimedia journalist in the Pacific Northwest reporting on the future of aviation at FutureFlight.aero and Aviation International News and was previously the Editor for Spaceflight and Astronomy news here at Space.com. As an editor with over 10 years of experience in science journalism she has previously written for Scholastic Classroom Magazines, MedPage Today and The Joint Institute for Computational Sciences at Oak Ridge National Laboratory. After studying physics at the University of Tennessee in her hometown of Knoxville, she earned her graduate degree in Science, Health and Environmental Reporting (SHERP) from New York University. Hanneke joined the Space.com team in 2016 as a staff writer and producer, covering topics including spaceflight and astronomy. She currently lives in Seattle, home of the Space Needle, with her cat and two snakes. In her spare time, Hanneke enjoys exploring the Rocky Mountains, basking in nature and looking for dark skies to gaze at the cosmos.
With contributions from
- Tariq MalikEditor-in-Chief
