How measuring tiny movements between two satellites revealed the Moon's hidden interior
In short: To understand what lies beneath the lunar surface, NASA didn't just look at the Moon; they felt its pull. By tracking the subtle dance of two twin spacecraft, scientists mapped the lunar gravity field with unprecedented precision, uncovering the secrets of the Moon's internal structure.
Launched on September 10, 2011, from Cape Canaveral, the GRAIL (Gravity Recovery and Interior Laboratory) mission deployed two identical spacecraft, named Ebb and Flow. These probes were designed to map variations in the lunar gravitational field to determine the Moon's internal structure, from its crust to its core, and to study its thermal evolution. The mission was a significant milestone as the 11th in NASA’s Discovery Program.
The mechanism relied on a principle similar to the Gravity Recovery and Climate Experiment (GRACE) used on Earth. As Ebb and Flow flew in tandem around the Moon, they moved slightly toward and away from each other as they passed over areas of varying gravity. A specialized instrument measured these changes in their relative velocity.
By using this information, researchers produced the most detailed gravity chart ever created for a space object. The spacecraft, each weighing 446 pounds, utilized avionics derived from the Mars Reconnaissance Orbiter and were designed as an evolution of the Air Force’s XSS-11 satellite. They followed a low-energy translunar cruise, passing near the Sun-Earth L1 Lagrange point before reaching the Moon. While the nominal mission was planned for three months, the primary science phase lasted from March to May 2012, with an extended science phase beginning in August 2012. The mission concluded on December 17, 2012, when both probes were purposely crashed into the lunar surface.
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Sources
- Summary by Intellect · source: NASA's GRAIL Spacecraft Launches on Lunar Mission
- Also: GRAIL (Ebb and Flow) - NASA Science
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