How two twin spacecraft mapped the hidden interior of the Moon
In short: To see inside a celestial body, scientists didn't just look at its surface; they measured how it pulled on everything around it. By tracking the subtle dance between two orbiting probes, NASA created the highest resolution gravity map of any celestial body in history.
The GRAIL mission (Gravity Recovery and Interior Laboratory) utilized a dual-spacecraft design to probe the Moon's internal structure. Named Ebb and Flow—titles won by fourth-grade students in Montana—the two identical 446-pound probes flew in tandem around the lunar surface. Their primary objective was to map variations in the lunar gravitational field, which serves as a blueprint for the Moon's composition and interior layout. The mission was designated as the eleventh project within NASA's Discovery Program. The spacecraft design evolved from a 2005 U.S. Air Force satellite system. Their avionics were actually adapted from the Mars Reconnaissance Orbiter.
The mechanism relied on precise relative motion. As the spacecraft passed over areas of varying gravity, they moved slightly toward or away from one another. Instruments measured these minute changes in velocity, providing data similar to the technology used by the GRACE mission to map Earth's gravity field between 2002 and 2017. This allowed scientists to achieve unprecedented detail regarding what lies beneath the lunar crust.
The mission was a feat of orbital logistics, involving a low-energy translunar cruise that passed near the Sun-Earth L1 Lagrange point before reaching the Moon. Ebb entered orbit on December 31, 2011, followed by Flow about 25 hours later on January 2, 2012. After completing both their nominal three-month science phase and an extended mission beginning in August 2012, the probes were intentionally crashed into an unnamed mountain on December 17, 2012, once their fuel levels became too low for further operations. The probes utilized several specific instruments including the Lunar Gravity Ranging System and a radio science beacon. They also carried a Moon Knowledge Acquired by Middle School Student imaging system. The primary science phase concluded in late May of 2012.
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Sources
- Summary by Intellect · source: NASA Grail Twins Make Impact with Moon
- Also: GRAIL (Ebb and Flow) - NASA Science
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