Gravity Recovery and Interior Laboratory

Gravity Recovery and Interior Laboratory. It was a pioneering NASA Discovery Program mission designed to create the most accurate gravitational map of the Moon to date. Managed by the Jet Propulsion Laboratory and led by principal investigator Maria Zuber of the Massachusetts Institute of Technology, the mission employed twin spacecraft in a precise formation to measure the Moon's gravity field with unprecedented detail. The resulting data provided profound insights into the Moon's internal structure, thermal evolution, and the origin of impact basins across the Solar System.

Mission overview

The mission's core concept was derived from the highly successful Gravity Recovery and Climate Experiment (GRACE) mission, which mapped Earth's gravity field. By deploying two identical spacecraft, named Ebb and Flow by winning students from Bozeman, Montana, in tandem orbit around the Moon, scientists could measure minute changes in the distance between them caused by gravitational anomalies. This innovative approach allowed for the mapping of gravitational variations across the entire lunar surface, including the far side, which is not visible from Earth. The primary science phase commenced in March 2012 and lasted for 89 days, followed by an extended mission that lowered the orbital altitude for even higher-resolution measurements.

Spacecraft design

Each nearly identical spacecraft was built by Lockheed Martin Space based on a compact, low-cost design philosophy. The satellites, each with a launch mass of approximately 202 kilograms, were cube-shaped and utilized a Xenon-fueled propulsion system for trajectory adjustments and orbit maintenance. Key instruments included the Lunar Gravity Ranging System, which used Ka band radio links to measure the minute velocity and range changes between the two craft with a precision of less than a micron per second. Power was supplied by body-mounted solar arrays, and communication with Earth was handled via the Deep Space Network using an S-band transponder.

Scientific objectives and results

The primary scientific goal was to determine the structure of the lunar crust and lithosphere and to understand the Moon's asymmetric thermal evolution. The mission successfully revealed that the lunar crust is much thinner than previously estimated, averaging between 34 and 43 kilometers, and is extensively fractured by ancient impacts. Data confirmed the "magma ocean" hypothesis for the early Moon and provided evidence that large impact basins, like the Mare Orientale, were filled by magma from the mantle. Furthermore, the gravity maps identified numerous mascons (mass concentrations) and provided clear evidence that the Moon's interior was hot enough to be at least partially molten shortly after its formation.

Mission timeline and operations

Launched together on a Delta II rocket from Cape Canaveral Space Force Station on 10 September 2011, the two spacecraft took a low-energy, three-month trajectory to the Moon, arriving in orbit in December 2011 and January 2012. After a commissioning period, the science phase began on 7 March 2012. The spacecraft operated in a near-polar orbit, eventually lowering their altitude to an average of 55 kilometers and then to just 23 kilometers during the extended mission. The mission concluded on 17 December 2012, when both spacecraft were commanded to impact a mountain near the north pole, specifically a crater adjacent to Mouchez, in a controlled maneuver to prevent contamination of historic sites.

Legacy and impact

The mission's legacy is encapsulated in its transformative gravitational models, named GRGM900A and later refinements, which became the standard reference for all subsequent lunar science and exploration planning. The high-resolution data is critical for future mission design, including those by Artemis and other international partners like CNSA and ISRO. The mission also had a significant educational outreach component, featuring cameras aboard each spacecraft called MoonKAM, which were used by middle school students across the United States to request images of the lunar surface. The findings continue to inform studies of terrestrial planets and the early history of the Solar System.

Category:NASA space probes Category:Discovery Program Category:2011 in spaceflight Category:Lunar spacecraft