Grail-A and Grail-B

The Grail-A and Grail-B spacecraft, collectively known as the Gravity Recovery and Interior Laboratory (GRAIL) mission, were twin lunar probes launched by the National Aeronautics and Space Administration (NASA) to study the internal structure and gravitational field of the Moon. The mission, part of NASA’s Discovery Programme, represented a significant advancement in lunar science and gravity mapping, improving understanding of the Moon’s composition, evolution, and geological history.

Background and Mission Objectives

The GRAIL mission was conceptualised to address key scientific questions left unanswered by previous lunar missions, such as the internal layering, crustal thickness, and thermal evolution of the Moon. It aimed to measure variations in the Moon’s gravitational field with unprecedented precision, allowing scientists to infer details about its internal structure.
Prior to GRAIL, gravity data from earlier missions—such as Lunar Orbiter, Clementine, and Lunar Prospector—were limited in resolution and coverage, particularly on the far side of the Moon. The GRAIL mission was therefore designed to provide a complete, high-resolution map of the lunar gravity field.
The primary scientific objectives were to:

• Determine the structure of the lunar crust and mantle.
• Understand the thermal evolution and tectonic history of the Moon.
• Improve knowledge of lunar impact basins and the processes that shaped them.
• Provide data to support future lunar exploration missions.

Design and Spacecraft Configuration

The GRAIL mission comprised two nearly identical spacecraft, designated Grail-A and Grail-B, each weighing approximately 200 kilograms. The spacecraft were built by Lockheed Martin Space Systems and managed by NASA’s Jet Propulsion Laboratory (JPL).
Key components included:

• Ka-band ranging system (KBR): The primary scientific instrument, capable of measuring the distance between the two spacecraft with an accuracy of a few micrometres.
• Radio science system: Used for communication and tracking via NASA’s Deep Space Network.
• Solar arrays and batteries: Provided power to onboard instruments.
• Propulsion system: Enabled trajectory corrections and orbital manoeuvres.
• Star trackers and gyroscopes: Assisted in precise attitude determination.

The two probes were designed to fly in tandem in a near-polar orbit around the Moon, maintaining a constant separation distance of roughly 175–225 kilometres. Small variations in the distance between the two spacecraft, caused by changes in the Moon’s gravitational pull, were used to map gravity anomalies.

Launch and Orbital Insertion

The GRAIL mission was launched on 10 September 2011 aboard a Delta II rocket from Cape Canaveral Air Force Station, Florida. Unlike most lunar missions that take a few days to reach the Moon, GRAIL followed a low-energy, 3.5-month trajectory to conserve fuel.

• Grail-A (later nicknamed Ebb) entered lunar orbit on 31 December 2011.
• Grail-B (later nicknamed Flow) followed on 1 January 2012.

After orbital insertion, both spacecraft were placed in nearly circular, near-polar orbits approximately 55 kilometres above the lunar surface.

Science Operations and Data Collection

The primary mission phase began in March 2012 and lasted about three months, during which the probes conducted high-resolution gravity mapping of the entire lunar surface. The extended mission phase, which lasted until December 2012, involved lowering the orbital altitude to about 23 kilometres to obtain even more detailed gravity measurements.
The two spacecraft continuously transmitted microwave signals to each other, allowing scientists to monitor minute changes in their relative distance. These variations reflected gravitational differences caused by lunar surface features such as mountains, craters, and mass concentrations (known as mascons).
The GRAIL mission also included an educational component, the MoonKAM (Moon Knowledge Acquired by Middle School Students) programme, which allowed students to request images of specific lunar sites, fostering public engagement in space science.

Major Discoveries and Scientific Contributions

The GRAIL mission revolutionised lunar science through its precise mapping of the Moon’s gravity field. Key findings included:

1. Thin Lunar Crust: The Moon’s crust was found to be significantly thinner than previously estimated—ranging from 34 to 43 kilometres in thickness. This suggested that the early Moon experienced extensive melting.
2. High Porosity of the Crust: Data revealed that the lunar crust is highly fractured and porous, likely due to the heavy bombardment during the Moon’s early history.
3. Structure of Impact Basins: GRAIL provided new insights into large lunar basins such as Mare Orientale and South Pole–Aitken Basin, improving understanding of impact formation and crustal rebound processes.
4. Distribution of Mass Concentrations (Mascons): The mission confirmed that mascons—dense regions beneath large basins—formed as a result of volcanic fill and crustal thinning following massive impacts.
5. Improved Lunar Gravity Models: The resulting gravity maps were the most accurate ever produced for a celestial body other than Earth, aiding future landing missions and navigation systems.

These results also helped refine models of the Moon’s internal differentiation, offering analogies for studying other terrestrial planets such as Mars and Mercury.

End of Mission

After completing their scientific objectives, both Ebb and Flow were intentionally crashed onto the lunar surface on 17 December 2012 near the Moon’s north pole, in a controlled manner to avoid contaminating scientifically important regions. The impact site was later named the Sally Ride Impact Site, in honour of Dr Sally Ride, the first American woman in space and a key participant in the MoonKAM project.
The mission officially concluded after this deliberate termination, marking a successful end to one of NASA’s most precise and cost-effective lunar explorations.