Grail-A and Grail-B

GRAIL refers to Gravity Recovery and Interior Laboratory.

Recently, NASA has succeeded in putting the first of two gravity mapping satellites in orbit around the Moon. The Grail-A spacecraft fired its main engine to slow itself sufficiently to take up an elliptical path around the lunar body. Its twin, Grail-B, will attempt exactly the same manoeuvre. The second Gravity Recovery and Interior Laboratory (GRAIL-B) began orbiting the Moon. GRAIL-A reached its lunar orbit on 31 December, 2011.

Together, the satellites will make measurements that are expected to give scientists remarkable new insights into the internal structure of the Moon. Beginning in March, the two unmanned spacecraft will send radio signals that allow earth-based scientists to create a high-resolution map of the Moon’s gravitational field, helping them to better understand its sub-surface features and the origins of other bodies in the solar system.

This new data should clarify ideas about the Moon’s formation and resolve many questions, such as why its near and far sides look so different, test a hypothesis that there was once a second Moon that fused with ours etc.

Scientists believe that the Moon was formed when a planet-sized object crashed into the Earth, throwing off a load of material that eventually became our planet’s airless, desolate satellite. How it heated up over time, creating a magma ocean that later crystallised, remains a mystery, despite 109 past missions to study the Moon since 1959 and the fact that 12 humans have walked on its surface.

GRAIL: Some Important Points

• The GRAIL spacecraft are expected to provide important clues to what the Moon is like deep inside by mapping variations in its gravitational field with much higher resolution than before.
• When the science phase of the mission starts in March 2012, the two satellites will be flying in formation, one behind the other, at a height of about 55 km. The spacecraft will then transmit GPS-like radio signals so that the distance between them can be accurately measured.
• That gap changes as they fly over areas with greater or lesser gravity. Scientists will then need to take the data and combine them with other information, such as topographical maps prepared with data from other satellites, in order to try and figure out the structure of the Moon’s interior and its composition.
• Such analysis can provide insights into many aspects of Earth’s natural satellite. Why, for instance, are the two hemispheres of the Moon so different from each another? The side we see is flat with lava-filled basins that appear as dark patches. The farside, on the other hand, is mountainous and has a much thicker crust.
• One explanation for this asymmetry, based on computer simulations and published recently in a scientific journal, suggested that the Moon once had a smaller companion that subsequently crashed into it. The GRAIL data could help substantiate or rule out such a scenario. Likewise, getting a handle on the distribution of materials in the interior will be invaluable in understanding the Moon’s evolution. As always, such quests of scientific discovery can reveal the unexpected.
• Grail is the 110th mission to target the moon since the dawn of the Space Age including the six Apollo moon landings that put 12 astronauts on the surface. Despite the attention the moon has received, scientists don’t know everything about Earth’s nearest neighbour.
• Grail is expected to help researchers better understand why Moon is asymmetrical and how it formed by mapping the uneven lunar gravity field that will indicate what’s below the surface. Previous missions have attempted to study gravity with mixed results. Grail is the first mission devoted to this goal.