GRAIL Will Use Gravity Data To Track Planetary Evolution

Colorful image of technician checking bottom side of solar panel shows internal components of spacecraft. Horn shaped feature at center is swiveling star tracker. Photo Credit: Lockheed Martin

Two identical NASA GRAIL spacecraft are about to dive under the south pole of the Moon to enter lunar polar orbit circling the Moon as low as 30 mi. altitude and flying in precise trail as if on the same railroad track.

But before lining up over the Moon they are completing a transit from the  L1 Lagrangian point where they have been circling above and below the ecliptic plane since shortly after launch from Cape Canaveral Sept. 10 on a Delta 2 Heavy booster.

The mission will create the most accurate gravitational map of the moon to date improving our knowledge of near-side gravity by 100 times and of far-side gravity by 1,000 times, according to NASA.

Accurate knowledge of the moon’s gravity will also be an invaluable navigational aid for future lunar spacecraft. Finally, GRAIL will also help provide a broader understanding of the evolutionary histories of the other rocky planets in the inner solar system: Mercury, Venus, Earth and Mars, NASA geologists say.

But the science work of the mission will not start until March. The objective now is to precisely tweak each spacecraft’s orbit so that by March they will be spaced so precisely that differences in lunar gravity, reflected in tiny changes to spacecraft separation, can be measured to within the diameter of a human hair, says principal investigator Maria Zuber of MIT.  The spacecraft arrived in lunar orbit Dec. 31 and Jan. 1

That “human hair” level of precision collected over the entire lunar surface will reveal the Moon’s deep interior structure. Once that data is acquired for the entire Moon, the information should also provide insight about the great bombardment of asteroids and large meteorites that took place after the initial planetary surfaces had solidified. Many of those mountain sized objects now lie buried producing strong gravity signatures.

Launched Sept. 10 2011, from Cape Canaveral by a United Launch Alliance Delta II Heavy, these two Lockheed Martin / Jet Propulsion Laboratory spacecraft were christened GRAIL A and B for “ Gravity Recovery And Interior Laboratory” satellites. But feedback to NASA’s Outreach program showed “GRAIL” to be such a bland non descriptive name that NASA decided to change it by way of a national student contest to rename it. Each spacecraft weighs 677 lb.

Overhead view looking down on Earth, Moon and L1 Lagrangian point at far left shows the 2.5 million mi. trajectory flown by twin Grail satellites to approach lunar south pole for lunar orbit insertion Dec. 31 and Jan. 1. Image Credit: JPL

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