A Tale of Two Moons

Last month, planetary science suggested a new explanation of the dramatic asymmetry of our Moon’s two sides; one, “our” side, is flat and low, while the other (the “dark side”) is a mountainous terrain. The dichotomy could arise, researchers say, from a collision between our moon and a smaller companion. As a smaller body, had the companion moonlet crashed into the moon at a low enough speed, it would have cooled more quickly, avoiding vaporization and simply smearing itself across the impact crater it created. Visually, it would go something like this:

The Two Moons theory would explain the vast differences in both composition and geography, but can it be proven?

Perhaps so. Early this morning, high-level winds delayed the launch of NASA’s GRAIL mission (Gravity Recovery and Interior Laboratory), a twin spacecraft designed to determine the structure of the lunar interior.

The twin spacecraft are now scheduled to begin their mission to the moon on Sept. 9, lifting off from Cape Canaveral Air Force Station’s Launch Complex 17B aboard a United Launch Alliance Delta II heavy rocket. There are once again two instantaneous (one-second) launch windows. Friday’s launch times are 8:33:25 a.m. and 9:12:31 a.m. EDT. The launch period extends through Oct. 19, with liftoff occurring approximately 4 minutes earlier each day.

Friday, NASA will attempt another launch, sending the GRAIL on its four-month journey to the moon by way of a new route that takes the spacecraft pair first on a one-lap tour of the planet, then after separating, toward the Sun. At the point when the Earth’s gravity balances the pair in orbit, they’ll hang out for a couple months before heading to the moon. The timing is important–the spacecraft have to avoid two lunar eclipses, which would block the sunlight needed to power the GRAIL probes.

Once there, the twin probes will utilize the same technology as GRACE, the mission which mapped the Earth’s gravity. Information from a full lunar gravity scan will give clues to the Moon’s composition, and, just maybe, tell us whether or not we did in fact once have two moons.

Live launch coverage will begin tomorrow morning (Friday, Sep 9) at 5:45 a.m. on NASA TV and on the web at www.nasa.gov/ntv and www.nasa.gov/mission_pages/grail/launch/grail_blog.html.

[sources: 1|2|3]

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