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]





4 Responses to A Tale of Two Moons

  1. Not to be completely off topic, but I wonder what kind of effect two moons would have had on our mythology had they not collided to form one (and had their conflicting effects on the tides and weather not made the world too unstable for civilization/life)

  2. Ok, this is a genuine question and I hope I don't sound like an idiot for asking.

    Isn't it just as likely a large meteor struck the moon?

    And if one did, could pieces of the moon (and pieces of the meteor that got caught in the earth's gravity) fall towards the earth causing a chain reaction of massive destruction? Perhaps enough to cause the extinction of many dinosaurs and maybe trigger the beginning of the ice age?

    Like I said, honest to goodness question. I'm seriously curious if it's possible for that to have happened.

    • Nicole: The difference between this impact and that of a meteor or other large body is the speed. Had a meteor a third of the size of the moon hit the moon, it would have done so at a very high speed, essentially vaporizing the contents of both, yielding a uniform lunar content when it re-coalesced and cooled. Because the moon has such a markedly different composition and topography, the theory suggests that any impact which caused it would have to occur at a speed low enough to leave the two bodies unvaporized–the simplest explanation (and the one that GRAIL will find evidence for or against) is that a smaller body traveling in the same orbit collided with the moon. Because they're traveling in the same direction at roughly the same speed, the impact isn't great enough to obliterate either of the moons, but instead kind of squishes them together.

      As for the question of a meteor or errant asteroid striking the moon: yes, it would almost definitely send material toward the earth. The good news is that when this happened, there were no dinosaurs yet, so they were ok. :)