Man in the moon to get 'CT scan'

Dana Mackenzie, contributor

Measurements of the distance between GRAIL's two probes will reveal variations in the strength of the moon's gravity (Image: NASA/JPL-Caltech)

I'm blogging today from the Orbit Cafe at the Kennedy Space Center in Florida, where you can't watch a space shuttle launch any more but you can still get a good chicken sandwich.

Tomorrow, if the weather cooperates ? about a 40 per cent probability ? a Delta II rocket will start the GRAIL satellites (a.k.a. the Gravity Recovery and Interior Laboratory) on their four-month journey to the moon.

GRAIL is designed to make the most sensitive measurements ever of the lunar gravitational field ? 100 times more accurate on the near side and 1000 times more on the far side.

In some ways this is the perfect mission at the perfect time. Just a month ago, planetary scientists suggested a new explanation of the dramatic asymmetry between the moon's flat, low near side and its rugged, mountainous far side.

The new hypothesis states that Earth started with two moons, and the smaller one ran into the larger one.

The impact, a relatively low-energy one in cosmic terms, caused the smaller one to "wrap around" the larger one, depositing most of its contents on the far side. According to deputy mission scientist David Smith of MIT, if this idea is correct then GRAIL ought to see denser than expected material near the surface of the lunar far side ? the innards of the now-defunct second moon.

In general, the mission will provide the data that scientists need to "see inside" the moon, and answer such still unresolved questions as whether the moon has a molten core.

GRAIL uses the same twin-satellite technology as another mission, GRACE, which mapped Earth's gravitational field. But the moon's is much more interesting; it's the lumpiest world that we've been to.

For techno-geeks, there are other things to get excited about. GRAIL has a one-second launch window. That's right, one second. Either it launches at 8:37:06 tomorrow morning, or at 9:16:12, or else it waits until Friday.

You've also got to love a mission that heads towards the sun to get to the moon. After taking a ceremonial lap around the Earth, the twin spacecraft will separate and head for a dot in space about 1 per cent of the way to the sun. There, Earth's gravity balances the sun's, allowing objects placed there to circle the sun in lockstep with our planet.

This never-before-used route to the moon offers two advantages. First, it uses less fuel and thus requires a smaller rocket. Second, it allows the spacecraft to park themselves for a month or two before heading moonward. This is important because the spacecraft must arrive on New Year's Eve, no matter what day they launched.

Why New Year's Eve? "It's just to give us a pain!" jokes Smith.

Actually, GRAIL needs to squeeze its mission between two lunar eclipses, one on 12 December and the other in June. During a lunar eclipse, the Earth comes between the moon and the sun, blocking the sunlight that powers the GRAIL probes. Smith is optimistic that GRAIL will survive the eclipse and be able to continue its mission beyond June ? but the scientists cannot count on it.

Finally, this launch will be the last hurrah on the East Coast for the Delta II, a rocket with a perfect 48-for-48 launch record dating back to 1960. We'll have to hope that the rockets that replace it, such as the Falcon 9 developed by SpaceX, will be equally reliable.

Source: http://feeds.newscientist.com/c/749/f/10897/s/181363d7/l/0L0Snewscientist0N0Cblogs0Cshortsharpscience0C20A110C0A90Cman0Ein0Ethe0Emoon0Eto0Eget0Ect0Escan0Bhtml0DDCMP0FOTC0Erss0Gnsref0Fonline0Enews/story01.htm

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