NASA aims to hit, smash comet

This year, NASA is planning a spectacular fireworks display on the Fourth of July. Unfortunately, you'll need a telescope to see it, as the explosions will take place over 268 million miles away. Obviously, this is far from the usual observances of our nation's birth. But NASA's plans do indeed have something to do with a birth: not just the birth of the United States, but our entire solar system.

I honestly have no idea if the timing of this mission is coincidence or not, but this July 4th, NASA is going to smash a comet, and observe the material that flies off in the explosion. The name of this mission is, appropriately enough, Deep Impact, and when it whops into the comet Tempel 1 this summer, the resulting smash will be equivalent to blowing up 4.8 tons of TNT, and is expected to blow a football stadium-sized crater about 7 stories deep into the dirty ice of the comet.

Now, you've got to admit, that sounds pretty cool. But aside from patriotic spectacles, there is a very good reason to do this.

Scientists are learning more and more about how planets form in young solar systems. Ten years ago, there were only a handful of stars that we knew had planet-forming disks of gas and dust around them (astronomers call them protoplanetary disks).

Now, we know of hundreds, due to a large part to the observations coming down from the Spitzer Space Telescope, an all-infrared observatory launched in 2003. Warm disks of planet-forming material show up bright and clear to a heat-light sensitive telescope, and for the first time, astronomers are able to look at the process of planet formation using large numbers of examples.

But the only example we have of a planet-forming disk that really did the right thing (i.e. formed a stable solar system capable of life) is our own. With so many examples of how planet-forming gets started, its important to compare the new systems we're discovering to what we must have looked like, a few billion years ago.

But there's one inconvenient fact: we've moved on since then. The dust and gas of our protoplanetary disk is long gone, gathered into planets or leaked away into the emptiness of space. Where could we find a sample of the material that was around during the actual formation process of our system?

Hey, guess what? Inside a comet!

Comets are a relic of our early solar system. In a way, the leftovers of planet formation - chunks of dirt and ice that never got incorporated into a larger planetary body. Astronomers have been interested in the chemistry of comets for some time now, as being like a time-machine to look back and study the chemistry of our solar systems before the planets even formed.

Up until now, we've only been able to study the material near the surface of comets - the stuff blown off in the tail or, more recently, a few near encounters with the icy body itself. But the surface material has been altered over time: blasted by the Sun's heat each time a comet makes its rounds through our solar system. If we want to see a pristine sample of the stuff of our early selves, we've got to dig down. Or even better, blast down!

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