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!

The idea behind the Deep Impact mission is this: send a spacecraft to intercept the comet Tempel 1and launch some ballistic object at it to carve out a crater deep enough to get down to the pure, unaltered comet material beneath the surface. While that's going on, observe the chemistry of the comet before, during, and after the impact, and study the structure of the crater for clues about how the comet is made up.

Deep Impacts strategy may seem rather brutal, but there is actually quite a bit of delicacy involved. The spacecraft has two parts, a fly-by craft and an impactor.

The fly-by spacecraft will travel millions of miles out into space to pull up about 536,000 miles away from Tempe 1. On July 3rd, it will release the impactor and train its sensitive cameras toward the surface of the comet. The impactor, as its name suggests, is the bit that will actually hit the comet, an 820 pound bullet of copper and aluminum.

But it is far from just a lump of metal; the impactor will have just 24 hours to travel half a million miles and insert itself directly into the path of Tempel 1, a comet only 4 miles across that will be barreling through space at incredible speeds. When the impactor hits the comet, it should crash into Tempel 1 with speeds around 23,000 miles per hour. The impactor also has to hit the comet on its sunny side, or we wont get a very good view of the crater or the debris that gets blown off.

The impactor is actually a mini-spacecraft unto itself, able to steer itself by the stars and alter its path accordingly with its own thrusters. The thrusters aren't hugely strong either; the idea is basically to get the impactor into the orbital path of the comet, and then let the comet plow into it. Watching from a safe distance of 536,000 miles away, the fly-by part of the spacecraft will be constantly sampling the chemistry of the comet, and observing the debris that gets thrown off during the collision. Some of the Earth's largest telescopes will also be pointing up at the celestial fireworks, hoping for a first glimpse inside a comet.

What do we expect to see?

To begin with, the size and shape of the crater will tell us something about the structure of the comet, whether it's denser in the middle, or if heavy material is spread more evenly throughout the whole body. That data, in turn, will tell us a surprisingly important fact about the processes that were going on in our ancient protoplanetary disk.

Were objects allowed to accrete together smoothly over huge spans of time (which would create comets with denser cores with lighter layers of ice deposited on top), or were things being mixed around more, blending heavy and light materials together over shorter timescales?

What molecules were present in the early solar system? Many of the protoplanetary disks we're observing now seem to have plenty of water, methane, carbon dioxide and other organic molecules enriching the gas of the disk. Was our chemistry, which ultimately led to life, the same or different?

At any rate, if you happen to be watching fireworks on the Fourth of July this year, look up into the sky and give a thought to the other, more distant fireworks taking place. We've traveled millions of miles to celebrate the beginning of something truly special.