I watched Deep Impact as she was hurled to the stars atop her Delta II rocket last January from my porch. Any launch is a spectacular and potentially destructive event living this close to the Cape. This one was made all the more so because my sister-in-law had worked on that launch vehicle.

Cocooned snugly in the tip of that Delta was Deep Impact, a big box of electronics sporting ungainly maneuvering nozzles and furled antennas, about the size of an SUV. The space craft probe itself is actually two vehicles in one. The mother ship with the communications and imaging gear, and the impactor, which will be fired early on Jul 3 to hopefully strike the comet's surface and set of an explosion with the force of 5 tons of TNT. From this brief flash scientists expect to learn something about the comet's internal composition.

Comets were usually perceived as an omen of bad news in ancient cultures. But we now know that comets are just dirty snowballs. Like a glacier about the size of a medium city spattered with black hydrocarbons and dotted with primeval dust and rock. According to the Solar Nebula Theory the earth, all the other planets, and even many satellites, were formed to a great extant by trillions of comets coalescing into planets around the embryonic sun 5 billion years ago. Cometary impacts it seems can both create and destroy. It may have been a comet that knocked out the dinosaurs and plunged the surface of our worlds into an ecological nightmare, but which also liberated early mammals from under the saurian claws.

But not all the comets were swept up into larger bodies over this time. A number of them escaped that fate and have gone their own way since our solar genesis. As life on earth stirred, then floated, then crawled out of the muck and onto land, comets are thought to have barely changed at all over that ocean of time. These ancient, pristine objects thus play the role of fossils in planetary astronomy.

Most comets mark out a lonely orbit in the dark cold reaches of the distant solar system far away form the central solar fire, in regions called the Kuiper Belt or the Oort Cloud. Occasionally a passing star or planet will perturb on of the trillions of members which populate these bodies, and it will begin a long trek sunward. Over time, that elegant long orbit, sometimes lasting millions of years, becomes altered by planets such as mighty Jupiter, and the objects will adopt a celestial path that bring them back again and again, until they melt away form repeated passes around the hot sun or hit a planet.

On the night of April 3, 1867 Ernst Wilhelm Leberecht Tempel of Marseilles France while visually searching for comets, observed a faint object which moved over the course of a few nights; the sign of a comet. Christened Comet Tempel 1, the object is one of those regular visitors. A short period comet which orbits the sun every 5.5 years. And it is this comet which will play target to the cosmic dart fired from the Deep Impact mothership.

Jet Propulsion Laboratories makes it look too easy. What they're attempting to do is hit a bullet with a bullet. Comet Tempel 1 is traveling at almost 40,000 kilometers an hour compared to earth, about six miles per second, and the relative velocities between the comet and the impactor will be of this magnitude. July 4, at 1:52 AM EDT will be a violent date indeed for both comet and craft.

"We are really threading the needle with this one," said Rick Grammier, Deep Impact project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "In our quest of a great scientific payoff, we are attempting something never done before at speeds and distances that are truly out of this world."

Deep Impact carries an instrument platform featuring cameras, spectroscopes, targeting sensors, and recorders, so that we can see what the fireworks look like close up as well as imaging the comet nucleus. In addition to the impactor itself, there is another small second probe that will detach from the mother By taking a spectra of the flash, scientists hope to gain detailed insight into the material which makes up the comet and thus get a better handle on the formation and state of the early solar system.

Right now the mission is on time and five-by-five in the pipe; I'll provide some updates when the time arrives. But it should be a great start and a new twist for us science geeks to another star-spangled, explosive Fourth of July!