Comets are relics from the origin of the solar system, carrying material about 4.5 billion years old. Sometimes described as “dirty snowballs,” they’re a mixture of rocks and ices of various compounds, with a surface layer altered by meteor impacts and solar heating. To expose the primordial material, a University of Maryland-led NASA mission named Deep Impact was launched in January of 2005. The mission’s two co-joined spacecraft (see drawing) were designed to journey to Tempel 1, where the “impactor” craft would separate, crash into the comet, and eject inner cometary matter into nearby space.

An outburst of sublimated gases on Tempel 1, as imaged by the Deep Impact probe about ten days before impact. (Image credit: NASA/JPL-Caltech/UMD/T. Farnham)

The Deep Impact probe, showing the impactor and the flyby spacecraft;. HGA: High-Gain Antenna; MRI: Medium-Resolution Instrument; HRI: High-Resolution Instrument (image courtesy of NASA).

The orbit of comet Tempel 1, located in the region between the orbits of Mars and Jupiter (image courtesy of Tony Farnham/University of Maryland)

A comet’s nucleus, coma, and tail (image courtesy of NASA)

Most comets inhabit the two great “clouds,” one in the vicinity of the orbit of Neptune—where Tempel 1 originated—and one extending hundreds of times further out than the orbit of Pluto. The inner cometary material ought to be very similar to the material of the outer planets. The comets we see were presumably disturbed by the gravity of some other star and now move in elliptical (non-circular) orbits, such as the orbit of Tempel 1 shown in the drawing. This comet, the target of Deep Impact, orbits the sun every five and a half years, moving between the orbits of Mars and Jupiter .

As a comet’s nucleus (see the image of comet parts) nears the sun, its temperature rises because of the absorption of solar radiation. The comet’s ices sublime—that is, convert directly from solid to gas—spitting out gas in jets that drag dust and solids along to form the coma. The series of images shows one of these outbursts in Tempel 1, captured by Deep Impact about a week before their head-on collision.

To determine the chemical composition of the nucleus, astronomers study the spectral lines in the light given off by the coma. This isn’t as straightforward as it sounds, though, because after these molecules sublime, they promptly dissociate (split) due the absorption of sunlight, so only the products of the dissociation can be identified. What the comet is made of must be inferred from these by-products, unless the spectrometer is very close to the nucleus.

In recent years, probes have captured detailed images of comets, but getting information about the comet’s chemical composition has been much more difficult, and that’s where Deep Impact comes in. It was designed to place a probe in the path of comet Tempel 1 so that the resulting crash would break through the outer layer of the comet. And it carried an infrared spectrometer designed to observe the comet’s spectra both before and after impact.

On July 3, 2005 , twenty-four hours before impact with Tempel 1, the washing-machine-size impactor and the flyby craft separated, with the impactor relying on its own computer and steering jets to hit the target. The 6 km diameter comet was then 860,000 km (about twice the Earth-moon distance) away but closing fast, at 10 km/s (23,000 mi/hr). After three bursts from the steering jets, the impactor scored a direct hit. The crash and its aftermath were imaged by a network of telescopes both on Earth and in space.