Ken Tapping, July 13, 2005

On 12^th January a spacecraft was launched from Cape Canaveral. Actually, it was a double spacecraft. The main vehicle was the size of a Volkswagen Beetle, and the other about the size of a stove. They had a unique mission, to make the first actual physical contact with a comet. Why do that?

Our Solar System was born about 4.5 billion years ago, when a huge cloud of cosmic gas and dust collapsed. We see these clouds all over our Galaxy and in others; one of the main projects at our observatory is to map them. Sometimes, a cloud becomes unstable and collapses. First it forms a spinning disc. The dust and gas particles coagulate into small grains, which then coalesce into bigger ones, then to grit, pebbles, and finally to planets. The middle part formed the Sun. The material we see around us on the Earth, Moon, Mars, and in meteorites, is material that has been changed. It is nothing like what we started with.

There is a lot of unused construction material on our Solar System building site, but it lies beyond the outermost planet, Pluto. However, on occasion, lumps of this stuff collide with others, getting deflected into the inner Solar System. As these lumps get closer to the Sun, the more volatile chemicals start to evaporate, so that a dull lump of cosmic construction material a few kilometres across develops a beautiful, glowing tail millions of kilometres long; it has become a comet. However, besides the spectacle, comets bring that primordial material within our reach.

That brings us back to those spacecraft. After a travel time of 172 days, they arrived in the path of the comet known as Tempel 1, which was hurtling up behind, overtaking the spacecraft at about 37,000 kilometres an hour. As the comet got closer, the two spacecraft separated. Using its jets, the smaller one manoeuvred itself exactly into the path of the comet, while the main one remained at a safe distance, watching. The small spacecraft, the "impact probe" bore some instruments and a camera, but was mainly a big lump of copper. As the comet bore down on the probe, the camera was used to take pictures of the comet's surface. Five minutes before impact, the camera showed what looked like a cratered potato. The last picture, taken a few seconds before the collision, showed two craters.

The impact produced a large explosion, which ejected clouds of comet material into space. Instruments on the surviving spacecraft observed the expanding cloud, making it possible to determine what comets are really made of. The reason the impact probe was made mainly of copper is so that any copper detected can be discounted, and everything else that turns up can be assumed to be comet. It has been assumed for a long time that comets are a soft, friable foam of ice, grit and chemicals. The impact explosion suggests that comets might be more like dirty ice cubes than dirty snowballs.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca