When NASA's Deep Impact probe hits the comet Tempel-1 on 4 July, there are several possible outcomes.

The impactor will be released around 24 hours ahead of its scheduled crash. On Monday morning at 04:52 (GMT) the fly-by craft will make a minor course correction to make sure it misses the comet's nucleus.

The most likely result of the crash is that the impact of the 360kg probe will form a gravity-controlled crater around 20-30m deep, and around the size of a football stadium. It might also form a muc small compression-crater, which would result in the release of far less material.

However, there are some other scenarios that are possible, although far less likely. The probe could split, or shatter the nucleus; or it could even pass straight through the middle.

Dr. Andrew Coates from the Mullard Space Science Laboratory at UCL, speaking today at the British Festival of Space 2005, in Birmingham, said that the kind of collision really doesn't matter.

"We know so little about comets that almost anything we learn from the collision will be useful," he said.

"What is definite is that material will be evolved from the comet. In 15 minutes, approximately one month's worth of gas will be released from Tempel-1."

There is, however, no danger that the comet will be sent off course and hurtling towards Earth. Coates explains that the collision is the equivalent of a mosquito ramming into a Boeing 767 plane, so although it will be a powerful impact, it will have little or no effect on the path of the comet.

"We used to be afraid of comets. The dinosaurs should have been afraid of comets. Now it is the comet's turn to be afraid," Coates said. ®

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