Just before seven o'clock on Monday morning next week, a 360kg lump of copper will smash into comet Tempel-1, in a collision that should answer many of the questions scientists have about the precise nature of comets.

Some of the first images of the collision will be captured by the UK-operated optical telescopes on Hawaii: UK school kids will be involved in the initial processing and analysis of these images, along with some of their counterparts on the Hawaiian islands.

NASA will be gathering data using telescopes on the islands as well, but their interest is in the spectroscopy. Although this is ultimately more interesting for scientists, Roche argues that to grab the public's interest, visual snaps are needed.

Deep Impact will send pictures back to Earth as well, but the fly-by section of the craft will have to shut down for an hour or so, just 800 seconds after impact, as it will be passing so close to the comet that its instruments will be at risk from the dust and gas.

"This is why the ground-based observation is going to be so important," Roche says. "We are hoping to get the before, during, and after pictures so we can put together an animation of the collision event."

Eager comet watchers should not get too excited, however. The early pictures are not likely to reveal much scientific data. "It looks like a fuzzy blob at the moment. Once deep impact hits, it will be a bigger fuzzy blob," Roche concedes.

However, UK scientists will be involved in spectroscopic analysis from two hours after the impact when the telescopes in Australia come online. At this point, the interesting question is what happens to the water vapour that will be released by the impact.

After another 13 hours, telescopes in La Palma will be in darkness, and will join the analysis of the impact's aftermath. They will watch the material expelled from the comet get blown into the comet's tail. The tail effectivey becomes a windsock for measuring the speed of the solar wind.

After two-and-a-half days, the Earth will be in a position to see the same area of space again. Taking new observations should give an indication of the behaviour of the solar wind in that particular area of the solar system.

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 impactor will make course corrections too, at 90 minujtes, 30 minutes and 12.5 minutes before impact. The probe is scheduled to hit at 06:52, and pictures from the fly by craft should arrive at NASA's HQ 7.5 minutes later.

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. ®