Comet's impact likely to be masked by cloud

A tiny probe is hurtling through space on a collision course with a potato-shaped comet but the "deep impact" smash is likely to be obscured for New Zealand astronomers by cloud.

The Deep Impact mission, which scientists say they named before the 1998 Hollywood movie about a comet threatening life on Earth, will be followed by millions of space enthusiasts round the world on the internet and snapped by the Hubble orbiting telescope.

New Zealand astronomer Grant Christie said no one knew exactly what would happen at 5.52 this evening when the table-sized, 350kg copper-fortified "impactor" smashed into Tempel 1 at 37,000 km/h.

"At the moment the best money is on seeing a momentary flash but it will be a challenge for amateur astronomers to see it without electronic aids," he said.

The collision has been likened to a fly slamming into a jumbo jet but the interior of a comet has never been explored in this way before.

"We don't know the mechanics of comets, whether they are like a loosely-compacted snowball or they are quite solid, and that's the purpose of this impact," Mr Christie said.

Comets, a mix of ice and dust particles, are debris left over from the formation of the solar system. The material deep inside Tempel 1 has remained untouched for 4.5 billion years.

Scientists are expecting a cone-shaped plume of debris to sheer off after impact, adding to knowledge of how the solar system formed.

The impactor is expected to blast a hole about the size of Eden Park into Tempel 1's surface.

The "Deep Impact" spacecraft that carried the copper impactor will have 13 minutes to capture images and data before it weathers the blizzard of particles produced from the explosion.

Keen amateurs will be aiming their lenses at the collision 133.6 million kilometres away in the constellation of Virgo in the northern sky near the star of Spica.