Close encounter with comet set for Independence Day

Scientists are making the final preparations to shoot a comet with a self-guided copper missile travelling at 100 times the speed of a bullet.

The explosive encounter is set for 4 July - American Independence Day - and it will be observed closely by astronomers around the world who hope that the event will shed light on the origin of the planets.

By deliberately firing a relatively large object into the icy interior of a comet, scientists hope to dig out and analyse the primordial material that was around when the Solar System formed more than 4 billion years ago.

It has taken six months for the $358m Deep Impact space probe to travel the 268 million miles from Earth to comet Tempel-1. It will take a further 24 hours for its copper missile to make the final trip from mother craft to impact site.

Comet Tempel-1 is nearly nine miles long and 2.5 miles wide - about half the size of Manhattan - and scientists insisted that its course around the Sun will remain unaltered in the collision with the half-ton bullet.

The mission is named after the 1998 film in which a former astronaut, played by Robert Duvall, attempts to stop a massive comet colliding with Earth, yet there is no risk of the controlled crash causing a similar collision with Tempel-1, scientists emphasised.

Nevertheless, the copper block, which measures about three feet in diameter, will be travelling at 22,700 miles an hour and is expected to pack a punch big enough to knock a hole the size of a football stadium into the comet's surface.

Cameras on board the projectile will record details of the event in the seconds before impact and a further set of instruments on the Deep Impact mother ship will analyse the plumes of gases and debris that are expected to result from the collision.

Michael A'Hearn of the University of Maryland, the mission's principle investigator, said that there is so little known about the nature of comets that almost any scenario is possible, from the impactor creating a relatively small dent in the comet's surface to causing its total disintegration.

"We don't have a clue what's going to happen, to be honest. My personal estimate [for the crater size] is at the large end of a large-size football stadium, perhaps 150 metres in diameter. It could be larger," Dr A'Hearn said.

The copper impactor is in effect a battery-powered spacecraft that is capable of operating independently of the Deep Impact mother spacecraft by making fine adjustments to its flight path as it approaches its moving target.

After it is released, the flyby mother ship will make its closet approach to the comet at the relatively safe distance of 300 miles, which is close enough to monitor the collision but far enough away to avoid being destroyed by the blast.

The impact crater is expected to be several stories deep and should reveal pristine cometary material that has remained untouched since the birth of the planets when the Solar System formed.

"Only the internal material of a comet is unchanged from the beginning of the Solar System. But there are no data on the interior, and that's what we're hoping to solve with Deep Impact," Dr A'Hearn said.

Comets are considered to be giant "dirty snowballs" of ice and dust and their tails are caused by trails of frozen debris which spew out away from the Sun.

Comets could contain so much water in the form of ice that it is possible that the Earth's oceans were created by an impact with one or more comets billions of years ago.

One of the aims of the Deep Impact mission is to assess the nature and quantity of water that comet Tempel-1 may possess to assess the plausibility of this theory.

Tempel-1, which was discovered by German astronomer Wilhelm Tempel in 1867, orbits the Sun once every five and a half years and often passes through the inner Solar System, although its path takes it well clear of the Earth's orbit around the Sun.

Dust driven from a comet's nucleus reflects sunlight and generates the typical cometary tail that has been viewed throughout history as an ill omen when they can be seen from Earth.

Over time, some comets can become less active and may even appear dormant. Scientists would like to know whether this is because they exhaust their supply of dust and gas or whether it is sealed inside the inner core of the nucleus.

They would also like to learn about the structure of a comet's interior and how it differs from its surface. The controlled impact with Tempel-1 promises to provide answers to all these questions, Nasa said.

- INDEPENDENT