Close encounter with a comet set for Independence Day

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

Scientists are preparing 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 - Independence Day in America- and it will be observed by astronomers around the world who hope it will shed light on the origin of the planets.

By 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 four billion years ago.

It has taken six months for the £140m Deep Impact probe to travel the 268 million miles from Earth to comet Tempel 1. It will take a further 24 hours for its missile to make the final trip from mother craft to the impact site. Comet Tempel 1 is nearly nine miles long and 2.5 miles wide and scientists insistits 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.

The copper block, measuring about 3ft in diameter, will be travelling at 22,700 miles an hour and is expected to knock a hole the size of a football stadium into the comet's surface.

Cameras on the projectile will record the event and instruments on the Deep Impact mother ship will analyse the gases and debris from the collision.

Michael A'Hearn of the University of Maryland, the mission's principal investigator, said so little was 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. 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 capable of operating independently of the mother spacecraft by making fine adjustments to its flight path as it approaches its moving target. After the impactor is released, the mother ship will makes its closest approach to the comet at the relatively safe distance of 300 miles, close enough to monitor the collision but far enough away to avoid being destroyed.

The impact crater should revealmaterial 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 the Earth's oceans were created by an impact with one or more comets billions of years ago. One of the aims of the 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, discovered by the German astronomer Wilhelm Tempel in 1867, orbits the Sun once every five and a half years.

Dust from a comet's nucleus reflects sunlight and generates the cometary tail. Over time, some comets 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.