Deep Impact could blow a hole the size of a football stadium into Tempel 1. Credit: © NASA / P. Rawlings

What's the best way to see inside a comet? Shattering it with a chunk of metal could be the answer, if a NASA mission due to be launched next week goes to plan.

The Deep Impact spacecraft is due to take off from Cape Canaveral in Florida at about 18:48 GMT on Wednesday 12 January and will meet comet Tempel 1 roughly 134 million kilometres from Earth, just beyond the orbit of Mars. It will then release a 372-kilogram copper probe into the path of the comet.

On 4 July, the comet and probe will collide at about 37,000 kilometres per hour, blasting a deep hole in the comet's nucleus that should reveal what lies beneath the icy surface. The porous surface of the comet should shatter on impact, spraying detritus outwards to leave a crater that could be ten storeys deep.

The probe carries a camera that will relay pictures of its death dive back to the Deep Impact mother ship, which will stay at a safe distance of at least 500 kilometres to film the crash and use an array of instruments to analyse the debris. "We will be capturing the whole thing on the most powerful camera to fly in deep space," says astronomer Michael A'Hearn, principal investigator for the project, from the University of Maryland, College Park.

NASA's orbiting telescopes Chandra, Hubble and Spitzer will also watch the event, and the explosion generated by the $267-million mission should even be visible to telescopes on Earth.

Ancient snowballs

Comets are made of material left over from the formation of the Solar System, and astronomers say that studying the interior of Tempel 1 will allow them to step back more than 4 billion years in time, giving them clues about the chemicals that formed Earth and its neighbouring planets.

The probe, roughly the size of a washing machine, is made of copper to avoid contaminating the spray of ejecta caused by the impact. "Copper is an element that [no one] trying to work out the origin of the Solar System cares about," says science-team member Jay Melosh of University of Arizona, Tucson. "It's not characteristic of any particular process."

Astronomers know very little about the internal structure of comets, so tracking the impact should reveal much about their sturdiness. They are also unsure why comets eventually lose their characteristic tails, which are normally generated by the volatile chemicals that boil away from the surface as comets approach the Sun. Breaking into Tempel 1 could reveal whether these volatiles can become trapped by changes inside a comet's core.

Delayed impact

Because they are shrouded in huge clouds of gas, the relatively tiny nuclei of comets are best seen close up. Three previous space missions have investigated the surfaces of comets Halley, Borrelly and Wild 2. The European Space Agency launched its own comet mission on 2 March 2004, and expects the Rosetta craft to arrive at comet Churyumov-Gerasimenko in May 2014, when it will drop a lander to study chemicals on the surface in more detail.

Tempel 1 makes a convenient target for a faster mission because it passes through the inner Solar System once every five-and-a-half years. Discovered by French astronomer Ernst Tempel in 1867, it is about six kilometres wide.

Deep Impact's launch has already been delayed by problems with the craft's software, and a fault with the Boeing Delta II rocket that will boost it into space. Mission scientists hope that it will launch on 12 January, but it has until 28 January to leave Earth if it is to reach Tempel 1 in time to create the biggest 4 July fireworks display ever seen.