On Friday, a tiny United States robot spacecraft will make a dramatic encounter in deep space.

The probe will sweep past a comet known as Hartley 2 and take detailed measurements and photographs.

The results that will be sent back by Nasa's Epoxi spaceship are keenly awaited - previously scientists have found the more they learn about comets the more baffling and mysterious they appear to be. They hope this time around some of those mysteries may be resolved.

In recent years, space scientists have discovered one comet that contains minerals created at extremely high temperatures, another that belches gases containing cyanides, one that has a surface fluffier than snow and another that has a firm surface peppered with craters.

"Comets were once thought to be roughly similar in shape and structure," said Dr Mark Bailey, of Armagh Observatory in Northern Ireland. "But the more probes that we fly past them, the more differences we find. Goodness knows what we are going to find this time."

Comets are formed from space rubble - dust, ice and small pieces of rock - left over from the making of our solar system billions of years ago and vary in size from a few hundred metres in diameter to tens of kilometres.

Most orbit the Sun at the very edge of the solar system and only occasionally sweep close to Earth. When they do, radiation from the Sun causes their surfaces to heat up and release gas and dust.

This material forms the comet's tail and creates their distinctive appearance.

Scientists believe that water and complex organic materials detected on comets may have played a key role in the evolution of life on Earth.

"Each of the four comets visited by spacecraft have looked very different from each other, in size and shape and detailed surface features," said Dr Lori Feaga, an astronomer at the University of Maryland, and a member of the Epoxi science team.

After Deep Impact swept past its first target, the mission was renamed Epoxi and sent on an orbit that will bring it to within 700km of Hartley 2 this week.

Already the mission has revealed fresh surprises. Last month, its instruments showed the comet was emitting a toxic gas called cyanogen whose output increased fivefold over eight days before slowly decreasing again. Scientists are still arguing over this data.

Many of these new mysteries may be solved in 2014 when the European Space Agency's Rosetta probe reaches the comet Churyumov-Gerasimenko and follows it, for more than a year, as it passes near the Sun. On the other hand, said Bailey, the mission may simply throw up even more puzzles.


Four space probes - one European and three American - have been sent on missions to rendezvous with comets.

* In 1986, Europe's Giotto probe passed close to Halley's comet and sent back photographs of a dark surface from which jets of glowing material were emerging.
* In 2001, Nasa's Deep Space 1 probe swooped close to Borrelly's comet and revealed a much smaller object with a more varied surface that appeared to be warm and dry.
* In 2004, Nasa's Stardust probe flew past the comet Wild 2 and returned to Earth with particles from its tail. These were found to include crystalline silicates that are formed only at high temperatures.
* In 2005, a comet called Tempel 1 was visited by Nasa's Deep Impact probe. The spacecraft fired a copper projectile into the comet to study the debris thrown up. It also carried out a detailed photographic survey of the comet which showed it was littered with craters that must have been caused by smaller objects striking it in the past.