Borrelly: A mottled mountain flying through space
The first close-up images of Comet Borrelly - a visitor from the cold and dark Kuiper Belt beyond the planets - confirm that its nucleus is long, dark, mottled and dry.
Borrelly's nucleus is only the second to be seen from close quarters (the first was Comet Halley's in 1986).
The images were gathered by the Deep Space 1 (DS1) spacecraft, which passed within 2,000 kilometres (1,240 miles) of the comet in September 2001.
Given the primitive nature or comets, the DS1 observations could shed light on the conditions that existed during the formation of the Solar System about four and a half billion years ago.
Mottled and dry
Data collected by the combination camera/spectrometer on board DS1 as it flew past Borrelly show the nucleus to have a dark, rough surface with little water or hydrated minerals.
The nucleus is about eight km (five miles) long, with a mottled appearance. Numerous faults, ridges and elevated areas can be seen but no craters.
The lack of craters is attributed to the weak surface that is constantly changing.
The pictures also show jets and fans of dust and gas actively vaporising from the comet's surface.
Scientists believe that the dryness of the nucleus is a consequence of its relatively high temperature - about 25 deg C - caused by solar heating, which is likely to have driven off any water.
Relic of the past
It is believed that comet Borrelly is a member of the minority of comets that originate in the Kuiper Belt, just beyond the orbits of the most distant planets.
Its composition reflects the cold conditions in which it formed four and a half billion years ago.
Studying cometary composition is important because it is believed that comets delivered water to the primitive Earth. They may even have seeded the Earth with organic material - the building blocks of life.
The only other comet to be seen so close up, Comet Halley, is somewhat different to Borrelly. Halley is an intermediate comet that originally came from the much more distant Oort cloud, which stretches partway to the nearest star.
In 1986, astronomers were amazed at the pictures of Comet Halley's nucleus returned from the Giotto spaceprobe. They showed Halley to be dark, uneven and riven by jets of gas erupting from beneath the surface.
The latest Borrelly research is described in the journal Science.