A story of comet death

Comet Linear fragmented before our eyes.

By BBC News Online science editor Dr David Whitehouse

Astronomers have been describing their most detailed view yet of the break-up of a comet.

Comet Linear came apart in July 2000, just a few days before its closest approach to the Sun. Many of its fragments were too small to see.

Data from Linear's demise has helped astronomers get a sense of the comet's component parts and where and how they may have come together.

"We think watching it come apart was a lot like seeing how the comet was put together in the first place played out in reverse," says Hal Weaver, of the Johns Hopkins University in Maryland in the United States.

Dirty snowball

Comets are mountain-sized balls of ice, dust and rock, which clumped together in the cold, distant regions from the Sun when the Solar System was forming.

"We have tended to think of cometary nuclei as being made up of roughly equal parts snow and meteoritic material, or 'dirty snowballs'," Weaver said.

Like building planets, but in reverse.

However, observations with the Hubble Space Telescope (HST) suggests that Linear appears to have been mostly meteoritic material with some ice.

According to Weaver, that could mean that the comet was initially formed relatively close to the Sun, perhaps in the region where Jupiter now orbits, where not as much ice would have condensed upon the meteoritic material.

Using images from the HST, astronomers detected about 16 large pieces among the debris.

Images of Linear's scattered fragments suggest it split apart because of its relatively rapid rotation. "It's possible that this rotation, combined with Linear's approach to the Sun, could have contributed to the comet's demise," Weaver says.

Small fragments

But there was a puzzle. After the break-up, most of the comet appeared to go missing. Estimates of the mass of Linear's nucleus, based on the amount of icy material on its surface, ranged as high as 300 billion kilograms (660 bn pounds).

However, the estimated total mass in the largest fragments observed after the disintegration event was only about 3 billion kg (6.6 bn lbs).

Comet Linear broke-up in July 2000

Spectroscopic observations of the comet, before and after the break-up, made by ground-based telescopes, show that most of the comet's original mass is now in pieces between one millimetre (0.04 inches) and 50 metres (164 feet) in size and is simply too small to be seen from Earth.

Analysis of the light from the comet also suggests that carbon-containing compounds were relatively scarce within it, hinting once more that it formed in a warmer environment than most other comets.

Its warmer origins would have allowed volatile carbon-rich molecules to escape into space. If other comets turn out to be more like comet Linear, researchers may have to reconsider their theory that comets delivered the precursor molecules for life to Earth.