A green beast with a gossamer tail is charging towards the sun. If it survives, we could soon see the sky show of the century
Since its discovery last year, comet ISON has been heating up as it heads towards the centre of the solar system. It is now on track to swing behind the sun and come within 1.2 million kilometres of the solar surface on 28 November.
As comets get close to the sun their ices vaporise, releasing gases and dust that make the objects shine with reflected light. This will be ISON's first trip to the inner solar system, and its pristine ices had many observers initially hopeful that it would release enough material to outshine the full moon.
But the comet keeps wrong-footing sky-watchers. Until last week, it looked like ISON might be a total dud. "It wasn't quite as bright as we would have hoped, and it didn't appear to be brightening up at the rate that we would have liked," says Karl Battams of NASA's Comet ISON Observing Campaign.
ISON displayed dramatic changes last week, becoming visible to the naked eye as a faint smudge in eastern skies just before sunrise. A few days later, an outburst caused the comet, a ball less than 4 kilometres wide, to sprout a spectacular, multi-part tail that extends 16 million kilometres – 12 times the diameter of the sun.
But there is a chance that the comet is being ripped apart as a result of the sun's heat. The outburst that lengthened the tail could have been the result of the core breaking apart, or it could have been triggered by fractures on the surface releasing jets of fresh ice and dust, says Battams. It will be difficult to know for sure until the comet comes into view of the sun-watching STEREO A spacecraft on 21 November.
"When comets fragment, it's not like they explode and send fragments in all directions. They just fall apart and the chunks slowly drift," says Battams. However, images taken from Germany already show wing-like features in the comet's atmosphere that hint at two fragments drifting away from the rest of the core. If ISON disintegrates before it comes back into view in early December, this week's display may be the best show we will get.