June 27, 2005

Hubble spies icy comet Tempel 1 belching dust

WASHINGTON (Reuters) - The volatile nucleus of comet Tempel1 blew off a stream of dust that was captured in an image bythe Hubble Space Telescope, scientists said on Monday.

The dust jet could be a preview of what astronomers see onJuly 4, when NASA's Deep Impact space probe is set to collidewith the comet, giving the first glimpse inside the heart of acomet, the scientists said in a statement.

The collision on the comet could cause a similar dust plumeon Tempel 1's surface.

Hubble captured the images when it was 75 million milesaway from Tempel 1. The orbiting telescope's views complementclose-up pictures being captured by Deep Impact's cameras as itspeeds toward the comet.

The two images snapped by Hubble were taken seven hoursapart on June 14. One shows a view of the comet before theoutburst; the other shows the jet, which extends about 1,400miles.

Comets often show bursts of activity, but astronomers donot know why. It might be because Tempel 1 is moving closer tothe sun and the increased heat could have opened up a crack inthe comet's crusty surface, allowing trapped dust and gas toescape.

Another theory is that part of the comet's crust lifted offthe nucleus because of the pressure of heated gases beneath thesurface, and the crust may have quickly crumbled into smalldust particles, producing a fan-shaped jet.

Astronomers hope the July 4 smashup will release moreprimordial material trapped inside the comet, which formedbillions of years ago.

Comets are thought to be "dirty snowballs" made up of iceand rock.

Images and more information are available online athttp://hubblesite.org/news/2005/16 andhttp://www.spacetelescope.org.