NASA's Spitzer tries to unravel mysterious comet explosion

Washington, Oct 15 : NASA's Spitzer Space Telescope has deeply observed comet Holmes to find out why it suddenly exploded in 2007.

Observations taken of the comet by Spitzer deepen the mystery, showing oddly behaving streamers in the shell of dust surrounding the nucleus of the comet.

The data also offer a rare look at the material liberated from within comet Holmes' nucleus, and confirm previous findings from NASA's Stardust and Deep Impact missions.

"The data we got from Spitzer do not look like anything we typically see when looking at comets," said Bill Reach of NASA's Spitzer Science Center at the California Institute of Technology, Pasadena, California.

"The comet Holmes explosion gave us a rare glimpse at the inside of a comet nucleus," he added.

Every six years, comet 17P/Holmes speeds away from Jupiter and heads inward toward the sun, traveling the same route typically without incident.

However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes exploded as it approached the asteroid belt, and brightened a million-fold overnight.

In an attempt to understand these odd occurrences, astronomers pointed NASA's Spitzer Space Telescope at the comet in November 2007 and March 2008.

By using Spitzer's infrared spectrograph instrument, Reach was able to gain valuable insights into the composition of Holmes' solid interior.

In November of 2007, Reach noticed a lot of fine silicate dust, or crystallized grains smaller than sand, like crushed gems.

"Comet dust is very sensitive, meaning that the grains are very easily destroyed," said Reach. "We think the fine silicates are produced in these violent events by the destruction of larger particles originating inside the comet nucleus," he added.

When Spitzer took spectra of the same portion of the comet again in March 2008, the fine-grained silicate dust was gone and only larger particles were present.

"The March observation tells us that there is a very small window for studying composition of comet dust after a violent event like comet Holmes' outburst," said Reach.

According to Jeremie Vaubaillon, a colleague of Reach's at Caltech, pictures snapped from the ground shortly after the outburst revealed streamers in the shell of dust surrounding the comet.

Scientists suspect that they were produced by fragments escaping the comet's nucleus after the explosion.

When Spitzer imaged the same streamers in March 2008, they were surprised to find them still pointing the same direction as five months before, even though the comet had moved and sunlight was arriving from a different location.

"We have never seen anything like this in a comet before. The extended shape still needs to be fully understood," said Vaubaillon.

ANI