Hubble probes a comet's heart
Posted: Thursday, November 15, 2007 2:30 PM by Alan Boyle
A. Dyer / Alberta, Canada |
Comet Holmes appears in all its fuzzy glory with a faint tail trailing off to lower right
in a picture taken by Canadian amateur astronomer Alan Dyer on Nov. 1.
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Comet Holmes is turning into the star of the night sky, thanks to a huge cloud of dust that makes it look more like a cosmic fuzzball than a dirty snowball. But all that dust has obscured the things you usually expect to see in a comet, such as a tail and a bright nucleus. Now the Hubble Space Telescope has cut through the clouds to make out the structure of the comet's dusty heart.
The comet was a run-of-the-mill celestial traveler until Oct. 23, when the object suddenly flashed to a million times its previous brightness. Scientists assumed that a chunk of the nucleus - the "dirty snowball" at the heart of every comet - had broken off and disintegrated. (Check out this mini-graphic to learn more about the anatomy of a comet.)
The debris from the breakup spread out to become a hazy cloud surrounding the nucleus, and astronomers say the sunlight scattered by all that ice and dust is responsible for Comet Holmes' fuzzy brightness.
Over the past three weeks, Comet Holmes' cloud of haze has spread out to a diameter of 900,000 miles (1.4 million kilometers), which is wider than the diameter of the sun, astronomers at the University of Hawaii Institute for Astronomy noted this week. That's not unprecedented for a comet, but this cloud has such a spherical shape that it's easy to imagine the comet as an insubstantial, ghostly star haunting the constellation Perseus.
H. Weaver / JHU-APL / NASA / ESA |
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This Hubble view shows a bow-tie structure in the
cloud surrounding Comet Holmes' nucleus. Click on
the picture to see Hubble's view in a wider context.
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Hubble has been periodically checking in with Comet Holmes, and its latest picture - taken with the Wide Field Planetary Camera 2 on Nov. 4 and released just today - shows an intriguing bow-tie shape at the core of the dust cloud. "We may finally be starting to detect the emergence of the nucleus itself," Hal Weaver of the Johns Hopkins University Applied Physics Laboratory, who led the Hubble investigation, said in today's news release.
The bow tie indicates that about twice as much dust lies along the east-west direction as along the north-south direction - hinting at the genesis of the breakup. In Hubble's Oct. 29 image, astronomers could see three spurs of dust emanating from the center, and an Oct. 31 image revealed an outburst of dust just west of the nucleus.
Ground-based imagery reveals that Comet Holmes' cloud is offset somewhat from the nucleus. That serves as an additional hint that a large piece broke off and disintegrated after moving some distance away.
Although the comet doesn't yet have a well-defined tail, you can see one forming to the lower right of the nucleus in an image captured by Canadian amateur astronomer Alan Dyer. In fact, comets can have two tails - one composed of dust, the other of ionized gas. This picture of Comet Hale-Bopp provides another classic view of the double-tail effect.
For additional perspectives on today's Hubble picture, click on over to the European Space Agency's Hubble site as well as Johns Hopkins University's Applied Physics Laboratory.
Comet Holmes is now 149 million miles (238 million kilometers) from Earth, and that means Hubble should be able to spot features as small as 33 miles (54 kilometers) across. Even that resolution won't be fine enough to resolve details of the nucleus itself. But there are other ways to unlock the secrets of a comet's heart.
Before last month's brightening, astronomers estimated that the nucleus was just 2.1 miles (3.4 kilometers) across, based on its brightness. Once the dust clears a bit more, they should be able to get a fix on how much of the nucleus is still left - and fill out the story behind the breakup.
In the meantime, Comet Holmes should be visible to the naked eye for weeks longer. It helps to get away from city lights and peer into clear skies. Bring along a pair of binoculars or a small telescope if you can. This weekend, the annual Leonid meteor shower reaches its peak - and seeing the comet in all its fuzzy glory provides just one more reason for getting out and looking up.
Update for 7:45 p.m. ET: JHU's Hal Weaver added some new angles to the tale of Comet Holmes during an interview with Reuters. He said astronomers suspect that volatile ices within the nucleus heated up during the comet's swing around the sun, building up pressure that was released months later by an explosion.
The explosion might have sheared off a pancake-shaped slab of the nucleus that crumbled to dust and created the bright cloud of debris.
Weaver noted that Comet Holmes went through a similar outburst in 1892. Astronomers thought that earlier display might have been caused by a collision with another celestial object - perhaps another comet, or a small asteroid. The collision was thought to have sliced off a piece of Holmes' nucleus that slammed back onto the main body, throwing up a spray of dust.
"Now we step forward to 2007, and the same thing is happening again," Weaver told Reuters. "It indicates that the [earlier] hypothesis is incorrect."
