A mysterious and explosive flare-up in a normally dim visitor from outer space is giving Maryland stargazers their first good look at a naked-eye comet since 1997.
Usually far too faint to see without a telescope, Comet 17P/Holmes unexpectedly blossomed in the evening sky a week ago, expanding into an odd, round shape that looks like neither a star nor a comet. And it's got scientists baffled.
"Consider the [comet's] nucleus," said Brian Marsden, a senior astronomer at the Smithsonian Astrophysics Observatory in Cambridge, Mass. "If it's only a mile and a half or two miles across, it's expanded into something in the tens of thousands of miles now - that's a tremendous explosion."
"I know of no other comet that has flared up in quite this way," he added. "This is really a remarkable event."
The flare-up has made the comet as bright as the stars of the Big Dipper, delighting backyard astronomers around the world.
"Naked eye, it looks like a new star in [the constellation] Perseus. ... It's easy to see, even with the moon up," said Tim Hickman, an amateur astronomer who snapped a photograph of Holmes over the weekend from his backyard in Timonium.
"In binoculars, you can see it is a fuzzy ball and not a star-like point," Hickman wrote in an e-mail to The Sun.
Through his 12.5-inch telescope, he added, "it has a small, star-like nucleus with a large glowing ball around it, inside of which is an off-centered bright area which could be the beginning of a tail."
Canadian astronomers revealed this week what they called "tantalizing" photographic evidence for the beginnings of a tail on Comet Holmes. But no one is sure how the comet's appearance will evolve in the coming days.
On Sunday, Baltimore's street-corner astronomer, Herman Heyn, introduced Inner Harbor tourists to the comet as it rose above the World Trade Center.
"People were impressed," he said. "A lot of them said, 'This is the first comet I've ever seen.' And I didn't have to apologize that it doesn't have a tail."
As long as it stays bright, and skies stay clear, Comet Holmes can be seen this week in the northeastern sky after sunset, rising higher as the night advances.
Stargazers should look below the familiar W-shaped constellation Cassiopeia, and above - and slightly to the right of - the bright star Capella, which is the brightest star above the northeast horizon this time of year.
A sweep of that stellar neighborhood with binoculars should easily reveal Holmes, a gray blob among sharp pinpoints of light.
The Maryland Science Center's observatory at the Inner Harbor will turn its telescope on Holmes during its open house Friday, said observatory manager Rich Stein.
"We looked the other night with the Alvin Clark refractor [the observatory's 8-inch telescope], and it looked very nice," he said. "I was able to see the small, starlike point, then the inner bright disk and the outer, fainter disk."
Holmes was named for its discoverer, British astronomer Edwin Holmes, who spied it in 1892. It circles the sun once every seven years, but it's normally so dim that astronomers have problems finding it.
Scientists believe a flare-up in 1892 similar to this month's led to its discovery. Holmes was observed in 1899 and in 1906, then disappeared until 1964, when astronomers acting on a prediction by Marsden rediscovered it.
Holmes made its most recent swing around the sun in May and is moving away, a situation that makes its sudden flare-up even harder to explain, Marsden said.
On Oct. 23, it was a mere 17th-magnitude object, thousands of times too faint to be seen with the naked eye. The next night, astronomers were astonished to see it had brightened to 7th magnitude. It quickly reached magnitude 2 or 3 - as bright as the stars of the Big Dipper.
"I've never seen anything like it," Marsden said.
It's not unusual for comets to flare as they approach the sun and their ices warm up, vaporize and escape. The gas and dust are swept into a broad halo, or "coma," and often away from the sun to form a classic comet "tail." That makes them much more reflective and brighter as seen from Earth.
But Holmes, in 1892 and last week, was five months past its closest approach to the sun. And that was more than 200 million miles at its nearest, or twice Earth's distance from the sun.
"What it all means, we don't know," Marsden said.
The late comet expert Fred Whipple once theorized that Holmes might have had a satellite that crashed into it in 1892, causing that year's odd flare-up.
But even if he were right then, Marsden said, that satellite is long gone. And yet the same thing has happened again, and at the same, outbound leg of Holmes's orbit.
Whatever is going on, he said, "it has to be internal to the comet. But what is it? You can imagine a dirty snowball that has cracks in it, and maybe they open up and a little solar radiation gets inside and lands on some ice, which vaporizes pretty quickly. But somehow it's doing it in a very explosive manner."
"We're at a loss," he said.
For an online look at the comet's flare-up, visit http:--antwrp.gsfc.nasa.gov/apod/image/0710/holmes_allen_big.gif.