For the next few weeks, millions of stargazers around the world may get a rare treat: seeing two comets in the night sky without using telescopes or binoculars. One comet is called "NEAT" an acronym for the "Near Earth Asteroid Tracking" observatory in Hawaii that first spotted the comet. The other comet bears another acronym: this one is called "LINEAR" for the Lincoln Near Earth Asteroid Research project in New Mexico.
NEAT and LINEAR will join a very small group of comets that have been visible to the naked eye, including the famed Halley's Comet, whose 76-year orbit around the sun last brought it near the Earth in 1986. Halley's comet was disappointingly dim, but as one astronomer quipped, "Comets are like cats. They have tails and they do precisely what they want."
Comets are sometimes described as "dirty snowballs" or "cosmic icebergs." But University of Maryland astronomy professor Lucy McFadden says comets and their asteroid cousins- are actually bits of rock and ice, relics of cosmic history:
"Comets are part of the solar system. They're in orbit around the sun as is everything else in the solar system," she said. "But they're special because they're from the outer solar system. They formed 4 ? billion years ago at the very outer edges of the solar system where, you can imagine, it's very cold. When they come close to the sun which they do because they're in elliptical orbits- the sun heats them up and the frozen ices go through a process called sublimation from a solid to a gas. We see fragments, dust and gas that are drawn off from the comet and driven by the heat of the sun."
Comets were first recorded by Chinese astronomers in 240 B.C. and depicted in the famed 11th century Bayeux Tapestry in Normandy, France. In modern photographs, comets are shown as bright darts against a dark sky. They are sometimes confused with meteors, but unlike meteors, comets don't "shoot" across the sky like fast-moving "stars," as Professor McFadden explains.
"They appear in the sky as stationary, bright spots with dramatic tails that look like they should be moving. But when you see them, they're not moving perceptibly to us. When people see things that are remarkable, and they don't understand them, they leave an indelible mark in people's minds," she said.
Indeed, some ancient cultures believed the sight of a comet meant the end of the world or some other ominous portent. But Ms. McFadden who has an asteroid named after her because of her comet and asteroid research- says she was awestruck seeing her first comet.
"The first comet I saw was when I was in college in 1973: Comet West, which I saw when I was on a geology field trip in Arizona. The neat thing about comets is that, yes, they do look like the pictures. You see comets either as the sun is setting or as the sun is rising and all of a sudden, it appears because the contrast between the dark or darkening sky and the emitting scattered light from the dust and gas of the comet. It's very dramatic, with this bright spot and a huge tail that shoots up many degrees above the horizon where you're looking. It's breathtaking," she said.
As an indication of how rare it is to see a comet with the unaided eye, University of Maryland astronomy student Sandy Grabowski of Annapolis, Maryland, has seen only one comet. "I think the first comet I remember seeing was in middle school. It was the first time I saw anything like it and before I knew much about astronomy. I remember waking up early one morning; my mom woke me up to see it. It was amazing - like a picture in a book."
The 1995 comet Ms. Grabowski saw was called "Hale-Bopp," named after amateur astronomers Alan Hale of New Mexico and Thomas Bopp of Arizona. Indeed, spotting comets and having them named after the discoverer used to be one of the few ways that amateur astronomers could make a real contribution to science. But some observatories have installed powerful new automated telescopes that can scan the skies tirelessly and detect comets much sooner than the average comet hunter. And that's cut down on the number of amateur discoveries. Gary Nalven of New York City, another astronomy major at Maryland, says, half-kidding, that auto-tracking has taken some of the fun out of astronomy.
"I'm always disappointed that I wasn't born 500 years ago. It seems there was so much more to be discovered back then. So much of the sky hadn't been catalogued, so much of even the earth hadn't been discovered. So right now, discovering new objects in the sky is still amazing to me."
Although newly-discovered comets NEAT and LINEAR were found by auto sky-tracking telescopes, Maryland professor McFadden says some sky watchers in more remote areas still have a chance at finding new comets. "I think the statistics are showing that the automated telescope programs are finding more comets. That's because they can find them when they're further away and fainter. They still do miss [some]. Look at how Mr. [Bill] Bradfield discovered one a few weeks ago that was missed by the automated systems. Mr. Bradfield was in southern Australia in the Southern Hemisphere, where there aren't as many automated programs. So an amateur has a better chance of finding a comet in the Southern Hemisphere now," he said.