Three comets are poised on our celestial doorstep this week, and backyard astronomers are trying not to be seduced by predictions that all three could become as easy to see as the Big Dipper.
"The vast majority of these comets are a lot of fizzle," said Joe Rao, a Space.com columnist and lecturer at the Hayden Planetarium in New York. All the same, he said, "it's an exciting time for those people who especially like comets."
The last time Marylanders had a chance to see a naked-eye comet was in 1997, when Hale-Bopp became visible even in light-polluted urban areas. And that came after an equally spectacular appearance by Comet Hyakutake in 1996.
The current visitors, NEAT, LINEAR and Bradfield, could present the first opportunity to see two or more bright comets at the same time since comets Brooks and Beljawsky graced the sky in 1911. "It certainly doesn't happen very often," Rao said.
Comets are balls of rock dust and frozen gas, a mile to dozens of miles across. They formed as the solar system condensed more than 4.5 billion years ago.
Some, kicked loose from the farthest reaches of the sun's realm, sail around the sun and off again into deep space, never to return. Others, like Hale-Bopp and Halley, have fallen into more compact orbits that bring them back with predictable regularity.
As comets near the sun, their frozen gases vaporize and form a bright halo or "coma" of dust and gas around the nucleus. The material is then swept out into a characteristic "tail" by the solar wind.
Naked-eye, or "bright," comets appear every 20 years or so, on average. Fainter comets are common - astronomers currently can see as many as 13 with binoculars or telescopes.
Two of the approaching comets have been expected for years, and astronomers predicted they would get as bright as magnitudes 2 or zero.
Although it sounds counterintuitive, the lower the magnitude number, the brighter the object. The dimmest objects visible to the unaided eye are magnitude 6 or 7. Most Big Dipper stars are magnitude 2. The brightest star, Sirius, has a magnitude of minus 1.5. Planets vary, but Venus at its brightest is a minus 4.4.
Astronomers' highest hopes rest with Comet NEAT, officially designated "C/2001 Q4." It was discovered in August 2001 by the automated Near Earth Asteroid Tracking program, which has found many new comets.
NEAT is already a naked-eye object for observers in the Southern Hemisphere. It has been brightening steadily as it nears perihelion late in May - its closest approach to the sun.
But there are no guarantees. NEAT is probably making its first pass around the sun, and first-timers are notorious fizzlers. Comet Kohoutek was a case in point, fading to near-invisibility in 1974 after months of news stories about the "Comet of the Century."
Once forecast to reach magnitude 1 or better, NEAT is expected to be no brighter than a 2. "A second-magnitude comet is still a relatively bright object," Rao said. NEAT also has an unusually large coma, and it will be a mere 30 million miles from Earth on May 7 - closer than Mars during its brilliant appearance in August.
Observers can start searching for it May 6, low in the southwestern sky after sunset. Look for the three stars of Orion's belt. They'll point you to the left, to the bright star Sirius.
"If you can find Sirius," Rao said, "you should be able to see NEAT off to the left and above Sirius, hopefully second magnitude and sporting a conspicuous dust tail," pointing upward.
The comet will move higher in the sky each night in May, growing fainter daily before disappearing after midmonth.
Comet LINEAR (C/2002 T7) was first spotted in October 2002 by professionals with the Lincoln Near Earth Asteroid Research program at the Massachusetts Institute of Technology. LINEAR, too, was brightening as it plunged toward its closest approach to the sun Friday.
Under ideal conditions, it's already visible low in the eastern sky just before dawn. But "its situation is actually worsening," said Gary Kronk, an amateur astronomer in St. Louis and the author of four books on comets and meteor showers.
LINEAR will remain within 5 or 6 degrees of the eastern horizon before dawn through May 6, then reappear low in the southwest after sunset from late May to late June. But haze typically obscures the horizon, And daylight hours are lengthening. "As each day goes by, the comet is a little deeper into twilight," Kronk said.
Once forecast to reach a striking magnitude of zero, "it now looks like it may not even break magnitude 2 - still a great comet but a far cry from what we were expecting," he added
The wild card this spring is Comet Bradfield(C/2004 F4), first spotted only a month ago from Australia by the octogenarian comet-hunter William A. Bradfield.
On April 17, Comet Bradfield passed the sun inside Mercury's orbit, and many astronomers expected it to break apart. Instead, it erupted with a brilliant plume of dust and gas. It was too close to the sun to be seen directly. But astronomers watching online, via NASA's sun-gazing Solar and Heliospheric Observatory (SOHO), estimated its magnitude at minus-3, as bright as Jupiter.
Comet chasers have been up before dawn ever since, watching for Bradfield to emerge from the sun's glare. If the comet hasn't broken apart and vanished, stargazers can look for it low in the east-northeast, 60 to 90 minutes before sunrise, any morning this week.
"Every day that passes, it will jump up in the sky, higher and higher," Kronk said. Most astronomers expect it will reach a naked-eye magnitude of 3 or 4. But binoculars will boost the chance of spotting it.
Even if it survives, Bradfield will fade quickly next month as it races away from the sun and the Earth.