A greenish comet named 103P/Hartley 2 flew within 11 million miles of the Earth on Wednesday, one of the closest approaches by a comet in centuries. But hardly anyone is likely to see it until a Maryland-led NASA mission sends a spacecraft past on Nov. 4.
While the comet has been visible in the northern sky for weeks, it has been a difficult target for backyard stargazers, especially in light-polluted suburban skies.
"This is probably one of the best comets of the year," said Armen Caroglanian, a member of an employees' astronomy club at the NASA Goddard Space Flight Center. But it's nothing like comets Hale-Bopp (1997) and Hyakutake (1996) — big comets that were hard to miss, even from urban locations, despite being much more distant.
Hartley 2's light, while considerable, is spread out across a wide patch of sky, making it a very faint fuzzball against the black of space.
Caroglanian first spotted it in a small telescope Sept. 10 at a star party under dark skies in West Virginia. But when he tried again a week later in light-polluted skies at his home in Silver Spring, it was invisible. He managed to pick it up again last weekend in binoculars as it drew closer to Earth and brightened. But it wasn't easy.
"It was not visible to the naked eye, and you'd have to know where to look for it with binoculars," he said. "I wish it were something better."
Alan MacRobert, senior editor at Sky & Telescope Magazine, said the comet is now near the bright star Capella, in the northeastern sky in the evening. It has grown "quite large, but dim," he said.
"It has a large coma, or outer atmosphere, which shows up especially well in photos, which are more sensitive to dim light than the human eye," he said. But stargazers' chances to see it from the backyard are fading fast. "The comet's as close as it's going to get," and bright moonlight is now going to wash out the view until early November.
By then, if all goes well, NASA's EPOXI spacecraft will have flown within 435 miles of the comet's nucleus and sent back a wealth of close-up photos and scientific data.
The mission's principal investigator, University of Maryland astronomer Michael F. A'Hearn, says his science team is already reporting very slow outbursts of gas from the comet. "That's a behavior we haven't seen before. We don't know what it means yet," he said.
"We've been very pleased with the performance of the spacecraft," A'Hearn said. Designed for an 18-month mission, it has now passed five years. Its scientific instruments are still performing well, and scientists on the team "are ecstatic" about all they've learned so far.
The spacecraft was launched in January 2005 as NASA's Deep Impact mission, sent to hurl a 750-pound projectile onto the surface of a comet called 9P/Tempel, to study its interior composition.
After the craft completed that task in July 2005, its mission was renamed EPOXI, with orders to study extra-solar planets; the Earth and Mars during a series of flybys in 2008 and 2009, and finally to buzz Comet Hartley 2 on Nov. 4, 2010.
EPOXI will photograph, analyze and map Hartley 2's surface composition, features and gas vents, model the nucleus's shape and map its temperatures.
NASA TV (nasa.gov/ntv will provide live coverage and movies made during EPOXI's 12 kilometer-per-second approach to the comet. The first five images taken during closest approach will be released an hour later.
Hartley 2 was discovered in 1986 by Australian astronomer Malcolm Hartley. Less than a mile wide, it is a "short-period" comet, circling between the orbit of Jupiter and the sun once every 6 1/2 years.
A comet's icy nucleus is too tiny to see from such distances. But as it approaches the sun, an expanding cloud of gas and dust escaping from the surface makes the comet visible.
In Hartley 2's case, MacRobert said, "it seems that the entire surface is blowing off gas and dust, rather than the stuff coming out of a few isolated vents … as is usually the case."
The comet's greenish color in photos reveals the presence of cyanogens, gases composed of carbon and nitrogen that fluoresce in green when struck by ultraviolet light from the sun.
See more on EPOXI and Comet Hartley 2 at epoxi.umd.edu.