Comet Hartley 2 was still too faint to be seen with the unaided eye when it was captured in this photograph at a distance of about 18 million miles from Earth on Sept. 28, 2010 by NASA astronomer Bill Cooke.
Credit: NASA/Bill Cooke
Since the start of October, many people with small telescopes have followed the path of Comet Hartley 2 across the night sky. Some skywatchers are reporting an unimpressive sight so far, but the best chances to spot this comet still lie ahead.
The overall consensus suggests that seeing Comet Hartley 2 is strongly dependent on your location, weather conditions and proximity to city lights. The comet will make its closest approach to Earth on Oct. 20, when it will be 11.2 million miles (18 million km) away.
This video sky map shows where to look this month to see the green Comet Hartley 2, but a small telescope is advised.
From locations plagued by light pollution, actually seeing the comet is a difficult to near-impossible task, but even those who are blessed with dark and starry skies, finding the comet is proving a bit of a challenge. This is because the comet is large in angular size and very diffuse, spreading its light out over a large area.
Most who ultimately have located it in their telescopes or binoculars typically describe it as a nearly circular cloud, comparable in angular size to the full moon. Some photographs have shown a slight elongation of the comet's head (called the coma) but hardly the kind of tail exhibited by other comets that attain 5th or 6th magnitude.
Unimpressive green comet
Carl Hergenrother, a professional astronomer at the University of Arizona's Lunar and Planetary Lab, said his attempts to observe the comet have yielded an unimpressive show. [Best Comet Appearances of All Time]
"At my home the comet is a faint, barely discernible fuzzball in 10?50 binoculars. At least the comet's location is relatively easy to find due to the large number of bright stars available for star-hopping," Hergenrother said. "Still the comet has not been an impressive sight and I'd guess that most inexperienced observers will have a hard time finding it."
Hergenrother tried to spot the comet as it passed by the Double Cluster in the constellation Perseus, and was surprised that it was hard to spot.
"Because it was located near a few bright stars, it was swamped by their glow. The proximity to the stars also ruled out any attempt at estimating its brightness," he added. "On one of these nights I need to drive out to a darker site and see how much of a difference that makes."
Some NASA telescopes and space observatories have been watching Comet Hartley 2 in advance of the anticipated rendezvous with the comet by NASA's Deep Impact spacecraft. The probe will zip by the comet on Nov. 4 to take a close look as part of the space agency's Epoxy mission.
Faint comet target 'underwhelming'
To escape light pollution, Long Island, N.Y., skywatcher Rich Tyson drove to Montauk Point to observe Comet Hartley 2 on Oct. 7 using a 10-inch reflector telescope.
"The best view was at 76x (magnification), appearing as an easily seen roundish ball of light with a brighter central condensation; the brightest part of the [coma] seemed to look slightly elongated," Tyson said. "Upon closer examination (171x) this elongation turned out to be a faint star seen next to the star like pseudo nucleus."
Another Long Island observer was Ken Spencer, who was attending the annual Astronomy Jamboree at the Custer Institute on Southold. He used an 8-inch telescope at 33x magnification.
"It was still very faint, and had a very slight amount of condensation," Spencer said. "A couple of other observers looked at it, and we felt it was a bit underwhelming."
Veteran comet observer Charles Morris saw Comet Hartley 2 just after midnight on Oct. 11 from Fillmore, Calif., with 10x50 binoculars. He estimated the faint coma as 30 arc minutes across, comparable in size to the full moon. The comet was 13 million miles (nearly 21 million km) from the Earth at that time, so its true length corresponds to 113,000 miles (almost 182,000 km).
Sam Storch, who this past August at the annual Stellafane Convention in Vermont received the Special Service Award from the Astronomical League for more than 40 years of contributions to astronomy, found Comet Hartley 2 from his son's home in Waterford, N.Y.
"On Oct. 10, the Milky Way was only barely visible, and I decided that I'd only have one chance to see the comet," Storch said. "It took no more than a minute to enjoy the easy spectacle of the Double Cluster again, and with some difficulty, we got Hartley 2 as a barely discernible grayish brightening of the sky at around 29x. We bagged it!"
Closest on Wednesday
Comet Hartley 2 will make its closest approach to the Earth within the next several days. At 3 p.m. Eastern daylight time on Wednesday, Oct. 20, the comet will be at its closest to our planet ? a distance of 11.2 million miles (18 million km).
It's quite unusual for any comet to approach this close to Earth; it happens on average perhaps three or four times a century. If it were a moderately sized comet, this proximity to our planet would make for a spectacular sky show. In spring of 1996, Comet Hyakutake made a similarly close approach and shone as bright as the brightest stars and was accompanied by a spectacular long tail that stretched halfway across the sky.
Not so, unfortunately, for Comet Hartley 2.
During the next 10 days, the comet will race rapidly southeast across the constellations Auriga and Gemini. It likely will continue to appear as a faint, diffuse fuzzball that will be best viewed under dark, pristine skies.
Under such conditions, the comet might even be glimpsed without any optical aid.
Skywatcher P. Horaluk of Ustupky, in the Czech Republic, reported that he could he see it with his naked eye just before local midnight on Oct. 10. For most of us, however, Comet Hartley 2 will continue to be a challenging object to locate.
Another fly-in-the-ointment is the presence of the increasingly bright gibbous moon that has been lighting up the sky during the first part of the night and seriously interfering with observations of the comet.
From mid-northern latitudes on Oct. 16, moonset is at 12:56 a.m. and dawn breaks at 5:37 a.m., which means the sky will be dark and moonless for 4 hours and 41 minutes.
But on the morning of Oct. 20 ? the day when Hartley 2 will be closest ? the nearly full moon will set at 4:51 a.m. with morning twilight beginning at 5:41 a.m., leaving just a scant 50 minutes of dark sky for comet viewing. After that, the moon will remain above the horizon until after morning twilight has begun.
So if you haven't tried to see the comet under a dark sky yet, you don't have much time left.
- Video Sky Map: How to Spot Comet Hartley 2
- Telescopes for Beginners
- New Comet Photos Show Icy Target for NASA Probe
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.