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Look for comet Lulin — it won’t be back for a million years
By Pete Spotts |
Throughout much of February, true early birds can catch a comet.
Comet Lulin — formally C/2007 N3 (Lulin) — is gracing the skies in the way-wee hours of the morning. It’s still not clear if it will be visible to the naked eye later this month. But at the least, it should be a sweet binocular sight.
And catch it while you can. It won’t be back for a million years. For a series of recent telescope-based images, you’ll find a nice photo gallery here.
As its formal title implies, the comet was discovered in 2007. NASA’s Tony Phillips has a nice little summary of the story behind the comet’s discovery here.
Its green hue comes from the gases it’s giving off as it approaches the sun and its ices begin to sublimate — change immediately from solid to gas. Two in particular — cyanogen and bound pairs of carbon atoms known as diatomic carbon — give the comet’s fuzzy halo, or coma, and its tail the green cast.
Up in New England, for instance, look for Lulin about 3 a.m., clear skies willing. (For a rough cut at what your skies may be like, Canada’s weather service has a nice “seeing forecast” site for North America here.)
The comet should sit about 20 degrees above the horizon in the constellation Libra. By Feb. 23, at the same time each morning, it appears in the southwestern sky about 40 degrees above the horizon near the planet Saturn. By then, according to the folks over at Sky and Telescope magazine, comet Lulin should be at its brightest.
For pros, this has been a tough comet to observe, said Lowell Observatory astronomer David Schleicher during a quick phone chat. It was first detected in July 2007 and through much of the second half of 2008 was too close to the sun — or behind it — to observe.
It popped back into view last month. But it did so when the moon was heading toward full, so the moon’s glare made it tough to observe.
Still, he and other astronomers are giving Lulin a serious once-over now that it’s getting brighter and the moon is on the wane.
Lulin turns out to be a very-long period comet, Dr. Schleicher explains; it orbits the sun roughly once every 1 million years. This puts it among the group of comets that originate in the Oort Cloud, a roughly spherical assembly of billions of icy objects that extends to about 18 trillion miles from the sun.
Schleicher and a colleague were able to grab photometry (light intensity) data last July as the comet was inbound and roughly 2.6 astronomical units — 2.6 times the distance between the Earth and the sun — away from the sun.
The ratio of dust to gas in the coma at the time indicated a “middle of the road” composition for the comet’s surface. And it was unleashing water at a heavy rate, some 20 octillion molecules per second, he said. (Actually, he said 2 times 10 to the 28th molecules per second.)
Within the past couple of weeks, they’ve also been able to gather images that will help them study the shape of the nucleus and coma.
He notes that others have been using one of the Keck Observatory’s 10-meter telescopes to study the comet at infrared wavelengths. So it shouldn’t be long before researchers have a more-complete picture of this once-in-a-million-year visitor.
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Comet Lulin in January and February 2009. 
For the next few days, a newly discovered, green-tinted comet will be visible by the naked eye in most parts of the Northern Hemisphere. Comet Lulin, with the official designation of C/2007 N3, was discovered in 2007 and astronomers say that this is the first time it has visited our solar system and may well be the last.

As it makes its way around the Sun, an astonishing 800 gallons (3 m3) of water will evaporate from the comet every second. In each 15 minute period, it will shed enough water to fill an Olympic sized swimming pool. It will come within 38 million miles of Earth at its closest pass, making it visible to the naked eye and even clearer with binoculars or a telescope. 

The best viewing time for people living in the northern hemisphere is after midnight when Lulin will be at its highest point in the sky, or 40 degrees from the morning horizon. Current estimates peg the maximum brightness at 4th or 5th magnitude, which means dark country skies would be required to see it. No one can say for sure, however, because this is Lulin's first visit.

Astronomers from NASA and the United Kingdom will use the Swift Telescope to study the comet and its composition. Astronomers also say to see it while you can because this could be the first and only time it passes through our solar system. It's estimated that if it returns, it will not be for another 1,000,000 years.

"We won't be able to send a space probe to the comet, but Swift is giving us some of the information we would get from just such a mission," said Jenny Carter, at the University of Leicester in England, who is leading the study.

The comet was discovered using the Lulin Observatory in Taiwan by astronomers Ye Quanzhi and Lin Chi-Sheng.