The Green Comet

You say that your new year’s resolution is to stop staring down at your phone so much? Here’s a reason to look up. This picture features Comet Lovejoy, a bright celestial ball of dust and ice probably borne from the Oort Cloud and currently traveling across skies in the Northern Hemisphere. While photographer Gerald Rhemann snagged the shot on December 22nd (from the Southern Hemisphere), casual observers in the top half of the world might be able to enjoy the show into February.

“Comets this bright [appear only] every few years, on average,” says Matthew Knight, a research scientist who studies comets at Lowell Observatory in Flagstaff, Arizona. Indeed, some viewers with keen sight might be able to spy it with the naked eye from a location far from urban lights (for example, the Arizona desert or Michigan’s Upper Peninsula).

But most skywatchers will probably benefit from a telescopic lens of some sort. The good news is, binoculars—say, a pair of 10 x 50s—should do the trick, according to Dean Regas, outreach astronomer at the Cincinnati Observatory. “The farther away from the city lights you can get, the better,” he adds.

Comets this bright [appear only] every few years, on average

Prime viewing conditions start right after dark, when bright stars emerge. Comet Lovejoy (officially designated C/2014 Q2) will appear highest in the southern sky at around 8 or 9 p.m.—and the loftier it is, the less atmosphere you’ll be peering through, and the better you’ll be able to see it, says Regas. This week, “when you see Orion’s belt, that’s an indication you can probably start looking for it,” he says. (This map from Sky & Telescope magazine charts the comet’s path.)

“It’s definitely bigger than the stars that are also in the field, [and] you can see it moving. If you’re there for half an hour, you can tell that it’s moved relative to the stars nearby,” adds Knight [if you’re using a telescope or binoculars].

You might even notice a green glow. How come? When ultraviolet light from the sun hits gas molecules in the comet’s coma, they get excited. To return to a stable state, those molecules release energy at different wavelengths, “and the one that we happen to see most easily with the human eye is the one that’s in the middle of where our eyes are [most] sensitive”—that is, the green region of the visible spectrum, says Knight. The molecules responsible for the verdance are pairs of carbon atoms, called diatomic carbon. (Other gas molecules in the coma emit energy when they’re excited, too—just not at wavelengths that we can see, or not at the same intensity as diatomic carbon’s. For more, check out Knight’s article about Comet ISON, which also glowed green.)

Regas says that many viewers won’t notice the green tint, however. Instead, they’ll see what he describes as a “gray fuzzball.” Humans don’t see colors that well in dim light, although “some people may have slightly different sensitivities,” says Alan MacRobert, a senior editor at Sky & Telescope, which has been keeping tabs on Comet Lovejoy. Using an instrument that lets in more light—for instance, a telescope rather than your basic binoculars—might help you detect the comet’s verdigris. And “cameras are more sensitive to colors in dim light than the human eye is,” he says, which explains the impressive images that photographers have been rolling out.

Gerald Rhemann created the image above using a remote-controlled private observatory—equipped with an astrograph (a telescope specifically designed for astrophotography), camera, and mount—located at the Southern Sky Lodge in Namibia, where there are “excellent observing conditions,” he wrote in an email. The picture is a composite of several photos taken with different filters.

On January 30th, Comet Lovejoy—the fifth comet discovered by amateur astronomer Terry Lovejoy, who located it from Australia last August—will reach perihelion, its closest point to the sun, at about 120 million miles. “That’s when it could be its absolute brightest,” says Regas, although it will be farther from the Earth, at slightly less than 70 million miles. “Keep an eye on it for the rest of the month” to see if its brightness changes, he suggests. “All comets are fickle beasts, that’s for sure. You’re not sure what they’re gonna do, how they’re gonna pan out.”

If you don’t catch the whizzing orb in the next few weeks, you’ll have to wait another 8,000 years or so before it comes back around. That’s the fascinating thing about comets, says Regas. “They are hidden from view for the vast majority of their life, ’cause they’re way in the distant part of the solar system, and we get to see them on those [fleeting] rare occasions.”

Have you spotted Comet Lovejoy? Send us your picture at website[at]sciencefriday.com.

*This article was updated on January 13, 2015, to reflect the following changes: The comet isn’t technically “streaking” across the sky, as the text originally stated. Also, the caveat was added that one must be using a telescope or binoculars to see the comet move with respect to the stars over the course of a half-hour. And finally, the photo caption originally stated that the comet has passed through Earth’s orbit before. The comet didn’t, and isn’t, passing through our orbit, but rather through the inner solar system.