Comet Lulin, a beautiful green comet with a double tail is fast approaching Earth. Tonight, and the wee hours of Tuesday morning, right around 1 AM, are you best bets for a view. You'll be able to spot Lulin, with the naked eye, barely, in rural areas without light pollution, but a decent pair of binoculars or a backyard telescope will work just fine for a great view. You'll want to look in the southwestern sky about 40 degrees above the horizon near Saturn, which should be fairly bright and easy to spot. The diagram here has a circled X to show the relative position of Lulin at about ten pm in North America on the Monday night, February 23, tonight. Here's a diagram showing Lulin and the nearby constellations. You should be able to see the bright, slightly greenish, center of the comet, and its two tails, one a dimmer gassy cloud, and the other a more distinct, brighter tail of dust. The two tails appear to be almost directly opposite each other. The greenish glow is the visual sign of ionized gases Lulin sheds in its passage and the sun warms the comet enough to thaw some of its ice, so that it changes from a solid to a gas. The water is shed, at about 800 gallons of water per second as it moves closer to the sun, according to NASA. As Lulin's outer layer thaws, and the water dissipates, the comet releases the gasses trapped in its ice since Lulin formed. Two gasses in particular dominate Lulin's composition, diatomic carbon (C2; pairs of carbon atoms bound together) and cyanogen (CN), a poisonous gas. They give the comet its fuzzy gas halo, and green coloration when they are illuminated by the sun. That's a picture of Lulin above, taken by astronomer Jack Newton. There's a gallery of Lulin images here.
Because the comet's orbit is in almost the same plane as Earth's orbit, it seems to be moving very rapidly. Lulin is actually moving "backwards," that is, Lulin's orbit is in the opposite direction from the planets. That means that from Earth, it looks as if Lulin is shifting position, relative to the stars in the background, in minutes instead of the more usual hours. Technically, the second, gassy tail, is an optical effect. Lulin is headed back in an orbit to its birthplace in the in the Oort Cloud, a collection of frozen leftovers from when our solar system was formed 4.6 billion years ago. The Oort cloud is roughly 18 trillion miles past our sun, with billions of frozen objects similar to Lulin.
Lulin, originally known as C/2007/N3, was spotted in July, 2007 by nineteen year old astronomer Quanzhi Ye, a student at China's Sun Yat-sen University. Quanzhi Ye was studying images taken by a fellow student Taiwanese astronomer Chi Sheng Lin. The comet has been moving steadily closer to Earth, and will be at it's closest in the early morning of Tuesday, February 24th. Lulin has grown progressively brighter, and will still be visible in the early morning sky through February, but by late February Lulin will be most apparent in the evening sky. Lulin will still be visible in March, but much more difficult to spot. Astronomer Dr. Dello Russo, from Johns Hopkins University will be making observations of Lulin using one of the Keck Observatory's two 10-meter telescopes on Hawaii's Mauna Kea. In fact, Lulin is giving Earth-based astronomers a unique opportunity; this will be the first time we've been able to see simultaneous ultraviolet and X-ray images of a comet, thanks to NASA's Swift Gamma-ray Explorer satellite, which is monitoring Comet Lulin as it nears Earth. It won't be possible to see Lulin from Earth again for a very very long time, since the comet's orbital path takes it around the sun once every million years.