Comet Lulin arrives tonight - break out your telescopes

Over the next three nights, skywatchers should expect their best views yet of Comet Lulin. It makes its closest approach to Earth -- some 38 million miles away -- on Tuesday, Feb. 24.

To the naked eye, the comet looks like a fuzzy patch of hazy light against the night sky. Use binoculars or telescopes, and you'll be able to pick out its brighter center, along with its dual tail -- a brighter tail of dust, and a dimmer one of ionized gases the comet sheds as its sun-warmed ices change directly from a solid to a gas.

Lulin, formally known as C/2007 N3, will dim quickly through March, thanks to the kick it's gotten from the sun's gravity. Then it's Oort-a here -- heading back out to its kin in the Oort Cloud, a vast collection of icy construction debris left over from the formation of the solar system some 4.6 billion years ago.

The far edge of the Oort cloud reaches some 18 trillion miles beyond the sun. That's a lot of space for literally billions of icy objects to inhabit. Scientists estimate that the combined mass of all those objects is about 40 times that of Earth.

While amateur skywatchers brave a chilly winter's night or two (in the higher latitudes of the northern hemisphere) with a thermos of hot chocolate and their favorite viewing tools, Neil Dello Russo is looking to the comet's flyby for a serious cosmic chemistry lesson.

Some serious comet-watching

Dr. Dello Russo, with the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., has snagged some telescope time on one of the Keck Observatory's two 10-meter telescopes on Hawaii's Mauna Kea. He and an international team are watching emissions from Lulin, as well as those from comet 22P/Kopff. Unlike Lulin, Kopff is a short-period comet, originating in a band of icy objects beyond Neptune's orbit. The band is called the Kuiper Belt.

Since an initial post on Lulin at the beginning of the month, you learn an extra thing or two -- such as the length of the comet's orbital period. It isn't just a million years; it's nearly 50 million years. Kuiper Belt objects like 22P/Kopff return every 200 years or less. Comet Halley is one high-profile example. Comet 22P/Kopff returns once nearly every seven years. It's next closest approach to the sun takes place in May.

Dello Russo and his colleagues are looking at Lulin and 22P/Kopff to compare the relative amounts of different gases they give off. Among them: organic compounds like ethane, hydrogen cyanide, acetylene, ammonia, methanol, and, of course, water.

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