Sky Show Tonight: Green "Two-Tailed" Comet Arrives

Victoria Jaggard
National Geographic News
February 23, 2009
A fresh new face has moved into our neighborhood, but once this green-colored comet swings by Earth tonight, it may never come back (picture of green comet Lulin).

Comet Lulin is currently sailing through the inner solar system and is getting closer to our home planet, with its nearest approach expected on February 24.

Although it's hard to glimpse with the naked eye, the comet "should be a fairly easy object [to see using] modest amateur telescopes or even binoculars," said Don Yeomans, a comet expert at NASA's Jet Propulsion Laboratory in Pasadena, California.

Astronomer Mark Hammergren of Chicago's Adler Planetarium added that the icy body has the potential to do something unexpected.

Comet Lulin is arriving from the far reaches of the solar system on a nearly parabolic orbit—"it's almost as if it comes from infinity and goes back out to infinity," he said. (Explore an interactive solar system.)

This means Lulin could be on its first pass by the sun, so the comet should still be encrusted in "fresh" ices preserved by the freezing environment of the outer solar system, Hammergren said.

As Lulin is exposed to the sun's heat for the first time, those ices are vaporizing—activity that could cause the comet to brighten rapidly or even break apart. Even now the comet is spewing cyanogen and diatomic carbon, both gases that glow green in sunlight out in the vacuum of space.

What's more, the comet's orbit is in nearly the same plane as Earth's, but the comet is traveling in the opposite direction. This causes Lulin to appear to move unusually fast and display a rare anti-tail—an optical effect that creates a secondary "tail" pointing toward the sun.

Green Comet Fuzz Ball

Quanzhi Ye, a student at China's Sun Yat-sen University, found comet Lulin in 2007 while examining images from the Lulin Observatory in Taiwan as part of an asteroid survey.

Lulin made its closest approach to the sun on January 10, 2009, and has since been getting brighter in the morning sky.

Tonight the comet will pass nearest to Earth—about 38 million miles (61 million kilometers) away—and will reach peak brightness and fastest apparent speed.

The comet's position on the night of closest approach "means we can observe it all night long from either hemisphere," Ye said. Lulin will appear in the constellation Leo just below the planet Saturn.

Viewers in very dark rural areas might even be able to see the comet with the naked eye, said Adler's Hammergren. And for sky-watchers using small telescopes, the comet "will look like a tiny fuzz ball in the sky."

(As an added bonus, four of Saturn's moons will pass in front of the planet around the same time, a sight that should be visible with small to medium telescopes.)

"In March it'll still be observable," NASA's Yeomans said of the comet, "but it's not going to knock anybody's socks off."

Comet With Two Tails

Stargazers able to take pictures of the green comet are already capturing images of Lulin's rare anti-tail.

Like other comets, Lulin is basically a combination of ices and embedded dust particles left over from the formation of the outer solar system.

Ordinary comets have tails because some of their ices vaporize as they near the sun. The resulting cloud of gases and dust is pushed away from the sun by solar particles and pressure from sunlight, Yeomans said.

But sometimes a comet is at such an angle that viewers on Earth also see a shorter tail that seems to poke out like a needle toward the sun.

This isn't really a second tail but a rare type of optical effect, Yeomans said. When Earth crosses the comet's orbital plane, we can see parts of the tail projected on both sides of the object's head.

In addition, comet Lulin is orbiting "backward" compared to the planets, so viewers on Earth should be able to see it shifting position against the background stars over a matter of minutes rather than hours.

Promising Target

Being a newcomer to the inner solar system makes Lulin a promising comet for scientific study, Yeomans said.

"New comets still have the ices they were born with," so studying their composition gives us a window into the early solar system, he said.

For example, regular visitors such as Halley's comet might dazzle stargazers, but for scientists, "once a comet has been around the sun several times, it gets to be not nearly as interesting," he said.

That's because the older comet will have lost much of its original ices, exposing a crusty surface of debris—a condition Adler's Hammergren compares to "a snowbank on the side of the road at the end of winter."

A pristine comet, however, will be full of ices that have been largely unchanged since the solar system formed 4.5 billion years ago.

On January 28 NASA's Swift Gamma-ray Explorer satellite trained its eyes on Lulin and saw that the comet was shedding about 800 gallons (3,030 liters) of water a second—enough to fill an Olympic-size swimming pool in less than 15 minutes.

Studying light that passes through the comet's aura of water and gases can help astronomers tease out the object's chemical makeup.

(Related: "'Deep Impact' Comet Spewed Tons of Water, Study Finds.")

"Radio, infrared, and optical astronomers will be very busy observing this one," Yeomans predicts.

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