Sky Show This Month: "Two-Tailed" Comet Nearing Earth
National Geographic News
|January 26, 2009|
A fresh new face has moved into our neighborhood, but once it swings by Earth next month, it may never come back.
Comet Lulin is currently sailing through the inner solar system and is getting closer to our home planet, with its nearest approach expected in late February.
Although it will probably be hard to see with the naked eye, the comet "should be a fairly easy object [to see with] modest amateur telescopes or even binoculars," said Don Yeomans, a comet expert at NASA's Jet Propulsion Laboratory in Pasadena, California.
In fact, notes astronomer Mark Hammergren of Chicago's Adler Planetarium, "it's visible right now with a small telescope if you know where to look."
But Hammergren notes 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.
As the object is exposed to the sun's heat for the first time, those ices will vaporize, possibly causing the comet to brighten rapidly or even break apart.
What's more, the comet's orbit is in nearly the same plane as Earth's but 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.
Cosmic 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 sky survey.
Lulin made its closest approach to the sun on January 10, 2009, and has since been getting brighter in the morning sky. Over the coming weeks stargazers will see it move westward from the constellation Libra into Virgo.
"It'll be good observing through February in the morning sky, and then it switches in late February to the evening sky," NASA's Yeomans said. "In March it'll still be observable, but it's not going to knock anybody's socks off."
On February 24 the comet will pass closest to Earth at about 38 million miles (61 million kilometers) and will reach its peak brightness and fastest apparent speed.
According to Lulin's discoverer Ye, the comet's position during closest approach "means we can observe it all night long from either hemisphere."
Viewers in very dark rural areas might even be able to see the comet with the naked eye, said Adler's Hammergren, who advised consulting star charts to know exactly where to look.
For sky-watchers using small telescopes, he said, the comet "will look like a tiny fuzz ball in the sky."
Stargazers able to take pictures of the 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, JPL's Yeomans said.
But sometimes the comet is at just the right angle that viewers on Earth also see a shorter tail that seems to poke out like a needle toward the sun.
This isn't a real 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.
Being a newcomer to the inner solar system also makes Lulin a promising target 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, leaving exposed a crusty surface of debris—a condition Adler's Hammergren compares to "a snow bank 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.
Taking samples direct from a comet can be challenging, but studying the light from the body as it passes by can help astronomers tease out a comet's chemical makeup.
(Related: "'Deep Impact' Comet Spewed Tons of Water, Study Finds" [April 4, 2006].)
"Radio, infrared, and optical astronomers will be very busy observing this one," Yeomans predicts.
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