Comet's breakup near Earth offers viewing spectacle

THE BALTIMORE SUN

Astronomers will have a ringside seat during the coming weeks as a dying comet with a tongue-twisting name flies past the Earth and literally falls apart in front of their eyes.

Comet 73P/Schwassmann-Wachmann 3 is buzzing the Earth -- closer than any comet in 23 years. It's gliding by as close as 5.5 million miles away, barely 20 times the moon's distance from the Earth.

Even better for scientists, SW3's icy nucleus is coming undone like the seeds of a dandelion in a stiff wind, revealing the physical and chemical secrets of its interior.

Astronomers have counted at least 59 fragments already, and there's no end in sight.

"It's driving us nuts; fragments of fragments of fragments," said Don Yeomans, manager of NASA's Near Earth Objects Program office at the Jet Propulsion Laboratory in Pasadena, Calif. It is his job to keep track of them all and tell astronomers where to find them.

"It's doing its best to make our lives miserable," he said, laughing.

As near as it is coming in astronomical terms, scientists insist there is no danger that SW3 will collide with the Earth, although its debris may produce a small meteor shower years from now.

The comet's fragments are too small to produce a naked-eye spectacle, like comets Hyakutake in 1996 and Hale-Bopp in 1997. But one or two may be visible in the eastern sky for the next two weeks. You will need binoculars and a dark location.

Look in the late evening. The comet is moving through the "Summer Triangle," formed by the bright stars Vega, Altair and Deneb. It will drop closer to the eastern horizon each night.

Many amateurs are already watching the comet's bust-up through backyard telescopes.

"I was able to hit it right away," said Tim Hickman, 60, who first photographed the comet's largest fragment April 15 from his backyard in Timonium. "It's kind of cool to see one breaking up, the end of the life of a comet. It looked like a miniature comet, with a little head and a little tail."

During several hours at his eyepiece, he said, "I could actually see its motion relative to the stars." But it's been too faint to see with the naked eye.

Hal Weaver, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory, had this advice: "The next two weeks is when it's going to be brightest. People should get out there with their binoculars."

Weaver has been leading a Hubble Space Telescope team watching the comet break apart. But he also plans to try to see SW3 with his own eyes. Why?

"My gosh, this thing is breaking apart," he said. "Maybe it's the last time people will be able to see it. You can say you were there."

The comet was discovered in 1930 by the German astronomers Arnold Schwassmann and Arno Arthur Wachmann. Subsequent calculations revealed that it circles once every 5 1/2 years between the sun and the orbit of Jupiter. But it wasn't seen again until 1979. Astronomers missed it in 1985, but found it again in 1990.

On its 1995 return, SW3 startled everyone, Weaver said. "It got dramatically brighter in a short period of time, and shortly after that people started seeing more than one nucleus."

The comet -- a hunk of ice and dust that astronomers estimate was once 1 to 3 miles long, had broken apart. Now they had four fragments -- A, B, C and D.

Only two were spotted on the comet's return in 2000. But astronomers had to peer all the way across the solar system to see it at all.

Weaver applied for observation time on Hubble to watch the comet's 2006 return, hoping that SW3's fragment C had survived.

"We were really sweating all through last year," he said. The comet was months late, but fragment C showed in late October, and B followed in January.

But then the fragments began multiplying, like the sorcerer's broom in Disney's Fantasia. By last month, astronomers began using double letters to name the fragments as the count topped 40. Now it's nearing 60.

"We may be witnessing the disintegration of the comet," Weaver said.

Astronomers are finding comet breakups quite common. The most memorable was Shoemaker-Levy 9, which broke into 21 fragments in 1992 as it passed close to Jupiter. Two years later, all 21 pieces fell into Jupiter's atmosphere as astonished scientists watched from Earth.

"It really does look like the natural way comets meet their demise," Weaver said. What makes this breakup special is its proximity to Earth's telescopes. It "gives us the opportunity to observe the demise of comets in a very intimate fashion."

The squadron of fragments from Schwassmann-Wachmann 3 is expected to make its closest approach to Earth between May 12 and 28. The nearest will glide by no closer than 5.5 million miles, NASA said. While that sounds a long way off, it's close enough to qualify SW3 as a "near-Earth object." NEO's are comets and asteroids that come uncomfortably close -- within about 9 million miles -- of Earth.

Weaver said NASA has undertaken a "very serious effort" to find all objects large enough to potentially hurt the Earth, "to catalog them and measure their orbits carefully so we can know far enough in advance if something will hit us."

The most sensitive all-sky surveys should detect threats far enough from Earth to provide the planet with "maybe an extra year or two" to mount a defense, Weaver said.

So far, Yeomans' group has catalogued 4,049 NEOs, and they are adding close to 700 each year, he said.

Among those is a subset of objects (771 at present) called "potentially hazardous objects," whose orbits bring them within 4 million or 5 million miles of Earth. Of those, 159 are larger than a kilometer in diameter -- large enough to cause global disasters if they struck.

So far, none poses any danger, Yeomans said. And many asteroids come much closer than Schwassmann-Wachmann 3.

"They can get pretty close," he said. "We have an asteroid predicted to pass within five Earth radii in 2029." That is well inside the moon's orbit -- closer, even, than the Earth's geosynchronous communications satellites.

Called 99942 Apophis, the asteroid is expected to be a naked-eye object when it skims past the planet Friday, April 13, 2029.

For now, astronomers are delighted by SW3's proximity. "It's not dangerously close, but it's close in the sense that observers will have their best opportunity in decades for observing a cometary breakup," Yeomans said.

In addition to ground-based observatories and backyard telescopes, the comet will be closely watched by NASA's Hubble and Spitzer space telescopes.

Intense observations may not solve the riddle of comet breakups, he said, but "I'd be surprised if there weren't some fresh ideas."

There are several theories. One possibility is that, as the comet approaches the sun's warmth, a "thermal wave" moves from the surface into the ice and dust of the interior.

Some scientists suggest the heat may reach interior ice, which then sublimates -- turns into a gas. As it expands, pressure builds until "basically you get a little explosion" that blows a chunk of the nucleus off into space, Weaver said.

Another theory suggests that escaping gas may act like a jet that starts the nucleus spinning until its centrifugal forces cause it to fly apart.

Still another blames "tidal forces" -- differences in gravitational forces across the breadth of the nucleus that cause it to fracture as it nears a large planet or the sun.

All these hypotheses suggest that comet nuclei are extremely weak, Weaver said. "If you had a chunk of it in your hand you could tear it apart easily."

Originally, NASA wanted to study SW3 with its Contour mission. But the APL-built spacecraft broke apart in August 2002, soon after its launch. So astronomers will have to do their best with telescopes. And they want to learn about more than just the mechanics of the comet's breakup.

"When astronomers point their telescopes at these things, they'll take spectra to find out what the chemical composition of the ices are," Weaver said.

He compared it with NASA's recent Deep Impact mission, in which scientists sent a robot spacecraft to comet Tempel 1. It released a massive impactor that blew open a patch of a comet's nucleus to reveal its interior composition.

"We think comets are among our best-preserved relics of what happened in the outer solar system during its formation," Weaver said. Any clues they find to the physical structure and chemical composition of comet nuclei "tell us something about the conditions in the early solar system."

frank.roylance@baltsun.com

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