Crashing a comet's secrets

When comets loop into view from the far edges of the solar system, they can ride the night sky like beautiful apparitions, pale tails streaming behind them. Or they can be barely discernible, disappointing blobs of fuzzy light you have to strain to see.

As long as humans have been watching the night sky, they've tracked comets, seeing them as omens for good or ill, depending on the tilt of the court astronomer's astrolabe. But we've only seen them from the outside, clouded by a stream of debris.

"Think of Pig Pen in the Peanuts cartoons,'' said Elizabeth Warner, director of the University of Maryland Observatory. "Or think of anyone walking around. We constantly throw off bits of hair and skin cells. And you can tell certain things about a person from those cells. But you can't tell whether we're squishy inside from them.''

At about 2 a.m. Monday morning - July 4th, our day to patriotically blow things up - NASA's $267 million Deep Impact mission plans look at a comet's inside for the first time by placing a probe in the path of Comet Tempel 1, and letting them smash together.

What astronomers around the world are hoping for is a big bang - a collision that blows a sizable crater in Tempel 1 and throws a huge cloud of dust into the space around it. It's possible the comet could break up. But even if Tempel 1 hits the probe, vaporizes it and sails on, untouched, it will tell astronomers vastly more about what comets are made of than they knew before.

That's because, right now, when it comes to the insides of a comet, astronomers don't know much at all.

"What we know now is that the structure of a comet ranges somewhere between cotton candy and concrete,'' said Brian Muirhead chief engineer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the initial project engineer for Deep Impact. "So even if the impactor just gets swallowed up by the comet - the cotton candy theory - it will really increase what we know about comets.''

And it turn, anything astronomers learn may help them understand something about the origins of the Solar System itself.

"Comets are sort of leftovers from the beginnings of our solar system,'' said planetary astronomer Heidi Hammel of Ridgefield. "They're out in the deep freeze. This is the first time we'll get a look at their interiors.''

Hundreds of pairs of eyes will be watching Deep Impact as it occurs. The impactor - the probe that Tempel 1 will smash into - will have a camera that will take pictures of the approaching comet until seconds before the two collide. The Deep Impact fly-by craft - which will releases the impactor 24 hours before the smash-up - will be positioned off to one side, about 5,300 miles away, recording what takes place.

About 60 big telescopes in 20 countries will be watching. So will the major space telescopes - Hubbell, Chandra, and Spitzer. Hammel, who is a senior research scientist with the Space Science Institute in Boulder, Colo., will be working at the Keck Observatory on Mauna Kea in Hawaii over the weekend, explaining to the world what astronomers are seeing.

And so will small-town observatories and backyard astronomers, who have registered with NASA's Small Telescope Science Project. Scattered across the country, they will monitor the collision in the hopes that lots of eyes watching from lots of different places will give NASA a broader view of the event.

The John J. McCarthy Observatory at New Milford High School is part of the project. Along with watching Tempel 1, the observatory will be open to the public both tonight and Monday at dusk.

The collision will occur when Comet Tempel 1 has gone below the horizon in the eastern United States, so local viewers will be out of luck.

"We won't see the impact,'' said Monty Robson of McCarthy Observatory. "But we'll see it before and maybe 18 to 20 hours after the impact occurs. We also hope to have the NASA TV link going. And as long as there's public interest, we'll stay open.''

Over the years, astronomers have gotten better looks at the outside of comets. In 2001, the Deep Space probe took some of the best pictures of a comet - Comet Borrelly - from a mere 1,320 miles away, showing it to be a sort of five-mile long space potato.

And, Hammel said, astronomers have a fairly good idea of what comets are made of - mostly ice, mixed with dirt, gravel and dust. In their swings through the Solar System, they stay frozen in the far reaches of space. But they warm up, melt, and throw off debris as they approach the sun. When we see a naked-eye comet, we see its coma - the cloud of debris that surrounds the nucleus - and the stream of debris that makes up the tail. The more debris, the more material to reflect light and the better the viewing.

Because of the complexity of the mission and its distance from Earth - 83 million miles away, about the distance of the Earth to the sun - there are hundreds of things that could go wrong. But if everything goes right, the Deep Impact fly-by craft - which is about a big as a Volkswagen Beetle - will separate from the impactor - as big as a coffee table - about 24 hours before impact.

Then, at about 1:52 a.m., Monday morning, Comet Tempel 1 - which swings into the Solar System every 5.5 years - will hit the impactor at the speed of about 23,000 mph., causing an explosion with the strength of 4.5 tons of TNT.

Warner said that the impactor will take photos of the approaching comet until just a few seconds before it's vaporized by the smash.

"For me, those pictures will be really cool,'' she said.

What the astronomers believe is the impact will blow a crater the size of a football stadium into Tempel 1. The fly-by craft, which will eventually swoop to within 300 miles of the comet, will get images of the explosion and the crater.

If Tempel 1 is as hard as a rock, the impactor will kick up only a small cloud of dust. If it's as soft as a giant snowball, that will tell scientists something, too.

"It's like having a pile of flour and a pile of gravel,'' Hammel said. "If you throw something into the flour, it will send up a lot of dust. If it's gravel, there won't be as much debris.''

In 1994, Hammel was on the team of astronomers that watched Comet Shoemaker-Levy break up into 21 pieces and plunge into the surface of Jupiter, causing spectacular flares of light as they hit.

"There were astronomers who, before Shoemaker-Levy hit Jupiter, said they didn't expect much to happen,'' Hammel said. "We are notoriously bad at predicting things. In this case, we have no idea what will happen.''



The John J. McCarthy Observatory at New Milford High School will be open to the public tonight and tomorrow from dusk to about 11p.m. For more information, call the observatory at (860)-354-1595.



Contact Robert Miller

at bmiller@newstimes.com

or at (203) 731-3345.