San Jose Mercury News

SAN JOSE, Calif. – Starting this Sunday, every major telescope on Earth and in space – and quite a few of the little ones – will point at one spot in the sky.

They’ll watch for a flash as an 820-pound projectile smashes into a comet half the size of Manhattan at 23,000 mph. Then they’ll look for a glow as the comet’s guts spill into space.

Scientists hope this faraway crash, the climax of NASA’s Deep Impact mission, will produce a Roman candle of gas and dust – our first look at the innards of a comet, the most primitive and pristine stuff in the solar system.

Not since 1994, when a string of comet chunks crashed into Jupiter, have astronomers from all over the world been so fixated on a single event.

“We’ve never done this before, so we don’t know exactly what will happen,” said Richard Puetter, leader of a team that will be watching the impact from Lick Observatory east of San Jose, Calif. “Possibly the flash will be bright enough to see by eye.”

Or maybe not: The probe could sink into the comet and disappear, like a bullet fired into styrofoam.

Many experts think it is unlikely that the average person on the ground will see anything, unless they’re in a very dark place with a good-size telescope pointed in exactly the right direction.

One thing’s for sure, scientists said: The impact will not shatter the comet or knock it off course. Since it’s 83 million miles away, there’s no danger that Comet Tempel 1 will menace the Earth in any way.

“This is very much like a mosquito running into an 18-wheeler,” said crater expert Peter Schultz of Brown University, one of the mission scientists.

Whether Deep Impact ends with a bang or with a whimper, researchers hope it will tell them how comets are put together.

Why does it matter? Because comets are leftovers from the birth of the solar system, the most primitive objects known. Scientists think that comets falling into the early Earth contributed critical materials, from carbon-based compounds to water, which helped create conditions conducive to life.

“They represent the raw junk out of which everything was made, way back when,” said Scott Sandford of NASA’s Ames Research Center in Mountain View, Calif. From that standpoint, he said, comets are more interesting than planets.

“What we want to do is blast into the comet and knock out some of the material from the inside, which is pristine,” said Puetter, an astrophysicist from the University of California-San Diego.

“If it’s really fluffy, like snow, then the impact will bury itself deep in the comet, and all the energy will get deposited deep in the comet and we won’t see anything,” he said.

“On the other hand, if it’s really hard, all the energy will deposit on the surface of the comet, and it will blow off a crater.”

Simulations show the crater could be as big as 650 feet wide and 150 feet deep and leak gas, dust, ice and rock for a long time to come.

Deep Impact was launched Jan. 12 from Cape Canaveral. On Saturday the spacecraft is scheduled to release a probe the size of a washing machine into the path of the comet, then maneuver into a safe position to watch what happens.

The impact, which was timed to occur on the East Coast on the Fourth of July, will take place about 1:52 a.m. EDT. The first signals from the spacecraft, which will hover nearby to take pictures and data, should arrive on Earth seven minutes and 26 seconds later.

The crash will be visible only from the West Coast and Hawaii. But astronomers throughout the world will be ready to observe the aftermath as the globe turns and the comet rolls into view.

They will be connected to an extraordinary degree by daily video conferences, a teleconference line and a Web site where observers will immediately post information, said astronomer Karen Meech of the University of Hawaii-Manoa, who will coordinate the network from the summit of Mauna Kea.

“I would be very surprised if there was any place on Earth that wasn’t observing,” she said. “I think a lot of the interesting science is going to develop in the two or three days following the impact,” with scientists analyzing the composition of the gases and dust leaking from the wounded comet.

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