Spacecraft races to rendezvous with a comet

Space: `Deep Impact' seeks its target tonight.

July 03, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

The first, and surely the biggest, blast on this Fourth of July will burst 83 million miles from the Inner Harbor.

Just before 2 a.m. tomorrow, NASA scientists led by University of Maryland astronomer Michael A'Hearn will send an 820-pound projectile from their Deep Impact spacecraft into the path of an onrushing comet.

They're hoping that the 23,000-mph collision - a wallop equivalent to igniting about 4 1/2 tons of TNT - will knock out some of the comet's stuffing, which scientists believe has not changed since the birth of the solar system.

"It's going to give us some information to piece together the puzzle of the formation of the solar system and what it was like 4 1/2 billion years ago at the outer edges," said University of Maryland astronomer Lucy McFadden, a member of the mission's science team.

The inner secrets of the comet, named Tempel 1, will help scientists map the content and conditions of the early solar system. They could also shed light on whether it was an ancient rain of comets that delivered the water and organic molecules that gave life its start on Earth.

Learning whether this comet is dense or loosely packed could even help determine how best to destroy or deflect any future comet that threatens Earth.

"There will be no definitive `Aha!' on these questions, McFadden said. Tempel 1 is just one comet among many, and Deep Impact will surely expose new riddles.

Once feared as omens of calamity, comets have been studied since the 16th century by scientists who discovered their periodic orbits around the sun.

Scientists believe comets formed at the birth of the solar system in the Oort Cloud - a frozen region hundreds of thousands of times farther from the sun than Earth - and remained mostly unchanged ever since.

Astronomers couldn't see a comet's tiny nucleus directly. But they could study the dust and gas that erupted from it as it neared the sun's warmth. It billowed into a vast cloud, or "coma," hundreds of thousands of miles wide, and was swept by the solar wind into a characteristic "tail" millions of miles long, all visible from Earth.

By the 1950s, spectroscopic analyses of comets' comas and tails had revealed the presence of water ice, frozen gases and carbon-based organic molecules. American astronomer Fred Whipple speculatively described the unseen comet nucleus as a kind of "dirty snowball."

Since 1985, scientists have moved in for a closer look, sending a series of spacecraft flying past comets Halley, Giacobini-Zinner, Borrely and Wild2. But they wondered whether what they saw on the comets' surface, or in their comas and tails, had been changed chemically by the sun.

What they really needed was a peek into a comet's pristine, insulated interior. A'Hearn proposed Deep Impact as a rare controlled experiment in astronomy - the first mission to look inside a comet by blowing a hole in it.

Tempel 1 became the target because it is convenient. Discovered in 1867 by German astronomer Ernst Wilhelm Tempel, it orbits the sun once every 5 1/2 years, coming within economical range of a spacecraft on every other pass.

The mission evokes a 1998 movie called Deep Impact in which actor Robert Duvall is dispatched to nuke a comet that threatens to erase civilization.

The real-life Deep Impact mission was launched from Cape Canaveral, Fla., on Jan. 12 of this year. It has traveled nearly 268 million miles toward tomorrow's rendezvous with Tempel 1.

The excitement at College Park, and at NASA's Jet Propulsion Laboratory in Pasadena, Calif., has been building for weeks. "It feels like we're moving at 67,000 mph [the speed of the comet through the solar system]. Things are happening fast," McFadden said.

Deep Impact's cameras have been snapping photos of the comet and its icy nucleus at an accelerating pace so controllers can fine-tune the craft's trajectory.

Pictures taken last year in visible and infrared light by the orbiting Hubble and Spitzer space telescopes revealed a pitch-black, peanut-shaped object 8.7 miles long and 2.5 miles wide, rotating once every 41 hours.

"We found out the comet is rotating very slowly, which is very good for us because the crater isn't going to rotate out of our field of view as we fly by," said mission scientist Carey M. Lisse, a senior research scientist at the Johns Hopkins University's Applied Physics Laboratory.

The information helped scientists plan their observations and set their camera exposures.

If all goes well, Deep Impact will release an "impactor" the size of a kiddie swimming pool just after midnight this morning and then veer off on a course that will carry it safely past the comet.

A kind of interplanetary geologist's hammer, the impactor has its own camera, steering and guidance. But at its heart is a 249-pound lump of copper, dead weight on board only to add mass and energy to the impact.

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