Comets, according to the most popular current theory, are "rubble piles," agglomerations of primordial ice and debris loosely held together by gravity as they hurtle darkly through the solar system, perking up only when close exposure to the sun broils them away like celestial Roman candles.
But the images of "Comet Wild 2," captured by NASA's Stardust spacecraft during a half-hour flyby Jan. 2, have left astronomers scratching their heads. This comet, at least, may be made of somewhat sterner stuff.
"I was expecting very bland and subdued features," rounded off as the sun peeled off layers of the comet, said University of Washington astronomer Donald E. Brownlee, lead scientist for the mission.
Instead, Brownlee and the Stardust research team reported in today's issue of the journal Science, Wild 2, is a hamburger-shaped mass, 2.8 miles across, of something tough enough to support cliffs hundreds of feet high, craters more than a mile wide, and assorted pinnacles, mesas and even overhangs.
"The material clearly has strength, and the overhang is surprising," University of Maryland astronomer Michael A'Hearn said. "It's 100 meters [about 325 feet] sticking out. I wouldn't have guessed it could sit there, even in the [tiny] gravity of the comet. We don't know why this thing is shaped the way it is."
Or why it behaves the way it does. Stardust's "Dust Flux Monitor Instrument," which measured the size and distribution of particles flying off the surface of Wild 2, found itself periodically enveloped by pulsing "swarms" of particles as much as a 0.06 inches in diameter.
"All the investigators were absolutely stunned; we didn't expect such erratic behavior," said physicist Anthony J. Tuzzolino, of the University of Chicago's Enrico Fermi Institute. "They came so suddenly. Nobody knows what happened."
Tuzzolino and other members of the team suggested in Science that the swarms might result from large chunks of material breaking free of the comet's surface only to disintegrate further into curtains of much smaller particles. Out of about 10,000 impacts detected on Stardust's shielding, the vast majority of the particles were "smaller than a pinhead," Tuzzolino said.
NASA launched Stardust Feb. 7, 1999, on a seven-year journey that has so far included about 2.3 billion miles and a loop around Earth to gain a gravity boost and to slingshot the spacecraft beyond Mars's orbit to reach Wild 2 (pronounced "Vilt"). The encounter occurred about 242 million miles from Earth on the far side of the sun.
Comets are ancient objects composed of dust, ice and debris from the formation of the solar system about 4.5 billion years ago and left largely unmolested in cold, remote solar orbits beyond the planet Pluto. By studying comets, scientists can find clues to the solar system's early history and perhaps the origins of life.
Occasionally their orbits are "perturbed" by the gravity of the sun, nearby stars or large planets, and they enter the inner solar system. Wild 2 swooped into range after Jupiter jogged its orbit in 1974.
Wild 2 "ran over" Stardust at a closing speed of about 14,000 miles per hour. Stardust -- about five feet tall and weighing 840 pounds at takeoff -- was designed not only to weather an expected battering by the comet's shroud of particles, but also to capture millions of these particles in a celestial "catcher's mitt" composed of a frothy material called aerogel.
This maneuver took place without a hitch, and the 101-pound return capsule with its precious cargo of primordial dust is en route home for a Jan. 15, 2006, parachute landing in the Utah desert. Stardust is the first mission to grab samples from space since the last moon landing in 1972.
Comets peel away through sublimation -- the sun's rays evaporate water and carbon dioxide ice in the comet and spew the fiery tail of dust familiar to Earth observers. Brownlee described comets as "large dirt balls with an ice component."
Spaceship flybys of previous comets -- Halley's in 1986, and Comet Borelli in 2001 -- had led scientists to expect that Wild 2's nucleus, the comet's main body, would have few discernible surface features and be covered with a coat of charcoal-like dust.
But Wild 2's high relief has disturbed the "rubble pile" interpretation: "Features like these have never been seen on comets," Brownlee said. "The comet probably formed as a rubble pile, but for some reason it has been hardened." Brownlee suggested the comet was composed of relatively rigid but brittle material, easily pocked but able to absorb impacts without flying apart.
"Something with cliffs that high would have to have the consistency of a souffle," added Donald Yeomans, a senior research scientist and comet expert at NASA's Jet Propulsion Laboratory. "I think the surface suggests that [the comet] is very porous and fragile."
And mysterious. Brownlee said that Wild 2, like most comets, is rich in carbon-based material that may have gelled to form a rigid crust over a mushier interior: "The common wisdom would be that any hardening process would happen when the comet [heated up] as it approached the sun."
"But the crater at the top of the comet is a kilometer [0.6 miles] in diameter," Brownlee added, and there is no way it could have been made recently. "This means the comet was somehow hardened when it was out there" in the solar system's nether reaches, he said.