Samples of rock dust retrieved from a comet called Wild 2 are forcing scientists to alter the way they think about these intriguing objects that streak through our solar system.
A chemical analysis of the samples brought back to Earth by NASA's Stardust spacecraft showed that the comet is much more like an asteroid than scientists had expected.
Comets are celestial bodies made of rock, dust and ice with characteristic tails of gas and dust streams that are formed in the solar system's distant, frigid reaches. A long-standing notion had been they were sort of a frozen time capsule of material from when the solar system formed 4-1/2 billion years ago, including stardust from other stars.
But this is not the case with Wild 2, scientists found.
A lot of the material detected in Wild 2's cometary dust was formed very close to the sun in the early solar system and was somehow later transported to the outer solar system, the scientists said.
The rock dust closely resembles material from bodies called chondritic meteorites from asteroids in the asteroid belt between Mars and Jupiter, they reported in the journal Science. Asteroids are fragments of ancient space rubble, made of rock and metal, that commonly orbit the sun in that belt.
"Overall, this comet, Wild 2, is looking a lot more asteroid-like than we had expected," Hope Ishii of the U.S. government's Lawrence Livermore National Laboratory, one of the scientists, said in a telephone interview on Friday.
"The material found in primitive objects just wasn't there in the samples," another of the researchers, John Bradley of the Lawrence Livermore National Laboratory, said in a statement. "I think this is science in action. It's really exciting because it's just not what we expected."
Wild 2 is named for Swiss astronomer Paul Wild (pronounced Vilt), who found it in 1978. Its diameter is 3 miles and it orbits the sun every 6-1/2 years.
Stardust, launched in 1999, intercepted Wild 2 in 2004 in the vicinity of the asteroid belt, collecting dust particles from it. The spacecraft returned to Earth in January 2006 with a cargo of the tiny particles for scientists to study.