Comets have long lit up the sky and the imaginations of scientists. Now these icy bodies from the beginnings of the solar system are finally ready for their close-up.
Six months after NASA scientists first peeked inside one comet from afar, they're bringing pieces of another to Earth for study under the microscope.
This weekend, the Stardust spacecraft will jettison a 100-pound capsule holding comet dust. It will nosedive through the Earth's atmosphere and — if all goes well — make a soft landing in the Utah desert.
The searing plunge is expected to generate a pinkish glow as bright as Venus that should be visible without a telescope across much of the West.
Comets — which astronomers consider to be among the solar system's leftover building blocks — have been scrutinized for centuries. But only in recent years have scientists had the technology to learn firsthand their ingredients.
Last July, the Deep Impact spacecraft released a probe that carved a crater in a comet, exposing its interior to NASA telescopes. The Stardust mission went a step further by retrieving the first samples from a comet named Wild 2, which was about 500 million miles from Earth when Stardust launched in 1999.
Comets are bodies of ice and dust that circle the sun. About 4.5 billion years ago, a cloud of gas and dust collapsed to create the sun and planets. Comets formed from what was left over, and scientists believe studying them could shed light on the solar system's birth.
But the capsule isn't home yet.
First it faces a blistering descent, piercing the atmosphere at a record-breaking 29,000 mph — the fastest re-entry of any man-made probe.
Its target is Dugway Proving Ground, a Rhode Island-sized Army base southwest of Salt Lake City where in 2004 the ill-fated Genesis probe crashed on live television after its parachute failed to open. Despite that crash, scientists recovered enough solar wind atoms for study.
To avoid another embarrassment, engineers checked Stardust's systems and believe they will work, said Ed Hirst, a mission system manager at NASA's Jet Propulsion Laboratory in Pasadena, which is managing the $212 million mission.
Stardust traveled nearly 3 billion miles halfway to Jupiter and back, looping around the sun three times. Along the way, it also captured interstellar dust — tiny particles thought to be ancient stars that exploded and died.
After five years, the 850-pound spacecraft finally reached Wild 2.
During a historic 2004 flyby, Stardust sped through the comet's coma — the fuzzy shroud of gas and dust that envelops it — to collect the microscopic samples. The particles were trapped by a catcher the size of a tennis racket, which has since been clammed up inside the capsule for the trip home.
Comet particles from Stardust would represent the second robotic retrieval of extraterrestrial material since 1976, when the unmanned Soviet Luna 24 mission brought back moon samples.
If all goes as planned, the main spacecraft will free the shuttlecock-shaped capsule about 69,000 miles from Earth late Saturday. Then the mothership will fire its thrusters and go into a perpetual orbit around the sun.
Early Sunday, the capsule will penetrate the atmosphere. As it tumbles toward the Utah desert, the temperature on its protective heat shield will spike to 365 degrees.
Traveling at supersonic speed, the capsule will release its first parachute at 100,000 feet, followed minutes later by a larger chute, which will guide it to a landing.
During Genesis, helicopters were deployed to retrieve the capsule in mid-air, but poorly installed gravity sensors on the capsule caused its parachute to fail.
For Stardust, helicopters will fly to the landing site only after the capsule has touched down. Crews will recover the capsule and bring it to a temporary clean room on the base before transferring it to the Johnson Space Center in Houston.
If the weather is too snowy or windy for helicopters to fly, NASA will send off-road vehicles to the landing site.
Scientists believe thousands of particles of comet and interstellar dust, most smaller than the width of a human hair, are locked inside the capsule.
To determine the makeup of the particles, scientists will slice the samples into even smaller chunks and probe them under powerful microscopes, said Brownlee, the mission's principal investigator.
"We are literally bringing back samples of the solar system as they were billions of years ago," he said.
If Stardust is not on target for a weekend re-entry, engineers can command the spacecraft to fire its thrusters to a backup orbit. That would postpone its return to Earth four years.