Guts of Comet Encounter
After a nearly five-year chase, the Stardust spacecraft will finally meet comet Wild 2 on the day after New Year’s. It’s a moment Donald Brownlee has anticipated for nearly 25 years.
"This could prove to be a pivotal time for science, a remarkable opportunity to gather evidence that might actually tell us how the planets formed and give us clues about how life on Earth began," said Brownlee, a University of Washington astronomer and principal investigator for the Stardust mission.
|First images of Annefrank asteroid from Stardust
Credit: NASA/JPL, U. Wash
On Jan. 2, 2004, the spacecraft called Stardust will fly within 75 miles of a cometary main body (called Wild-2), close enough to trap small particles from the coma, the gas-and-dust envelope surrounding the comet’s nucleus. Stardust will be traveling at about 13,400 miles per hour and will capture comet particles traveling at the speed of a bullet fired from a rifle.
The main camera, built for NASA’s Voyager program, will transmit the closest-ever comet pictures back to Earth. Launched in February 1999, Stardust was designed to capture particles from Wild 2 and return them to Earth for analysis. The spacecraft already has collected grains of interstellar dust. It is the first U.S. sample-return mission since the last moon landing in 1972.
"Comets have long been an unexplored wasteland," wrote astronomer Fred Whipple of the Harvard Center for Astrophysics. "Today we know that they are black and cold, consisting of ices and dust that coalesced from an interstellar cloud as it collapsed to form the solar system".
Wild 2 (pronounced Vilt) will overtake Stardust about 242 million miles from Earth, on the other side of the sun. "Their physical structure is poorly known," continued Whipple. "Some astronomers guess that comets have low densities, perhaps less than half that of water ice. This material would be crushable in one’s hands. Even the ratio of ices to dust remains a subject of uncertainty. Probably the physical structure varies over small distances such as a few meters".
The spacecraft will capture tiny grains streaming through the coma, the gas-and-dust envelope around the comet’s icy nucleus.
|Aerogel panel used to capture comet particles.
A tennis-racquet shaped collector, using a wispy material called aerogel, will harvest comet grains as they speed past. Meanwhile, a high-gain antenna will transmit close-up pictures and sensitive instruments will gather data about the comet.
The ball of dirty ice and rock, about as big as three Brooklyn Bridges laid end-to-end, was visually detected on November 13 by the spacecraft’s optical navigation camera on the very first attempt. The Wild 2 sighting was verified on November 18 using the second set of optical navigation images downloaded from Stardust. To make this detection, the spacecraft’s camera saw stars as dim as 11th visual magnitude, more than 1,500 times dimmer than a human can see on a clear night.
"With Mars and other planets, we know relatively well where the planets are," said Dr. Shyam Bhaskaran, a Stardust navigation specialist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. "This is not the case with comets, which are not easily observed because they are small objects with gas jets. It is much harder to predict their orbits, which is why we have a little extra help from a camera onboard the spacecraft."
Though the encounter will last about 12 hours, the really intense activity will be over in a matter of minutes. The action will be monitored closely in Stardust’s nerve centers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and Lockheed Martin Space Systems in Denver.
The comet particles will be traveling five times faster than a bullet from a rifle, but the aerogel will stop them in a fraction of an inch. However, because aerogel is as much as 99.9 percent empty space, it will not damage the grains or appreciably alter their characteristics, Brownlee said.
The spacecraft is scheduled to return to Earth in January 2006 and a capsule containing its treasure – less than an ounce of comet dust – will parachute to the Utah desert. The contents will be sent to NASA’s Johnson Space Center in Houston, and then parceled out to scientists around the world, who will begin trying to unlock secrets of the early solar system.
Thousands of tons of microscopic comet particles blanket Earth each year.
|Dust grain or IDP, interstellar dust particle
Credit: UWSTL, NASA Hubble
"Unfortunately, they are difficult to find among the earthly materials," Brownlee said. "And even when extraterrestrial particles can be found, they are cosmic orphans – there is no way to determine their origin."
And such particles cannot give the same kind of information as those taken directly from a comet like Wild 2, he said. That’s because Wild 2 only recently started orbiting close to the sun and so there hasn’t been enough time for the sun’s heat to destroy characteristics that have been preserved in the cold of deep space for billions of years.
Before 1974, the comet’s solar orbit extended from Jupiter to beyond Uranus. But Jupiter’s gravity altered Wild 2’s course, bringing it close enough – just beyond the orbit of Mars – to make the Stardust mission feasible.
"This gives us a real opportunity to find out if our long-held suspicions are right, that comets played a major role in the origin of life," Brownlee said. "No one really knows how life began, but we’re certain that carbon was key to the process. Comets are the most carbon-rich materials in the solar system, and we know they are full of organic compounds that fall on the Earth all the time."
|Icy-rock core of Halley’s Comet.
Credit: ESA/Giotto spacecraft flyby
In addition, comets delivered a significant share of the Earth’s water.
Brownlee, along with colleague Peter Tsou at the Jet Propulsion Laboratory, began considering the possibility of a comet mission in 1980. Five years later he and a team of NASA scientists tried to formulate a mission to Halley’s comet, but that proved to be unworkable. However, technological advances and the fortunate orbit change made a mission to Wild 2 possible.
Stardust, the fourth in the National Aeronautics and Space Administration’s series of Discovery missions and the first mission designed to return samples from beyond Mars, was launched from Cape Canaveral, Fla., on Feb. 7, 1999. It is currently on its third giant loop around the sun, and will have traveled some 3.1 billion miles by the end of its voyage.
In November 2002, the spacecraft successfully tested systems it will use in the Wild 2 encounter during a flyby of Asteroid 5535 Annefrank. During its nearly five years in space, it also has captured interstellar dust using the opposite side of the collector that will gather the grains from Wild 2.
Along with optical navigation, Stardust engineers employ standard Doppler and range tracking techniques during the mission’s cruise phase. The techniques, used by all interplanetary missions, involve relaying radio signals from the spacecraft to Earth via the Deep Space Network’s three worldwide tracking stations in Australia, Spain and California. These signals reveal details about the spacecraft’s orbital path when compared with a mathematical model of the solar system, and allow engineers to pinpoint the spacecraft’s position relative to Earth. Engineers are also able to adjust the path of the spacecraft based on this information.
"We don’t anticipate any surprises, but we have to be prepared, and that’s what makes this job anything but routine," Bhaskaran said. "With a little luck and a lot of skill, we should be able to meet the mission’s goal of flying by Wild 2 at a distance of 300 kilometers (186 miles)."
In the next 5 or so years, there will be no fewer than four encounters of spacecraft with comets and asteroids. All the following missions are fully funded, though only not all have already been launched (the others will follow in 2004):
|2001 Sept. 22||Comet||Borrelly||Deep Space One||(simple flyby)|
|2004 Jan. 2||Comet||Wild 2||Stardust||(coma sample return)|
|2005 July 3||Comet||Tempel 1||Deep Impact||(big mass impact)|
|2005 Sept.||Asteroid||1998 SF36||Muses-C||(sample return)|
Stardust is a collaboration of the UW, NASA and its Jet Propulsion Laboratory at the California Institute of Technology, and Lockheed Martin Space Systems. Other key members are The Boeing Co., the Max-Planck Institute for Extraterrestrial Physics, NASA Ames Research Center and the University of Chicago. Stardust, a part of NASA’s Discovery Program of low-cost, highly focused science missions, was built by Lockheed Martin Astronautics and Operations, Denver, Colo., and is managed by JPL for NASA’s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena. The principal investigator is astronomy professor Donald E. Brownlee of the University of Washington in Seattle.