December 16, 2003
Imagine driving through heavy fog to a place you've never been, guided only by a faint taillight in the distance. The challenge is similar to one NASA will take in January 2004
by flying its Stardust mission through the halo of dust that surrounds the nucleus of a comet.
"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."
One of three methods the Stardust navigation team is using to find their way, optical navigation involves placing a 200mm focal length camera onboard the spacecraft as it flies to its target, a comet called Wild 2 (pronounced Vilt). The camera photographs the view from the spacecraft about twice a week until 10 days before its encounter with the comet. It then photographs the view three times daily until 72 hours before encounter, when it begins taking one image per hour. These images continuously help engineers on the ground figure out where the spacecraft is in relation to the comet. Based on those images and other data, engineers can plan maneuvers accordingly and document the mission. This method is especially necessary since the comet flew behind the sun as viewed from Earth in May 2003, thus making Earth-based observations impossible.
"Trying to view the comet from Earth at this point would be almost like trying to find a firefly behind a fire," Bhaskaran said.
The Wild 2 comet is not expected to emerge until several weeks before its encounter with the spacecraft. Its fiery path aside, the comet is the mission's best available target. The decision to fly by this comet was based on a number of factors including fuel constraints and mission launch date, as well as a database of information established by ground-based astronomers.
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.
As of December 15, the spacecraft will be about 9.5 million kilometers (6 million miles) from the comet, and closing in at a rate of about 530,000 kilometers (330,000 miles) every day. The comet was first spotted by the spacecraft's camera on November 17, and more images have been coming in every few days, allowing the navigators to more accurately determine where the spacecraft is with respect to the comet. Comets, however, are unpredictable objects.
"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)."
Stardust's cometary and interstellar dust samples will help provide answers to fundamental questions about the origins of the solar system. More information on the Stardust mission is available at http://stardust.jpl.nasa.gov.
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