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.

View: Full Article | Source: Nasa.gov