Every day our neighborhood appears a bit more crowded--and dangerous. The band between Earth and Mars hosts swarms of swift-moving asteroids, some of which might eventually threaten our planet. The inner solar system is home to an estimated 1,000 to 1,500 asteroids a kilometer or greater in width, with perhaps a million rocks 50 meters and larger. Asteroid observations pour in at the rate of 15,000 or more a day.

The burden of keeping track of near-Earth objects (NEOs)--asteroids and the occasional comets that pass through our vicinity--falls on Brian Marsden. Since 1978 he has directed the Minor Planet Center (MPC) at the Smithsonian Astrophysical Observatory in Cambridge, Mass. Sky watchers from all over the world send putative sightings to the MPC, which operates on behalf of the International Astronomical Union (IAU). The MPC processes and organizes data, identifies objects, computes orbits, assigns tentative names and disseminates information on a daily basis. For objects of special interest, the center solicits follow-up observations and requests archival data searches. "We are the focal point," Marsden says. "All the observations come here."

   
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