On the power of fear: "I think a good scare is occasionally helpful."

Of 42 near-Earth asteroids with nonzero impact risk, only one object, 1997 XR2, warrants careful monitoring. See http://neo.jpl.nasa.gov/risk/" data-pin-do="buttonBookmark">

BRIAN MARSDEN: HEADS UP IN DANGER

On national security: "There's a lot of concern about security these days, but does it go beyond terrorism and extend to outer space?"

On the power of fear: "I think a good scare is occasionally helpful."

Of 42 near-Earth asteroids with nonzero impact risk, only one object, 1997 XR2, warrants careful monitoring. See http://neo.jpl.nasa.gov/risk/ Image: JASON GROW

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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."

Marsden has served as the referee for all NEO sightings over the past 25 years--a period in which the total search effort has grown from a fledgling survey or two into a productive and efficient international network. At times this role has put him in the middle of controversy. Perhaps the most notorious incident occurred on March 11, 1998, when Marsden indicated on the MPC Web site that an asteroid discovered in December 1997 (then named 1997 XF11 and now called asteroid 35396) would make a close approach in 30 years. "The chance of an actual collision is small," he wrote, "but one is not entirely out of the question." That phrase set off a media circus that ranked among the 20 top science debacles of the century, according to Discover magazine.

Marsden admits that his word choice was "ill advised" but insists the calculation was correct at the time. He emphasized its uncertainty in the original notice and asked for more data. When the computations were redone a day later, incorporating orbital information from an old photograph, the threat vanished.

"Much as the incident was bad for my own reputation, we needed a scare like that to bring attention to this problem," Marsden remarks. "Many wondered whether I'd survive, but I'm still here." More important, he says, the field itself has prospered. In the wake of XF11 publicity, NASA increased funding for asteroid searches from $1 million to $3.5 million annually. In addition, groups at the University of Pisa in Italy and the Jet Propulsion Laboratory in Pasadena, Calif., began doing routine risk evaluations of potentially menacing objects. To date, the confirmed NEO total includes about 2,250 asteroids, a dozen comets that complete their orbits in less than 200 years, and 1,000 long-period comets (on orbits 200 years or longer) that pose no immediate concern.

It's a far cry from the early 1960s, when Marsden started adding to the list of some two dozen known NEOs with detections he made as a Yale University graduate student. After earning his Ph.D. in astronomy in 1965, he took a job at the Smithsonian, where he has worked ever since.

When Marsden began studying minor planets, "nobody cared about asteroids. They were dismissed as 'vermin of the sky.'" Now the study is a bona fide field, thanks to automated search programs such as the Lincoln Near-Earth Asteroid Research (LINEAR), run by the Massachusetts Institute of Technology Lincoln Laboratory, and NASA's Near-Earth Asteroid Tracking (NEAT), which collectively account for 90 percent of all NEO detections. The MPC is hard-pressed to keep up with the tide of incoming data, especially with a volume of main-belt asteroid observations 100 times as great as that for NEOs.

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