Even at the height of panicky reports that the world as we know it would end in 2028, courtesy of a one-mile ball of rock code-named 1997 XF11, Brian Marsden remained calm. The Smithsonian astronomer had published the first notice of the asteroid's very close encounter with Earth. But he knew more accurate information would follow, which he would disseminate just as quickly. Issuing such notices for the world's sky gazers is, in fact, Marsden's job as head of the International Astronomical Union's (IAU) Central Bureau for Astronomical Telegrams (CBAT) and Minor Planet Center (MPC). Along with the news, Marsden's fateful circular also requested more data, a routine precaution given that no one yet knew the exact orbit of XF11.

Unfortunately, Marsden's request was all but overlooked during the ensuing hysteria. Less than a day later, researchers found 1990 photographs that refined XF11's orbit and proved it would not only bypass us by a comforting 600,000 miles (more than twice the distance from Earth to the Moon) but stay away for millennia to come. Once again, Marsden and his colleagues Dan Green and Gareth Williams did what they always do: they let the world know.

"We're the UPI and the AP of the astronomy business," says Dan Green with a grin. Comparison to 24-hour international news agencies does sound a bit funny as we sit in Brian Marsden's conspicuously not-buzzing office at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. But it's no exaggeration. Nearly every time you've heard of a brilliant new comet (or an asteroid heading dangerously close), that news originated in these modest offices perched amid telescope domes and trees on the aptly named Observatory Hill.

As computers here silently compile information from astronomers, it's easy to feel far removed from 1847, when the then-largest telescope in America, the 15-inch "Great Refractor," was built and housed just down the hall. That same year, American astronomer Maria Mitchell discovered a comet, for which she was awarded a gold medal by the king of Denmark.

Still, it wasn't until 1882 that astronomers, prompted by the discovery of one of the brightest comets ever seen, established the first international bureau for reporting celestial discoveries. In 1922 it was succeeded by the Central Bureau for Astronomical Telegrams. The bureau moved from its Copenhagen home to the Smithsonian Astrophysical Observatory in 1964. A few years later, Brian Marsden began overseeing its reports of comets, novas and supernovas. (The Minor Planet Center, started in 1947 in Cincinnati to handle discoveries and observations of small orbiting bodies such as asteroids, came under Marsden's aegis in 1978.) As the world's astronomical news agencies, the CBAT and the MPC work with individuals and a host of other organizations, some for amateurs, around the globe to track celestial finds.

At first glance, what Marsden, Green and Williams do is outrageously simple. Every hour of every day, astronomers both professional and amateur e-mail or phone the CBAT or the MPC with information on something they have seen that is not on the charts. That information is compared with known objects at that location.

Observatories or individual sky watchers may be asked to take a look. Once convinced, the CBAT or the MPC will send the news out to subscribers. More notifications are sent as the object's orbit is refined or its spectrum analyzed. Despite being full of language only astronomers could know and love, these reports are hardly bland dispatches from the cold depths of space: they help determine priority in discoveries."There's a lot of competition among observers," notes Marsden, "so they often won't deal with each other. But they will deal with us." The fun, and worry, of naming newly discovered celestial objects goes with the territory. Like the famous Hale-Bopp and Shoemaker-Levy 9, comets are named by the IAU after their discoverers. Minor planets, on the other hand, can be named after someone their discoverers want to honor (hence rocker Frank Zappa's immortalization through minor planet Zappafrank).

Before anyone's claim to celestial fame is ensured, however, new objects need to be certified as such. That depends first on the observers themselves, to whom the CBAT and MPC give extensive guidelines on how and when to report observations. Such caution is born out of the proverbial dust on the telescope lens: as many as four out of five comet reports from amateurs turn out to be false alarms.

Even once an observer is sure of her discovery and reports it, astronomical immortality is at the mercy of further checking. For this, the Smithsonian trio depends on the good eyes of more than a hundred observers around the globe — up from fewer than a dozen just 20 years ago. Many in this valuable group are advanced amateurs. "It's only with the electronic revolution that amateurs can measure accurately on their images the positions of objects they see," explains Marsden.

Charge-coupled device (CCD) cameras are a big reason. Using silicon chips that convert the energy of incoming photons into electronic signals readable by computer, CCDs can quickly provide positions for even extremely faint objects. "Photographs had to be exposed for hours and took days to measure," says Dan Green, "and even then you might have had only three or four observations over a couple of nights. Now, people regularly send us CCD positions within minutes of observing an object."

The result is a huge increase in amateur and professional discoveries. "The number of supernovas has gone up by a factor of 4," notes Green. "In terms of comets, we're looking at a factor of 3."

Technology hasn't made only people better sky watchers, either. Across the hall from Marsden and Green, Gareth Williams swings his chair between two large computers, into which he swiftly types numbers. And more numbers. "I'm processing some 800 LINEAR observations," he says without pausing to look up, "from what was submitted from six nights of observation." Around him sit stacks and stacks of data on asteroids and other celestial objects found by an innovative computer- and CCD-driven telescope in Socorro, New Mexico, known as LINEAR. Since it began operations in 1997, LINEAR has discovered more than 50,000 minor planets in our solar system. (For anyone counting, that works out to about one-quarter of all minor planets discovered since 1925.)

As I watch, Williams' computers are engaged in an invaluable matching game, in which LINEAR's observations are compared with the orbits of known objects. The numbers are humbling. "This is not everything that LINEAR was doing for those six nights," Williams says. "We get 15,000 observations from them every day." He pauses long enough to grin ruefully at the paper mountains by his side. "I can get more from LINEAR in one night than I used to get in several months in 1990."

Of course, how news gets out to CBAT and MPC subscribers has undergone a revolution in itself. Less than 20 years ago, when e-mail was still evolving, telegrams were sent to the CBAT by astronomers with discoveries to report; the CBAT itself used telegrams to announce that news to the world. Those who could wait received their circulars as postcards.

Today, the "telegram" in the CBAT's name is a pleasant reminder of bygone technology. Astronomers rarely send reports by telegram when faster e-mail is available. And e-mail has supplanted telegrams as the main mode of distributing reports to subscribers, though postcards are still used. Not surprisingly, the Web is crucial. There, most of the CBAT and MPC reports are available, and ephemerides for unconfirmed near-Earth objects (NEOs) are posted for anyone to quickly update.

As the scare over XF11 proved, what the CBAT and MPC publish is hardly the stuff of astronomical esoterica. Before looking into relocating underground, however, one can get more details on close approaches in this century by looking at the MPC's compilation on the Web. There, I see, the half-mile-wide asteroid 1999 AN10 (discovered in January 1999 by LINEAR) could on August 7, 2027, come as close as 240,000 miles — as near as the Moon (gulp!).

"It can't hit us," Marsden reassures me, noting that the number listed is the smallest known distance from us. All information here is constantly updated by that valuable group of astronomers he, Green and Williams call on for just that. In AN10's case, helpful clarification of its minimum approach distance in 2027 came from amateur Frank Zoltowski. "We found that there was a lot of uncertainty about whether it will come close in 2027," Marsden says. "Frank's measurements made it clear that it would be well within a million miles. This actually increased the likelihood it could hit Earth in 2044. But with data gleaned by two amateurs from 1955 photographs, it became clear that AN10 will miss us by a large margin in 2044."

He pauses, then adds, "That's why we need to keep making observations: many people involved with discovery programs think that all you need to do is discover objects. But 1997 XF11 and 1999 AN10 show that we need to pay attention to what is found."

By Valerie Jablow