Who Plays Paul Revere When a Comet Is Coming?

At a press conference, the president of the United States announces that a year earlier astronomers had discovered a comet. They quickly found that the "dirty snowball" had Earth in its cross hairs. Impact, the president says, is a year away.

Everyone in the press corps - except for "Deep Impact's" intrepid MSNBC producer, who figured it out beforehand - is dumbfounded.

In a movie director's dreams!

These days, astronomers say, nobody could keep an impact forecast secret. Too many astronomers - including some amateurs - can track objects in space, tap existing orbit data, and calculate new orbits.

Nor can comets' orbits be pinned down with the speed and certainty they were in the film.

Researchers at the CalTech Jet Propulsion Laboratory in Pasadena, Calif., ran a simulation that tweaked the orbit of comet Hale-Bopp to put it on a collision course with Earth, notes Alan Harris, a senior member of the technical staff in the lab's Earth and Space Sciences Division. The evidence did not leave enough time to react..

"They knew for sure it would 'hit' only three months in advance" of the simulated collision, he says.

Still, astronomers acknowledge some vexing questions about potential impacts: How long should they wait to make the announcement? And how should the risk of collision be reported to avoid sensationalism?

Their case study is last March's false alarm involving asteroid 1997XF11. In 24 hours, a heads-up from the International Astronomical Union Minor (IAU) Planet Center (MPC) in Cambridge, Mass., that the asteroid could pass uncomfortably close to Earth in 2028 was replaced by an "all clear." More data on the object had been uncovered, and other astronomers double-checked the MPC's calculations. The asteroid would miss Earth by a wide margin.

"1997XF11 was important for bringing visibility to the problem" of near-Earth objects (NEOs), says Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, Mass. But the cost in lost credibility, he adds, was too high.

Since then, US astronomers and NASA have proposed interim guidelines for NEO watchers. An object that appears to be on an eventual collision course with Earth should initially be reported to colleagues only, who should try to confirm the object's orbit. That process should be completed within 48 hours.

So far, so good, several astronomers say. That represents a prudent backstop.

Indeed, the MPC, the world's clearing house for asteroid and comet discoveries and orbits, has reprogrammed its computers to help.They take freshly reported positions for an object and update its orbit. If the orbit looks suspicious, the computers temporarily remove the data from the center's daily reports, which are e-mailed worldwide.

The computers notify MPC officials, who solicit second opinions before releasing the new position and orbit information.

But researchers chafe at NASA's insistence that it be given an additional 24-hour's notice before an announcement of a possible collision is made public.

From the agency's viewpoint, it merely wants to avoid being caught flat-footed.

But, that stipulation "does not sit well with the international community," says Gareth Williams, MPC deputy director. "Observers around the world are aghast that a US institution would try to corner the market on this information."

The danger, he and others say, is that additional delays could be construed as a coverup.

"Much more so than NASA's people, we are concerned that there be no hint of secrecy or keeping things quiet," says Michael A'Hearn, an astronomy professor at the University of Maryland at College Park and the president of one of the IAU panels involved in drafting international guidelines.

The IAU goal, he adds, is to develop reporting rules that are acceptable to the international community, as well as to NASA headquarters.