THERE are a couple of astronomical anniversaries this month worth noting.
It's 191 years since a group of sky-scanners, who styled themselves the "celestial police," discovered Ceres, the first officially recorded asteroid. And it's just a year since asteroid 1991 BA whizzed past Earth at a distance of 170,000 kilometers - less than halfway to the moon.
The alarm bells that near collision set off publicized a growing concern among asteroid astronomers that the time has come to revive the celestial police.
Now the United States National Aeronautics and Space Administration is studying how to do it. NASA is responding to last year's congressional directive that the agency assess ways to increase surveillance of near-Earth asteroids (NEAs) and suggest possibilities for deflecting any that threaten our planet.
The agency is convening a study group this month at the Los Alamos National Laboratory in New Mexico to explore what might be done. Meanwhile, another NASA-sponsored international study group - members of the Near-Earth Asteroid Detection Workshop - are finishing a report on asteroid surveillance.
Workshop chairman David Morrison, who heads the Space Science Division at NASA's Ames Research Center at Moffett Field, Calif., says the group has looked at how to discover NEAs that are one kilometer across or larger. No one knows what the lower size limit is for an asteroid that could cause global-scale damage. Dr. Morrison explains that it may be at least several kilometers. The workshop picked one kilometer to be conservative.
The workshop report will recommend building a global surveillance network of six telescopes, each two to three meters in diameter. Cost is uncertain since several different countries would be responsible for the instruments. Morrison notes that telescopes of this size should cost $6 million to $8 million each. So the total cost might be on the order of $50 million. Such a network should be able to do a complete census of NEAs of concern in 10 to 15 years.
The NASA workshop participants are not alone. The International Astronomical Union has also set up an asteroid-threat working group. Meanwhile, Thomas Gehrels, David Rabinowitz, and James Scotti of the University of Arizona in Tucson have already begun a dedicated search for NEAs using the Spacewatch telescope on nearby Kitt Peak.
We should take the asteroid/comet threat seriously. It has existed since Earth was formed 4.5 billion years ago and organic life arose upon it. Major impacts have shaped the planet's surface. They may also have helped direct the evolution of life on Earth by causing occasional mass extinctions, including that of the dinosaurs. Throughout these billions of years, our planet has been a passive target.
Now we at last have the capability to foresee possible collisions, perhaps many decades in advance, giving us time to avert disaster. A change in an asteroid's orbital speed of as little as one centimeter a second could deflect it from a collision course. A well-placed nuclear missile might do the trick.
With such a potential payoff, spending $50 million or so on an international tracking network would be a wise investment. Furthermore, Morrison says it would generate knowledge that would delight solar-system astronomers. Let's make them happy and make ourselves a little safer.