Professor Richard P. Binzel has come up with a scale that assigns a number to the likelihood that an asteroid will collide with the Earth. Zero or one means virtually no chance of impact or damage; 10 means certain catastrophe.
Dr. Binzel, professor of earth, atmospheric and planetary sciences at MIT, created the scale to help scientists, the media and the public assess the potential danger of asteroids. He hopes it will assuage concerns about a potential doomsday collision with the Earth.
Professor Binzel's risk-assessment system is similar to the Richter scale used for earthquakes. It is named the Torino Impact Hazard Scale for the Italian city in which it was adopted at a workshop of the International Astronomical Union (IAU) in June.
The IAU announced on July 22 at the third United Nations conference on the exploration and peaceful uses of outer space (UNISPACE III in Vienna) that it has officially endorsed the Torino scale to gauge potential impacts with asteroids and comets, collectively referred to as near-Earth objects (NEOs).
"What I find especially important about the Torino impact scale is that it comes in time to meet future needs as the rate of discoveries of near-Earth objects continues to increase," said Hans Rickman, IAU assistant general secretary.
"The Torino scale is a major advance in our ability to explain the hazard posed by a particular NEO," said Carl Pilcher, science director for solar system exploration in the NASA Office of Space Science in Washington, DC. "If we ever find an object with a greater value than one, the scale will be an effective way to communicate the resulting risk."
"Naming the newly proposed hazard scale after Torino is a highly appreciated recognition of the Torino Astronomical Observatory's great deal of work over the past two decades," said Alberto Cellino, astronomer at the Torino Astronomical Observatory.
Based on the orbit trajectory for a given NEO, the scale takes into account the object's size and speed as well as the probability that it will come into contact with the Earth. The scale can be used at different levels of complexity by scientists, science journalists and the general public.
The scale assigns a number from zero through 10 to a predicted close encounter by an NEO. A zero, in the white zone, means that the object has virtually no chance of colliding with the Earth or that the object is so small it would disintegrate into harmless bits if it passed through the Earth's atmosphere. A red 10 means that the object will definitely hit the Earth and have the capability to cause a "global climatic catastrophe."
Close encounters in the green, yellow and orange zones with "scores" from one to seven are categorized as "events meriting careful monitoring" to "threatening events." Certain collisions fall in the red zone, with values of eight, nine or 10, depending on whether the impact energy is large enough to be capable of causing local, regional or global devastation.
No asteroid identified to date has ever made it out of the green zone by having a scale value greater than one. Several asteroids that had initial hazard scale values of one have been reclassified into category zero after additional orbit measurements showed that the chances of impact with the Earth became zero. All currently known asteroids have scale values of zero.
A COSMIC SHOOTING GALLERY
Professor Binzel, who has been developing the scale for five years, aims to give scientists a consistent way to communicate about the growing number of close-encounter asteroids being spotted. Increasingly sophisticated equipment, partially funded by NASA, such as the Lincoln Near Earth Asteroid Research (LINEAR) project at MIT's Lincoln Laboratory in Lexington, is being used to detect and track a growing number of the estimated 2,000 NEOs larger than about a half-mile (1 kilometer) in diameter.
The LINEAR project uses technology originally developed for the surveillance of Earth-orbiting satellites to detect and catalog NEOs. It has detected almost 250,000 asteroids to date, more than any other source. Of these, 228 are newly discovered NEOs.
While more asteroids than ever are being identified in the cosmic shooting gallery inhabited by our planet, Professor Binzel points out that there is no increase in the number of asteroids out there -- only in our awareness of them. Because we know about more asteroids, there is an increasing awareness that many of them can make close passes by the Earth.
"This doesn't mean that the Earth is in any greater danger," he said. "Fortunately, the odds favor that newly discovered objects will miss."
On the other hand, space-borne objects do hit the Earth. Tiny fragments as big as grains of sand bombard us constantly, and objects the size of a small car hit a few times a year. An asteroid bigger than a mile across might hit once every 100,000 to 1 million years. The planet bears scars from these encounters.
In the 1960s, Eugene Shoemaker of the US Geological Survey proved that a big dent in the Arizona desert is a meteor crater. Most scientists believe that the dinosaurs were wiped out by a massive object 65 million years ago. The well-documented collision of the Shoemaker-Levy comet with Jupiter demonstrates that impacts are still a reality in the solar system today, but, Professor Binzel noted, "No one has clearly documented deaths from a meteorite impact."
"If you tell a Californian that an earthquake registering one on the Richter scale was going to hit tomorrow, he would say, 'So what?'" Professor Binzel said. "If you were talking about a six, that would be different."
So he hopes it will be with asteroids. Nobody should lose sleep, he said, over an asteroid in the zero or one category, which accounts for the vast majority of them. He hopes to avoid sensationalism such as that surrounding the 1997 XF11 asteroid that led to the New York Post headline of March 13, 1998, "Kiss your asteroid goodbye."
"Scientists haven't done a very good job of communicating to the public the relative danger of collision with an asteroid," said Professor Binzel, who is a specialist on planetary astronomy. "Scientist-astronomers who are going to be confronted with this should have some means of clearly communicating about it so as to clearly inform but not confuse or unnecessarily alarm the public."
Once an asteroid is detected, scientists try to use information that shows a tiny section of its orbit to calculate where it will be in 10, 15 or 100 years. There is some uncertainty in this prediction because the orbit measurements are not perfect and the NEO may be altered by gravity if it passes close to the Earth or another planet, but "orbits generally behave like clockwork," Professor Binzel said.
As more information is gathered about a particular asteroid, its placement on the scale can be adjusted accordingly, he points out. "It is hoped that in all cases the placement will go to zero.
"What I hope the scale will accomplish is to put in perspective whether an object merits concern," he said. "This is a case of a high-consequence but low-probability event. It's difficult in human nature to figure out what level of anxiety we should assign to an approaching asteroid."
A version of this article appeared in the September 11, 1999 issue of MIT Tech Talk (Volume 44, Number 2).