Planetary scientists have developed a new means of conveying the risks associated with asteroids and comets that might collide with the Earth.
A risk-assessment scale, similar to the Richter scale used for earthquakes, will assign values to celestial objects moving near Earth. The scale will run from zero to 10. An object with a value of zero or one will have virtually no chance of causing damage on Earth; a 10 means a certain global climatic catastrophe.
The scale was created by Dr. Richard P. Binzel, professor of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology (MIT) in Cambridge, MA. It is named the Torino Impact Hazard Scale after the Italian city in which the scale was initially adopted by the International Astronomical Union (IAU) in June 1999.
"These events have a small probability of occurring, but if they happen they can have severe consequences," said Binzel. "It is difficult to figure out what level of anxiety we should have about an approaching asteroid or comet. I hope the Torino scale will put in perspective whether a Near-Earth Object merits public concern, just as the Richter Scale does with earthquakes."
The scale is being endorsed officially today by the IAU in an announcement at the United Nations' UNISPACE III conference in Vienna, Austria.
" 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 Dr. Hans Rickman, IAU Assistant General Secretary.
The scale takes into account the object's size and speed, as well as the probability that it will collide with Earth. The scale can be used at different levels of complexity by scientists, science journalists and the public.
Close encounters, assigned Torino-scale values from two to seven, could be categorized as ranging from "events meriting concern" to "threatening events." Certain collisions would merit values of eight, nine or 10, depending on whether the impact energy is large enough to cause local, regional or global devastation.
No asteroid identified to date has ever had a value greater than one, noted Binzel, who has been working on the scale for five years. Several asteroids that had initial hazard scale values of one have been reclassified to zero after additional orbit measurements showed that the chances of impact with the Earth were essentially zero.
"Nobody should lose sleep over an asteroid in the zero or one category," Binzel said. "Scientists haven't done a very good job of communicating to the public the relative danger of collision with an asteroid. The Torino Scale should help us clearly inform but not confuse the public."
Increasingly sophisticated equipment, partially funded by NASA, such as the Lincoln Near Earth Asteroid Research project at MIT's Lincoln Laboratory in Lexington, MA, is used to detect and track a growing number of an estimated 2,000 Near-Earth Objects larger than about a half-mile (one kilometer) in diameter. The project uses technology originally developed for the surveillance of Earth-orbiting satellites. It has detected almost 250,000 asteroids to date, more than any other source. Of these, 228 are newly discovered Near-Earth Objects.
Large asteroids are rarely a threat to the Earth. An asteroid bigger than a mile across might hit once every 100,000 to one million years on average. On the other hand, tiny meteorite fragments as big as grains of sand bombard Earth constantly, and objects the size of a small car hit a few times a year.
Once an asteroid is detected, scientists use tracking data from 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 path of an object may be altered by gravity if it passes close to Earth or another planet. As more information is gathered about a particular asteroid, its placement on the scale can be adjusted.
"The Torino scale is a major advance in our ability to explain the hazard posed by a particular object," said Dr. Carl Pilcher, science director for Solar System exploration in NASA's Office of Space Science, Washington, DC. "If we ever find an object with a value greater than one, the scale will be an effective way to communicate the resulting risk."
A more detailed explanation of the points on the Torino scale and related graphics are available on the Internet at:
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