A numerical measure of asteroid collision hazards similar to the Richter scale for earthquakes. The below was taken wholesale from http://neo.jpl.nasa.gov/torino_scale.html and appears to not be copyrighted.



Events Having No Likely Consequences (White Zone)
Level 0 : The likelihood of a collision is zero, or well below the chance that a random object of the same size will strike the Earth within the next few decades. This designation also applies to any small object that, in the event of a collision, is unlikely to reach the Earth's surface intact.

Events Meriting Careful Monitoring (Green Zone)
Level 1 : The chance of collision is extremely unlikely, about the same as a random object of the same size striking the Earth within the next few decades.

Events Meriting Concern (Yellow Zone)
Level 2 : A somewhat close, but not unusual encounter. Collision is very unlikely.
Level 3 : A close encounter, with 1% or greater chance of a collision capable of causing localized destruction.
Level 4 : A close encounter, with 1% or greater chance of a collision capable of causing regional devastation.

Threatening Events (Orange Zone)
Level 5 : A close encounter, with a significant threat of a collision capable of causing regional devastation.
Level 6 : A close encounter, with a significant threat of a collision capable of causing a global catastrophe.
Level 7 : A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe.

Certain Collisions (Red Zone)
Level 8 : A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years.
Level 9 : A collision capable of causing regional devastation. Such events occur between once per 1000 years and once per 100,000 years.
Level 10 : A collision capable of causing a global climatic catastrophe. Such events occur once per 100,000 years, or less often.

The scale was devised by Richard Binzel a professor of Earth, Atmospheric and Planetary Sciences at MIT. After five years of work the Torino scale was officially endorsed by the IAU on Thursday July 22nd 1999 at the United Nations' UNISPACE III conference in Vienna, Austria. The first version of the scale was callled "A Near-Earth Object Hazard Index" and was presented at a United Nations conference in 1995; a revised version of the "Hazard Index" was then presented in June 1999 at an IAU worshop in the Italian city of Torino. The participants voted to adopt the revised version, naming it "The Torino Impact Hazard Scale" to recognise the international cooperation at the conference.

Asked by the BBC why he decided to create the scale Professor Binzel said:
"Scientists haven't done a very good job of communicating to the public the relative danger of collision with an asteroid," he said. "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."

As a side note no asteroid identified to date has ever made it out of the green zone by having a scale value greater than one.

Resources
Professor Binzel's Homepage : http://www-eaps.mit.edu/faculty/binzel.htm
The BBC's report on the Torino scale : http://news.bbc.co.uk/1/hi/sci/tech/401777.stm
An MIT page on Binzel's research : http://web.mit.edu/giving/spectrum/fall99/briefs.html
A NASA page reporting the scale : http://neo.jpl.nasa.gov/news/news042.html

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