Measuring the Unimaginable

Now there's a Richter Scale for cosmic collisions called the Torino scale.

Related Resources • 2004 Torino Scale • Impact Links    Elsewhere on the Web • Space Frontier Foundation

Earth is a precarious place, but few of us take much action in light of that knowledge. One concrete step was taken in 1999 by researchers who created the first numerical scale for cosmic impact events—a "Richter scale" for meteorite strikes.

You can talk about global warming or giant volcanoes all you want, but the greatest natural threat to the human species is impacts from outer space. Only in the last decade, once the dinosaur-killing asteroid of 65 million years ago was accepted as fact, have geologists taken impacts seriously. And once impacts are permitted in geologic history, they must be considered a possibility for the geologic present.

There is tantalizing evidence from various ancient sources that whole civilizations have been destroyed by comets before the invention of writing. And if you accept that the legend of Noah's Flood is the long-preserved story of a real geologic event, then you have to admit that the other stories, including the legend of Atlantis and the myth of Phaethon, are suggestive too.

And the spectacle of comet Shoemaker-Levy smashing into the planet Jupiter in 1994 got a lot of people thinking. It seems prudent for a civilization like ours, which is starting to think about better ways of living with disasters for the long term, to get ready for that sort of thing. An estimated 2,000 near-Earth objects are orbiting out there, only a couple hundred of which have been found, that could collide with Earth and wipe us out.

In 1995 Prof. Richard Binzel of the Massachusetts Institute of Technology drew up a simple scale, from zero to 10, to indicate how threatening one of those objects might be. A comet rated zero is good for a show, with no chance of harming us. One rated 10, though, is certain to strike us and cause a global-scale catastrophe. The numbers in between signify greater and lesser chances of collision and different sizes of cataclysms.

Binzel presented his scale at a scientific meeting, discussion proceeded, and in June 1999 a gathering of specialists in Torino, Italy, ratified the final version of Binzel's scale. It's named the Torino Scale:

No Hazard (White Zone)	0	The likelihood of a collision is zero, or is so low as to be effectively zero. Also applies to small objects such as meteors and bodies that burn up in the atmosphere as well as infrequent meteorite falls that rarely cause damage.
Normal (Green Zone)	1	A routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger. Current calculations show the chance of collision is extremely unlikely with no cause for public attention or public concern. New telescopic observations very likely will lead to re-assignment to Level 0.
Meriting Attention by Astronomers (Yellow Zone)	2	A discovery, which may become routine with expanded searches, of an object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely. New telescopic observations very likely will lead to re-assignment to Level 0.
	3	A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of localized destruction. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.
	4	A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of regional devastation. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.
Threatening (Orange Zone)	5	A close encounter posing a serious, but still uncertain threat of regional devastation. Critical attention by astronomers is needed to determine conclusively whether or not a collision will occur. If the encounter is less than a decade away, governmental contigency planning may be warranted.
	6	A close encounter by a large object posing a serious but still uncertain threat of a global catastrophe. Critical attention by astronomers is needed to determine conclusively whether or not a collision will occur. If the encounter is less than three decades away, governmental contigency planning may be warranted.
	7	A very close encounter by a large object, which if occurring this century, poses an unprecendented but still uncertain threat of a global castastrophe. For such a threat in this century, international contingency planning is warranted, especially to determine urgently and conclusively whether or not a collision will occur.
Certain Collisions (Red Zone)	8	A collision is certain, capable of causing localized destruction for an impact over land or possibly a tsunami if close offsore. Such events occur on average between once per 50 years and once per several thousand years.
	9	A collision is certain, capable of causing unprecendented regional devastation for a land impact or the threat of a major tsunami for an ocean impact. Such events occur on average between once per 10,000 years and once per 100,000 years.
	10	A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often.

The scale has been used a few times, and the public response was such that Binzel felt the need to reword it in 2004, emphasizing that events "merit concern" by astronomers, not anyone else. That version is now the official one that appears in the Earth Science Scales list. For the rest of us it's enough to, as they said in the movie, keep watching the skies.

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If you feel spurred to just get off the planet, out of the way of comet collisions, try the Space Frontier Foundation. They quote Larry Niven to great effect: "The dinosaurs became extinct because they didn't have a space program."

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