Torino Impact Hazard Scale

See also: Palermo Technical Impact Hazard Scale

A risk-assessment scale, with integer values from 0 to 10, used to describe an approach of a particular asteroid to Earth in terms of the likelihood and consequences of its collision with Earth. The scale was devised by Richard P. Binzel and first presented at a conference in 19951. A revised version was approved at an International Astronomical Union (IAU) workshop in Torino (Turin), Italy in 1-4 June 1999 and formally adopted by the IAU in an announcement at the UNISPACE III conference in Vienna on 22 July 1999.

Category Index Description
Events Having No Likely Consequences 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 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 2 A somewhat close, but not unusual encounter. Collision is very unlikely.
3 A close encounter, with 1% or greater chance of a collision capable of causing localized destruction.
4 A close encounter, with 1% or greater chance of a collision 
capable of causing regional devastation. 
Threatening Events 5 A close encounter, with a significant threat of a collision capable of causing regional devastation.
6 A close encounter, with a significant threat of a collision capable of causing a global catastrophe. 
7 A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe. 
Certain Collisions 8 A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years.
9 A collision capable of causing regional devastation. Such events occur between once per 1000 years and once per 100,000 years.
10 A collision capable of causing a global climatic catastrophe. Such events occur once per 100,000 years, or less often.

The index number is derived from two factors: the kinetic energy of the asteroid, and the probability of a collision with Earth, according to the following chart.

Plot showing how combinations of collision probability and kinetic energy lead to numbers on the scale.

The left-hand scale is in megatons of TNT (but they define the megaton as 4.3 × 1015 joules). The secondary left-hand scale shows the diameter of the object, assuming typical encounter velocities. 

According to Binzel, speaking in 1999, no asteroid identified to that date had a value greater than 1.  It often happens that an approach's initial rating is downgraded as additional observations make it possible to define the asteroid's orbit more precisely.  For information on asteroids currently of interest, see: 

http://newton.dm.unipi.it/cgi-bin/neodys/neoibo

or

http://neo.jpl.nasa.gov/risk/index.html

The Torino Scale has been largely superceded by the Palermo Scale.

1. Richard P. Binzel.
A Near-Earth Object Hazard Index.
Annals of the New York Academy of Sciences, volume 822 (1997).

Resources

http://neo.jpl.nasa.gov/torino_scale.html

http://spaceguard.esa.int/tumblingstone/dictionary/torino.htm

Discusses as well the "Palermo Scale."

D. Morrison, C. R. Chapman, D. Steel, and R. P. Binzel.
Impacts and the Public: Communicating the Nature of the Impact Hazard.
In M. J. S. Belton, T. H. Morgan, N. H. Samarasinha and D. K. Yeomans, eds.
Mitigation of Hazardous Comets and Asteroids
Cambridge (UK): Cambridge University Press, 2004.

R. P. Binzel.
The Torino Impact Hazard Scale.
Planetary and Space Science, volume 48, pages 297-303. (2000)

R. P. Binzel.
Assessing the Hazard: The Development of the Torino Scale.
The Planetary Report, volume 19, pages 6-10. (Nov/Dec. 1999)

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