The Torino Scale

The Torino Scale is a “Richter Scale” for categorizing the Earth impact hazard associated with newly discovered asteroids and comets. It is intended to serve as a communication tool for astronomers and the public to assess the seriousness of predictions of close encounters by asteroids and comets during the 21st century

Why is the Torino Scale needed?

When a new asteroid or comet is discovered, predictions for where the object will be months or decades in the future are naturally uncertain. These uncertainties arise because the discovery observations typically involve measurements over only a short orbital track and because all measurements have some limit in their precision.

Fortunately, for the majority of objects, even the initial calculations are sufficient to show that they will not make any close passes by the Earth within the next century. However, for some objects, 21st century close approaches and possible collisions with the Earth cannot be completely ruled out.

How does the Torino Scale Work?

The Torino Scale utilizes numbers that range from 0 to 10, where 0 indicates an object has a zero or negligibly small chance of collision with the Earth. (Zero is also used to categorize any object that is too small to penetrate the Earth’s atmosphere intact, in the event that a collision does occur.) A 10 indicates that a collision is certain, and the impacting object is so large that it is capable of precipitating a global climatic disaster.

The Torino Scale is color coded from white to yellow to orange to red. Each color code has an overall meaning:

• White – “Events having no practical consequences,” meaning they are virtually certain to miss Earth or are so small that any impact would almost certainly dissipate in the atmosphere. White corresponds to category 0.
• Green – “Events meriting careful monitoring” refers to objects that have predictable close approaches with some very small, but not seriously concerning, chance of a collision. Nonetheless, prudence dictates their orbits should be tracked closely so that the collision chance becomes refined, and probably in all cases, will ultimately be reclassified within Torino Scale category zero. Green corresponds to category 1.
• Yellow – “Events meriting concern” are close approaches by objects that have higher collision chances than the Earth typically experiences over a few decades. These are object for which refinement of the orbit is of high priority. Yellow corresponds to categories 2, 3, 4.
• Orange – “Threatening events” refers to close encounters with objects that are large enough to cause regional or global devastation, where the chance of collision greatly exceeds the level that typically occurs within a given century. These are objects for which refinement of the orbits are an extreme priority. Orange corresponds to categories 5, 6, 7.
• Red – “Certain collisions” refers to objects that are certain to collide with Earth having sufficient size to likely penetrate the atmosphere with the capability to cause either local damage, regional devastation, or a global climatic catastrophe. Red corresponds to categories 8, 9, 10.

How does an object get its Torino Scale number?

An object is assigned a 0 to 10 value on the Torino Scale based on its collision probability and its kinetic energy (proportional to its mass times the square of its encounter velocity). Categorization on the Torino Scale is based on the placement of a close approach event within a graphical representation of kinetic energy and collision probability . An object that is capable of making multiple close approaches to the Earth will have a separate Torino Scale value associated with each approach. (An object may be summarized by the single highest value that it attains on the Torino Scale.) There are no fractional values or decimal values used in the Torino Scale.

Can the Torino Scale value for an object change?

Yes! It is important to note that the Torino Scale value for any object initially categorized as 1 or greater will change with time. The change will result from improved measurements of the object’s orbit showing, most likely in all cases, that the object will indeed miss the Earth. Thus, the most likely outcome for a newly discovered object is that it will ultimately be re-assigned to category 0. Any object initially placed in category 0 is unlikely to have its Torino Scale value change with time.

How did the Torino Scale get its name?

The Torino Scale was created by Professor Richard P. Binzel in the Department of Earth, Atmospheric, and Planetary Sciences, at the Massachusetts Institute of Technology (MIT). The first version, called “A Near-Earth Object Hazard Index”, was presented at a United Nations conference in 1995 and was published by Binzel in the subsequent conference proceedings (Annals of the New York Academy of Sciences, volume 822, 1997).

A revised version of the “Hazard Index” was presented at a June 1999 international conference on near-Earth objects held in Torino (Turin) Italy. The conference participants voted to adopt the revised version, where the bestowed name “Torino Scale” recognizes the spirit of international cooperation displayed at that conference toward research efforts to understand the hazards posed by near-Earth objects. (“Torino Scale” is the proper usage, not “Turin Scale”).

Additional Information

• Nasa – A colored picture of the Torino Impact Hazard Scale
• Wikipedia – A defintion of an impact event and a history detailing impact events on earth throuhgout time
• Science Mag – “And Now, the Asteroid Forecast.” vol. 285, p. 655 (1999).
• Nature – “Scaling the Degree of Danger from an Asteroid.” vol. 400, p. 392 (1999).