Torino scale

Advanced by Massachusetts Institute of Technology Professor Richard P. Binzel in 1995, the Torino scale is a revision of the Near-Earth Object Hazard Index. In 1999, the International Conference on Near-Earth objects adopted the scale at a meeting in Turino (Turin), Italy (from which the name of the scale is derived). The Torino scale is used to portray the threat to Earth of an impact with a particular comet or asteroid. The measurement scale is based upon agreement between scholars as a means to categorize potential hazards.

When a new comet or asteroid is initially tracked, an extrapolation of its projected orbital path is compared to predicted Earth orbital positions. The Torino scale assigns categories to the closeness with which an object will approach or cross Earth orbit. Because initial estimates can be greatly altered by refined data regarding the track of an asteroid or comet, it is possible that a particular asteroid or comet could be upgraded or downgraded with regard to the threat it poses Earth. In addition, a different scale designation can be made for each successive orbital encounter over a number of years or decades. Data is most accurate as related to encounters in the near-term because various gravitational forces and encounters with other celestial objects can alter the course of asteroids or comets .

The Torino scale is based upon a zero to 10 numbering system wherein a zero designates a statistically negligible threat of collision with Earth. At the other extreme, a numerical designation of ten would indicate certain impact. In addition to being based upon the probability of impact, scale numbers also incorporate a potential "damage" value. For example, a very small object with little chance of surviving a fiery entry into Earth's atmosphere will still be assigned a very low number (zero for very small objects) even if an impact was certain. At the other extreme, the designation 10 carries the ominous distinction of being reserved for a certain impact of cataclysmic proportions.

The size of an object is important because the force (kinetic energy) that it would carry in a collision with Earth is related to its mass and velocity. Like nuclear explosions, estimates of the energy of collision are given in units of megatons (MT).

The Torino scale also assigns colors to the potential hazard assessment. A "white" label means that the asteroid or comet poses no threat (i.e, will miss or not survive entry into the Earth's atmosphere). Green events designate orbital crossings with a small chance of collision. Yellow events designate more potential orbital crossings than average. A yellow designation would focus intense scientific scrutiny upon the track of the asteroid or comet. Orange events are "threatening" crossings or other encounters with asteroids or comets that have a potential to cause severe destruction. The designation is reserved for objects with a significantly higher risk of impact. Red events or collisions are certain and globally devastating.

Because risk assessments are difficult to quantify, another scale, the Palermo Technical Scale, is often used by astronomers to complement the Torino scale. The Palermo scale offers a more mathematical calculation utilizing the variables of probability of impact and energy of collision.

As of May 2002, with approximately 25% of Near Earth objects identified, no object rating more than a "1" on the Torino scale has yet been detected. For example, during February 2002, an asteroid designated 2002 CU11 was classified as a "1" on the Torino scale (a "green" code). Extrapolations of the orbital dynamics of the asteroid and Earth indicated a low probability (approximately 1 in 9000) of a potential collision in 2049.

As of May 2002, additional information regarding the Torino scale and up-to-date information on identified NEO (Near Earth objects) can be found at the Near Earth Objects Dynamic Site (NEODyS) (<http://newton.dm.unipi.it/neodys/>).

See also Astronomy; Barringer meteor crater; Catastrophism; Gravity and the gravitational field; Hubble Space Telescope (HST); Solar system, Space and planetary geology