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NEAR-Earth Object Hazard Index

scale asteroid impact torino

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 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 0-10 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 number also incorporate a potential "damage" value. For example, a very small object that little chance to 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 nucelar 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 not 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 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 March 2003, with approximately a third 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 the Earth indicated a low probability (approximately 1 in 9,000) of a potential collision in 2049.

Amors, Apollos, and Atens designate three categories of Near-Earth asteroids (NEAs) and are characterized by their orbital interface dynamics. Potentially hazardous asteroids (PHAs) are larger than 0.1 mile (0.16 km) in diameter and approach close enough to present a potential hazard.

Other Near-Earth objects may be manmade. In 2003 astronomers announced the tracking of an object designated J002E3 (first discovered in 2002). The object was thought to be either a small asteroid captured by the Earth's gravity, or a discarded rocket casing.

Calculated to be in a 50-day orbit, if J002E3 is determined to be an asteroid it would join the Moon and Cruithne as a natural satellite of Earth (Cruithne can be technically and temporarily classified as a natural satellite during periods of its bizarre orbital path) around the Sun. In 2003, astronomers provided evidence that J002E3 might be the remains of a portion of a Saturn V rocket from one of the United States Apollo lunar missions. Further analysis of J002E3's orbit indicates suggests that it may impact the Moon in 2003 or possibly enter Earth's atmosphere within the next two decades.



Bottke, W. F., Asteroids III. Tempe, AZ: University of Arizona Press, 2002.

Lewis, John S. Comet and Asteroid Impact Hazards on a Populated Earth: Computer Modeling. Academic Press, 1999.

Remo, John L., ed. Near-Earth Objects: The United Nations Conference on Near-Earth Objects (Annals of the New York Academy of Sciences V. 822). New York: New York Academy of Sciences, 1997.


NASA Ames Space Science Division. "Asteroid and Comet Impact Hazards, The Torino Scale" [cited March 12, 2003]. <http://impact.arc.nasa.gov/torino/>.

NASA Jet Propulsion Laboratory, California Institute of Technology. "Near-Earth Objects" [cited March 10, 2003]. <http://neo.jpl.nasa.gov/neo.html>.

K Lee Lerner

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