Near–Earth Object Hazard Index
Near-Earth Object Hazard Index
Advanced by Massachusetts Institute of Technology Professor Richard (Rick) 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 the 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’s 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 to 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 to 10 numbering system, wherein a zero designates a statistically negligible threat of collision with the Earth. At the other extreme, a numerical designation of ten would indicate certain impact. In addition to being based upon the probability of impact, the scale number also incorporates a potential damage value. For example, a very small object that has little chance of surviving a fiery entry into the 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 the 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 Earth’s atmosphere). Green events designate orbital crossings with a small chance of collision. Yellow events designate potential orbital crossings that are 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 one-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 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 artificially made. 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 the Earth around the sun. (Cruithne can be technically and temporarily classified as a natural satellite during periods of its bizarre orbital path.) In 2003, astronomers provided evidence that J002E3 might be the remains of a portion of a Saturn V rocket from one of the U.S. Apollo lunar missions. Analysis of J002E3’s orbit suggested that it might impact the moon in 2003 or possibly enter Earth’s atmosphere within the next two decades. However, since then, J002E3 has left its orbit around Earth. Astronomers have calculated that it might return to its orbit about Earth in around 2032.
On December 25, 2004, 2004 MN4, or 99942 Apophis, was assigned a rating of 4 on the Torino Scale. Two days later, astronomers estimated that there was a 2.7% chance of an impact with Earth on April 13, 2029. Over the next two years, the rating was raised and lowered many times based on further studies of the object. As of August 2006, its rating is 2.25, although further changes to the scale is expected.
The only near-earth object with a score greater than zero on the Palermo scale is (29075) 1950 DA. As of 2006, it is estimated to pass close to, or crash into Earth, in the year 2880.
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Ehrenfreund, P., B. Foing, and A. Cellino, eds. The Moon and Near-Earth Objects. Oxford, UK: Elsevier, 2006.
Ames Research Center, National Aeronautics and Space Administration. “Asteroid and Comet Impact Hazards.” <http://impact.arc.nasa.gov/> (accessed accessed October 18, 2006).
K Lee Lerner