A Richter Scale for Close Encounter Asteroids

A new tool to help scientists, the media, and the public assess the potential danger of asteroids and comets, collectively called near-Earth objects (NEOs), was created by Dr. Richard P. Binzel, professor of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology. His risk-assessment system is similar to the Richter scale used for earthquakes. It is named the Torino Impact Hazard Scale, after the Italian city (Turin), in which the scale was initially adopted by the International Astronomical Union (IAU) in June 1999 and officially endorsed by the IAU on July 22, 1999. Based on the orbit trajectory for a given NEO, the scale takes into account the object's size, speed, and the probability of it striking the Earth.

The Torino Scale uses numbers and color zones that range from 0 to 10, where 0 on the white zone indicates an object has virtually no chance of impact with the Earth. (Zero is also used to categorize any object that is too small to penetrate the Earth's atmosphere, in the event that a collision does occur.) A red 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 as follows:

• White (corresponds to category 0). “Events having no practical consequences,” meaning that they are virtually certain to pass the Earth or are so small that any impact would almost certainly dissipate in the atmosphere.
• Green (corresponds to category 1). “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 will become refined. These objects will almost certainly be reclassified within Torino Scale category zero.
• Yellow (corresponds to categories 2, 3, 4). “Events meriting concern” or close approaches by objects that have higher chances of collision than the Earth typically experiences over a few decades. These are objects for which refinement of the orbits is of high priority.
• Orange (corresponds to categories 5, 6, 7). “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 is an extreme priority.
• Red (corresponds to categories 8, 9, 10). “Certain collisions,” refers to objects that will definitely hit the Earth. The values of 8, 9, and 10 depend on whether the impact energy is large enough to cause either local damage, regional devastation, or a global climatic catastrophe.

Once an asteroid is detected, scientists use tracking data from a tiny section of its orbit to calculate where it will be in 10, 15, or 100 years. There is some uncertainty in this prediction because the orbit measurements are not perfect and the NEO may be altered by gravity if it passes close to Earth or another planet, but “orbits generally behave like clockwork,” Binzel said. As more information is gathered about a particular asteroid, its placement on the scale can be adjusted accordingly.

He pointed out that no asteroid to date has ever had a value greater than one. Large asteroids are rarely a threat to the Earth. An asteroid bigger than a mile across might hit once every 100,000 to one million years on average.

The risk from NEO impacts increases with the size of the projectile. The greatest risk is associated with objects larger than a half-mile to a mile (1 to 2 kilometers), which are large enough to perturb Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. An ocean impact could trigger large ocean waves or tsunamis.

As of July 19, 1999, 304 of the 803 known near-Earth asteroids have a diameter of about 0.6 mi (1 km) or larger. Of these NEOs, 183 have been classified as Potentially Hazardous Asteroids (PHAs). The potential to make close approaches to Earth does not mean that the PHA will crash into the Earth. It only means that there is a possibility for such a threat.