The Torino Impact Hazard Scale

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"The Torino Impact Hazard Scale"

Journal article

By: Richard P. Binzel

Date: 2000

Source: Binzel, Richard P. "The Torino Impact Hazard Scale." Planetary and Space Science 48 (2000): 297-303.

About the Author: Richard P. Binzel is a professor, chair of the program in planetary science, and Margaret MacVicar Faculty Fellow in the Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. His research interests include the collision of asteroids as well as the physical and surface features of the Pluto-Charon system. He earned a bachelor's degree in physics from Macalester College and a doctorate in astronomy from the University of Texas. Binzel, who published his first scientific paper at age fifteen, is a Fellow of the American Association for the Advancement of Science, the past winner of a Presidential Young Investigator Award, and recipient of the Harold C. Urey Prize from the American Astronomical Society. The International Astronomy Union has named asteroid 2873 after Binzel in recognition of his contributions to science.


The Torino scale was devised to convey the likely severity of a collision between Earth and an asteroid or comet, known generically as a near-Earth object or NEO, in its vicinity. Although the Torino scale is often compared to the Beaufort scale used to classify wind speed or the Richter scale used to quantify earthquake magnitude, it has several differences that arise from the nature of NEO impact hazards. The scale takes its name from a 1999 scientific conference held in Torino (also known as Turin), Italy, at which it was adopted by astronomers. A modified version was released in 2005.

Used to measure wind speed, particularly at sea and in coastal areas, the Beaufort scale was created by British Admiral Sir Francis Beaufort (1774–1857). It consists of twelve so-called forces that are defined by wind speed at a height of 3.3 feet (10 meters) above the surface. Force 0 represents calm whereas Force 12, the highest level, represents hurricane-strength winds. The Beaufort scale also includes qualitative descriptors, for example "smoke rises vertically," "whole trees in motion," and "slight structural damage occurs." The Richter scale, invented by American seismologist Charles Richter (1900–1985) and his lesser-known colleague Beno Gutenberg (1889–1960), standardizes the size of earthquakes based on the maximum heights of waves on a seismogram measured by a particular kind of seismograph located 62 miles (100 kilometers) from the earthquake epicenter. Each unit on the Richter scale corresponds to a tenfold change in the height of the waves and a thirty-two-fold change in the amount of energy released. An earthquake has only one magnitude regardless of the location at which it is felt. Another scale, the Modified Mercalli Intensity Scale, is used to quantify the severity of earthquakes as a function of location. In general, maps of Mercalli earthquake intensity form a bull's eye pattern around the earthquake epicenter with intensity decreasing with distance from the epicenter. The Mercalli scale is based entirely upon qualitative descriptors (for example, "felt indoors" or "heavy furniture moved") that allow earthquake intensity to be quantified based on the observations of non-scientists. Seismologists have also developed wholly quantitative measures of earthquake intensity that can be calculated from seismograms. All three of these scales—Beaufort, Richter, and Mercalli—measure the size or intensity of an event that has occurred, and none include any consideration of the likelihood of occurrence.

The Torino scale differs from the Beaufort, Richter, and Mercalli scales because it considers the size, likelihood, and time until a potential future event. Its values range from 0, indicating that the likelihood of a collision is zero or infinitesimally small and that the size of the object is small, to 10, indicating that a collision is virtually certain and that its impacts will be catastrophic. It can be broadly subdivided into five classes: white (indicating no hazard and corresponding to level 0), green (indicating normal hazard and corresponding to level 1), yellow (indicating a hazard that merits further attention of astronomers and corresponding to levels 2-3), orange (indicating a threatening situation and corresponding to levels 5-7), and red (indicating certain collision and corresponding to levels 8-10). Torino scale levels can be changed, in general to lower levels, as astronomers learn more about the trajectories and sizes of newly discovered near Earth objects. Torino scale 10 events typically occur on average only once every 100,000 years and correspond to global catastrophes that have the potential to end civilization as we know it. The Chicxulub impact near the Yucatan Peninsula, which led to the extinction of dinosaurs and marks the boundary between the Cretaceous and Tertiary Periods (often referred to as the KT boundary, where K is an abbreviation for Cretaceous used to avoid confusion with the older Cambrian Period) was a Torino scale 10 event. The inclusion of time to likely collision is another unique feature of the Torino scale. While it is impossible to predict with any accuracy the time until a severe storm or an earthquake, once a near-Earth object is discovered astronomers can calculate the time to potential impact very accurately. A collision that is likely to occur within a century, for example, would be rated higher than one that is not anticipated to occur for several hundred years because less time is available to plan for the collision.

The likelihood of collision depends not only on the number and true paths of near-Earth objects, but also upon errors in human observations and calculations. When a near-Earth object is discovered, little is known about its path through space and it is virtually impossible to determine whether it poses a hazard to Earth. As astronomical observations continue over time, however, its path becomes more accurately known and the likelihood of collision can be refined (and generally lowered).

The highest Torino scale ranking yet assigned to a near-Earth object has been 4, for asteroid 2004 MN4 (also known as 99942 Apophis). This asteroid, which is inferred to be approximately 1,300 feet (400 meters) in diameter, will pass within 22,600 miles (36,370 kilometers) of Earth on April 13, 2029. Within a week of the Torino scale 4 rating in 2004, a review of existing data led to the conclusion that the asteroid will not strike Earth. Its current Torino scale rating is 1.




Newly discovered asteroids and comets have inherent uncertainties in their orbit determinations owing to the natural limits of positional measurement precision and the finite lengths of orbital arcs over which determinations are made. For some objects making predictable future close approaches to the Earth, orbital uncertainties may be such that a collision with the Earth cannot be ruled out. Careful and responsible communication between astronomers and the public is required for reporting these predictions and a 0-10 point hazard scale, reported inseparably with the date of close encounter, is recommended as a simple and efficient tool for this purpose. The goal of this scale, endorsed as the Torino Impact Hazard Scale, is to place into context the level of public concern that is warranted for any close encounter event within the next century. Concomitant reporting of the close encounter date further conveys the sense of urgency that is warranted. The Torino Scale value for a close approach event is based upon both collision probability and the estimated kinetic energy (collision consequence), where the scale value can change as probability and energy estimates are refined by further data. On the scale, Category 1 corresponds to collision probabilities that are comparable to the current annual chance for any given size impactor. Categories 8-10 correspond to certain (probability > 99%) collisions having increasingly dire consequences. While close approaches falling Category 0 may be no cause for noteworthy public concern, there remains a professional responsibility to further refine orbital parameters for such objects and a figure of merit is suggested for evaluating such objects. Because impact predictions represent a multi-dimensional problem, there is no unique or perfect translation into a one-dimensional system such as the Torino Scale. These limitations are discussed. © 2000 Elsevier Science Ltd. All rights reseved.

1. Introduction

With the advent of expanded surveys for the discovery of asteroids and comets in the vicinity of the Earth, (collectively known as near-Earth objects or NEOs), there are increasing numbers of predictable close approaches to the Earth. Because orbit determinations are limited by finite precision in positional measurements and finite lengths of measured orbital arcs, there are natural uncertainties associated with close approach predictions. When an NEO is recognized to have a future close approach to Earth and has an orbital position uncertainty that intersects the known location of the Earth on a specific date, a collision probability can be calculated. (In general, the larger the orbital uncertainty, characterized as an "error ellipse," the lower the probability of a collision). While astronomers have a responsibility for publicly communicating the dates and circumstances of these close approach events, which typically have very low collision probabilities, this responsibility poses a risk communication challenge. The challenge arises because collisions of asteroids and comets with the Earth represent a topic so provocative and so prone to sensationalism that great care must be taken to assess and publicly communicate the realistic hazard (or non hazard) posed by such events. At the heart of this risk communication challenge resides the fact that low probability/high consequence events are by their very nature not within the realm of common human experience. Risk communication about asteroid and comet encounters is further different from other natural hazard predictions in that typically it is possible to specify with precision the date of the potential hazard, even if it is decades into the future.

The risk communication challenge posed by asteroid and comet close encounters was recognized by attendees of the "International Monitoring Programs for Asteroid and Comet Threat" (IMPACT) workshop held in Torino, Italy on 1-4 June, 1999. This paper reports the outcome of a proposal presented by the author for a simple hazard scale system designed to meet this challenge. The resulting system, now known as the Torino Impact Hazard Scale (named in recognition of the workshop's endorsement and the historical asteroid science contributions of the Torino Observatory), was discussed and debated and endorsed by the workshop sponsors, select science journalists, and was approved for public release by officials of the International Astronomical Union. The genesis of the Torino Scale can be traced to a proposal (Binzel, 1997) made at the 1995 United Nations International Conference on Near-Earth Objects.

2. Development of the Torino Scale

It is desirable that any system for risk communication should: (a) communicate the time window or exact date of the potential hazard; (b) provide a context for understanding the full range of the potential hazard; (c) have a rigorous basis in scientific calculation; and (d) be understandable at many different levels. Reporting the date of the close encounter satisfies the first requirement. A one-dimensional scale such as the 0-10 system adopted here, satisfies the other three requirements. Thus, responsible risk communication of asteroid and comet close encounters requires the inseparable reporting of both the encounter date and its hazard scale value. The 0-10 Torino Scale has qualities similar to other systems that are familiar to the public, such as the Richter scale for earthquakes and the Beaufort scale for wind velocities. By utilizing a one-dimensional scale bounded between 0 and 10, where 0 is "good" and 10 is "bad," the reporting of a single number provides, at a very basic level, some immediate sense of context for the hazard even if thee is no deep understanding of the construction of the scale. The combined reporting of the date of the encounter provides the most simple method for assessing the time interval until the event and hence the relative urgency.

Assessing the close approach of an NEO is a multidimensional problem, and as such, it is a problem that cannot be translated uniquely or perfectly into a one-dimensional scale. Beyond the encounter date, which is always reported as a coupled parameter, the multiple dimensions to be considered for translation include: the impact probability of the object, the kinetic energy of the object, the potential consequences should an impact occur, the time horizon for forecasting all potential impacts, and conveyance as to whether the specific threat posed is significantly higher or lower than that posed by the multitude of similar-sized objects that remain undiscovered. Although the one-dimensional 0-10 Torino Scale is inescapably a non-unique translation of these multidimensional aspects, when combined with the encounter date it does represent a carefully considered and endorsed system for clear and efficient risk communication about asteroid and comet close encounters….

3. Application and understanding of the Torino Scale

For an object making a close approach to Earth on a specified and reported date, assigning a corresponding Torino Scale value requires two numbers as input. The first is the best available number for the object's collision probability on the date of encounter. The second is the best available calculation (or estimate) for the object's kinetic energy. (Kinetic energy is proportional to mass times the square of velocity. The velocity is accurately calculable from the orbit, while the mass must be estimated from direct physical measurements.) The input numbers for collision probability and kinetic energy pinpoint a location within a labeled region, thereby yielding the Torino Scale value. The Torino Scale value is always reported as an integer. For solutions that place an object at a boundary between categories, or whose error bars substantially overlap more than one boundary, then a range (such as "1 to 2") may be reported. Solutions that place an object in the Category 0 region immediately below 3 and/or 8, might be referred to as a "possible 3" or "possible 8" if there is sufficient uncertainty in the kinetic energy that those ultimate categorizations are not reasonably ruled out. For an object that makes multiple close approaches over a set of dates, a Torino Scale value should be determined for each approach and reported with each date. It may be convenient to summarize the overall hazard posed by such an object by the greatest Torino Scale value within the set. The time window for the application of the Torino Scale is for close encounter events occurring one century into the future.

There are several essential aspects to the application and understanding of the Torino Scale. Primary among these is to understand that the Torino Scale is a predictive system whose application requires the specification of a close encounter date and that this system is based on observational measurements that accumulate and improve over time. Because the input parameters of collision probability and kinetic energy are calculated from observational data, the Torino Scale value can (and will) change as significant new data are obtained. In contrast, the precision of orbital mechanics will seldom result in substantial refinements in the date of any individual close approach event after the initial determination. Change in an object's Torino Scale value is inevitable since the ultimate outcome for any close approach event is binary: either it will hit the Earth or it won't. The challenge for astronomers is to obtain sufficient information on any object to place it securely within Category 0 or to conclusively determine that the object falls within the "certain collision" range (Categories 8-10). Fortunately, the odds tremendously favor the ultimate placement of objects within Category 0. Understanding and conveying the intrinsic variability that is inherent within a predictive scale is essential to mitigate the perception that scientists previously "got the answer wrong" when additional data allow revised Torino Scale value to be reported for a given encounter.

Finally, it must be strongly re-emphasized that the reporting of just a single Torino Scale value cannot convey, does not convey, and is not intended to convey, all of the information that is necessary for clear and responsible public communication regarding a future close encounter event. Most specifically and most importantly, the encounter date(s) always must be specified concomitantly so that an appropriate sense of urgency can be conveyed. In addition to this inseparable pairing of the Torino Scale value and the date, responsible public communication should also include: the name of the object, its estimated size, and the calculated collision probability value for each close encounter date.

For the unfortunate case of an event on a specified date that is a "certain collision," then a single Torino Scale value (such as 8-10) is wholly recognized as being grossly insufficient for communicating the consequences to a concerned public or for providing a basis for formulating a response by local, national, or international governmental agencies. Many additional parameters come into play for evaluating the urgency and likely consequences of a certain collision. These include: the impact date (and hence the time remaining), the exact location and nature of the impact site, details regarding the expected consequences of impact blast damage or tsunami effects, the exact nature of the colliding body, impact angle, season, etc. All of these details can only be addressed through extensive advisories based on the best available collision models and expert analyses. "Certain collisions" that fall just below 1 MT in estimated energy, and hence within Category 0, will also require substantial public advisories to address the level of concern that is merited, where these may be communicated as borderline "possible 8" or "unlikely 8" cases that have minimal likelihood of significant local damage.

4. Multi-level communication of the Torino Scale

A fundamental goal in the development of a risk communication tool such as the Torino Scale is to establish a system that can convey information at a variety of different levels, and in particular, provide information and context beyond the simple 0-10 scale value. To accomplish this goal, a color-coded text graphic for the Torino Scale, was developed as the primary "public release" product. Within this graphic, each scale value has a color code progressing from white to green to yellow to orange to red that attempts to further convey a context for the level of concern merited by a close encounter on a specified date. A subjective statement for the level of concern that is intended to be conveyed by each color appears as text along the left side. These levels range from (white) "Events Having No Likely Consequences" to (green) "Events Meriting Careful Monitoring" to (yellow) "Events Meriting Concern" to (orange) "Threatening Events" to (red) "Certain Collisions."

A still higher level of communication is provided by a text block for each scale value that provides a few sentences of qualitative explanation. Category 0 explains that the object is too small or the chance of a collision is too low to merit any practical concern. Category 1 attempts to convey the sense of a relatively normal close approach event that is sufficient to merit careful monitoring and refinement of the orbit, but not sufficient to merit any particular public concern. Category 2 also attempts to convey the sense of a "normal" close approach event, but one that merits some increased measure of concern relative to Category 1. For Categories 3-7, each explanation includes an indication of what the level of consequence would be in the case that the specified close encounter event becomes a certain collision. This indication is necessary because the compression of the multi-dimensional problem into a one-dimensional scale results in objects (having constant kinetic energy but evolving probabilities) not necessarily progressing uniformly upward or downward in scale value. Within the red zone that describes "Certain Collisions" (Categories 8-10) the qualitative terms for collision consequences escalate from "destruction" to "devastation" to "catastrophe" following the kinetic energy transitions described by Chapman and Morrison (1994). The explanations for these highest categories also attempt to convey information on how often such events are likely to occur, thereby providing an additional sense of context for the overall problem of impact hazards.

The same color-coding developed for the public release text graphic is also applied to the hazard space plot that defines the Torino Scale values. This resulting color-coded plot similarly conveys context of the problem at multiple levels by adding a text description for each color code and labeling the potential consequences associated with each kinetic energy range. This color-coded plot has also been developed as a "public release" product, intended to be interpretable by the scientifically interested laymen.

5. Public versus professional use of the Torino Scale

It is important to emphasize that the primary purpose for the development and dissemination of the Torino Scale is to provide a tool for public communication and assessment for asteroid and comet impact hazard predictions in the next century. Professional astronomers can use much more sophisticated metrics that assess the need for follow-up observations and orbital refinements for objects that fall within Category 0 but with collision probabilities that are not mathematically zero.

6. Conclusion

The establishment of the Torino Scale as a tool and common lexicon for public communication and assessment of NEO close encounter predictions helps to fulfill the responsibility of astronomers to provide clear and consistent public information on celestial impact hazards. When coupled with a close encounter date that conveys the relative urgency, the single most important aspect of the Torino Scale is that it provides an immediate sense of context for the potential hazard of the encounter by reporting a value on a 10 point scale. Additional color coding and descriptive wording allow a higher level understanding of the context for any scale value. The Torino Scale has natural limitations that arise from it being a one-dimensional translation of a multi-dimensional problem. Therefore responsible public communication that announces the dates of close encounter events that represent serious potential threats, requires significantly more hazard context information than just a Torino Scale value. Similarly, astronomers have a responsibility to monitor and refine orbits for all objects that can make future close approaches. Professional interest and professional communication toward this task is by no means intended to be limited by the establishment of the Torino Scale….


Although the Torino scale has not seen wide use, and has not established itself in the vernacular as solidly as the Beaufort or Richter scales, it establishes a rational framework for the analysis of and response to events that are rare but potentially catastrophic. Its potential utility lies in its ability to convey complicated and sometimes subtle information about the likelihood of future events in a simple format understandable to politicians, policy makers, and other non-scientists. Because the scale deals with future, rather than past or present, events, Torino scale ratings can be modified as additional information about newly discovered near-Earth objects becomes available.



Peebles, Curtis. Asteroids: A History. Washington, DC: Smithsonian Books, 2001.

Web sites

National Aeronautics and Space Administration. "Near Earth Object Program." 〈〉 (accessed January 27, 2006).

National Aeronautics and Space Administration. "Near Earth Object Program, Radar Observations Refine the Future Motion of Asteroid 2004 MN4." 〈〉 (accessed January 27, 2006). "Measuring Asteroid Threats." 〈〉 (accessed January 27, 2006).