The Torino Scale -
how asteroid risk is rated

Richard Binzel, a planetary scientist at MIT, designed the scale in the late 1990s. The problem he was solving was communication, not science. By that point, survey telescopes were finding near-Earth objects (NEOs - asteroids and comets with orbits that bring them within 1.3 AU of the Sun, where one AU is approximately 150 million kilometres) faster than ever. A handful of these discoveries generated alarming headlines based on early orbital calculations that subsequently resolved to nothing.

The media coverage of those events was chaotic. Scientists issuing probabilistic statements - "a 1-in-1,000 chance of impact" - found them reported as either definite threats or casually dismissed. A standardised scale with plain-language descriptions at each level would allow scientists to communicate risk consistently without ambiguity.

The International Astronomical Union adopted the scale at their 1999 General Assembly held in Turin, Italy - Torino in Italian - hence the name. A revised version was published in 2004.

The Torino Scale maps two variables onto a single number: the probability of an Earth impact on a specific predicted date, and the estimated kinetic energy that impact would release. Neither variable alone is sufficient. A tiny object with a high impact probability might score 0 because it would burn up harmlessly in the atmosphere. A large object with a very low probability might score 1 because the combination of size and non-negligible probability warrants monitoring.

The scale is designed for specific predicted impact dates, not cumulative risk. An object may have multiple predicted encounter dates over the coming decades; each date carries its own Torino rating. The highest of those ratings is generally reported as the object's current score.

Newly discovered objects often enter at Torino 1 while their orbital uncertainty is large. Follow-up observations over the days and weeks after discovery typically narrow the orbit enough to rule out Earth entirely, dropping the rating to 0. This cycle of discovery, brief elevation, and resolution is routine.

Apophis is the only object ever to reach Torino Scale 4. Discovered in June 2004, the asteroid was initially estimated at around 300 metres across. Early orbital calculations suggested a 2.7% probability of an Earth impact in April 2029 - a genuinely alarming figure given that Apophis is large enough to cause regional-scale destruction.

The Torino 4 rating was assigned in December 2004 and stood for a matter of weeks. Astronomers around the world trained their telescopes on the object, accumulating additional observations. The refined orbit showed that the 2029 approach would occur at roughly 38,000 kilometres from Earth's centre - closer than many operational satellites, but not an impact trajectory. By early 2005, Apophis had been downgraded to Torino 1, then 0.

The subsequent revision to Apophis's size - now estimated at approximately 370 metres - made the original concern more credible in retrospect, not less. The system responded correctly: an object with a meaningful impact probability attracted intense scrutiny, the orbit was determined precisely, and the threat resolved to zero.

The Palermo Technical Impact Hazard Scale is the scientific complement to the Torino Scale. Where the Torino Scale gives a single integer from 0 to 10 with plain-language descriptions, the Palermo Scale is a continuous logarithmic measure designed for technical comparison.

The Palermo Scale compares an object's predicted impact probability against the background rate - the average likelihood that any random impactor of similar or greater energy would strike Earth over the same time interval. A Palermo Scale value of 0 means the object is exactly as risky as the statistical background. A value of -2 means it is 100 times less likely than background. Values above -2 are considered to warrant monitoring; values above 0 would mean the object poses more risk than any random draw from the asteroid population.

No object on the current Sentry risk table has a Palermo Scale value above 0. Most sit far below -2. The Torino Scale communicates the result to the public; the Palermo Scale is how scientists compare risks with each other.