Risk Rating
How dangerous are
near-Earth asteroids?
Scientists use two scales to communicate asteroid impact risk: the Torino Scale for public communication and the Palermo Scale for technical analysis. Here is how both work - and what they actually tell you.
Current impact risk overview →The Torino Scale
The Torino Scale was adopted by the International Astronomical Union in 1999 as a standard way to communicate impact risk to the public. It runs from 0 to 10, combining two factors: the probability of a collision and the estimated kinetic energy if one occurred.
An object does not need to be on a direct collision course to carry a non-zero Torino rating. If orbital uncertainty leaves open the possibility of an impact, and the object is large enough to cause damage, it gets a non-zero rating until observations narrow the orbit and rule out Earth.
No hazard
No likelihood of collision, or object is so small it burns up. The rating for all currently tracked objects.
Normal
Routine discovery. Collision is statistically improbable. Merits careful monitoring.
Meriting attention
Close encounter with increasing probability of collision causing regional to global damage. Warrants public notice.
Threatening
Close encounter with significant threat of collision causing regional or global catastrophe. International planning warranted.
Certain collision
Certain collision. Impact energy ranges from local (8) to global mass extinction (10). International response essential.
The only object to reach Torino 4
Asteroid 99942 Apophis was discovered in June 2004. Initial calculations suggested a 2.7% probability of impact with Earth in April 2029 - an unprecedented finding that briefly pushed its Torino rating to 4 in December 2004.
The rating did not hold. Additional observations over the following weeks refined the orbit. By early 2005, Apophis had been downgraded to Torino 1, then to 0. The 2029 flyby will now occur at roughly 38,000 km from Earth's centre - a spectacular pass but with no impact risk.
The Apophis episode illustrates how the system is supposed to work. A newly discovered object carries uncertainty. That uncertainty produces a non-zero rating. More observations reduce the uncertainty. In almost every case, the threat resolves to nothing.
The Palermo Technical Impact Hazard Scale
The Palermo Scale is used by planetary scientists for more precise risk comparison. It is logarithmic and measures an object's impact probability against the background rate - the average chance of any random impact of similar or greater energy occurring over the same time period.
A Palermo Scale value of 0 means the object is exactly as likely to impact as the background rate would predict for something of its size. A value of -2 means it is 100 times less likely than background. Values above -2 are considered worth monitoring. Values above 0 mean the object poses more risk than a random draw from the asteroid population.
Most of the objects on CNEOS's Sentry risk table have Palermo Scale values well below -2. None currently sit above 0.
Related pages
Will an asteroid hit Earth?
Current impact situation and historical context.
Impact probability explained
What the statistics behind impact risk actually mean.
Planetary defence
How NASA tracks and responds to potential threats.
The Apophis asteroid
The only object ever to reach Torino Scale 4 - and its 2029 flyby.
