The Palermo scale or Palermo technical impact hazard scale is a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-Earth object (NEO). It combines two types of data—probability of impact and estimated kinetic yield—into a single "hazard" value. A rating of 0 means the hazard is equivalent to the background hazard (defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact). A rating of +2 would indicate the hazard is 100 times as great as a random background event. Scale values less than −2 reflect events for which there are no likely consequences, while Palermo scale values between −2 and 0 indicate situations that merit careful monitoring. A similar but less complex scale is the Torino scale, which is used for simpler descriptions in the non-scientific media. As of 2 April 2026, no asteroid has a cumulative rating for impacts above 0, and only two asteroids—(29075) 1950 DA and 101955 Bennu—have ratings between −2 and 0. Historically, three asteroids had ratings above 0 and half a dozen more above −1, but most were downrated since.
Scale The Palermo scale was devised for astronomers to compare impact hazards at a technical level, rather than for the general public. It was adopted at the meeting of the Working Group on Near-Earth Objects of the Scientific and Technical Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space which was held in Palermo, Italy, on 11–16 June 2001. The scale compares the likelihood of the detected potential impact with the average risk posed by objects of the same size or larger over the years until the date of the potential impact. This average risk from random impacts is known as the background risk. The Palermo scale value, P {\displaystyle P} , is defined by the equation:
P ≡ log 10 p i f B T {\displaystyle P\equiv \log _{10}{\frac {p_{i}}{f_{B}T}}}
where
p i {\displaystyle p_{i}} is the impact probability
T {\displaystyle T} is the time interval until the potential impact that is considered
f B {\displaystyle f_{B}} is the background impact frequency The background impact frequency is defined for this purpose as:
f B = 0.03 E − 4 5 yr − 1 {\displaystyle f_{B}=0.03\,E^{-{\frac {4}{5}}}{\text{ yr}}^{-1}\;}
where the energy threshold E {\displaystyle E} is measured in megatons, and yr is the unit of T {\displaystyle T} divided by one year. For instance, this formula implies that the expected value of the time from now until the next impact greater than 1 megaton is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of E {\displaystyle E} . However, another paper published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:
f B = 0.00737 E − 0.9 {\displaystyle f_{B}=0.00737E^{-0.9}\;}
This formula gives considerably lower rates for a given E {\displaystyle E} . For instance, it gives the rate for bolides of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years (or a 38% probability that it happens at least once in a century) as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.
For asteroids with multiple ( n {\displaystyle n} ) potential impacts, the cumulative Palermo scale rating, P c u m {\displaystyle P_{cum}} , is the rating that can be calculated with the sum of the probability ratios of the individual potential impacts (each calculated with a p i {\displaystyle p_{i}} probability and a T i {\displaystyle T_{i}} time until potential impact), which can also be expressed as the logarithm of the sum of 10 raised to the P i {\displaystyle P_{i}} Palermo scale rating of the individual potential impacts:
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