The 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 July 2021, two asteroids have a cumulative Palermo Scale value above -2: (29075) 1950 DA (-1.42), and 101955 Bennu (-1.42). A further four have cumulative Palermo Scale values above -3: 1979 XB (-2.79), 2000 SG344 (-2.84), 2009 JF1 (-2.88), and 2007 FT3 (-2.95). There are 24 more that have a cumulative Palermo Scale value above -4, two of them having been discovered so far in 2021: 2021 DG1 (-3.70) and 2021 EU (-3.78).
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, is defined by the equation:
The background impact frequency is defined for this purpose as:
where the energy threshold E is measured in megatons, yr is the unit of T divided by one year.
In 2002 the near-Earth object (89959) 2002 NT7 was given a positive rating on the scale of 0.06, indicating a higher-than-background threat. The value was subsequently lowered after more measurements were taken. 2002 NT7 is no longer considered to pose any risk and was removed from the Sentry Risk Table on 1 August 2002.
In September 2002, the highest Palermo rating was that of asteroid (29075) 1950 DA, with a value of 0.17 for a possible collision in the year 2880. By December 2015, the rating had been reduced all the way to −1.42.
For a brief period in late December 2004, with an observation arc of 190 days, asteroid (99942) Apophis (then known only by its provisional designation 2004 MN4) held the record for the highest Palermo scale values, with a value of 1.10 for a possible collision in the year 2029. The 1.10 value indicated that a collision with this object was considered to be almost 12.6 times as likely as a random background event: 1 in 37 instead of 1 in 472. With further observation through 2016 there is no significant risk from Apophis at any of the dates in question.
Use Unconstrained Settings, sort by Palermo Scale (cum.)
The primary reference for the Palermo Technical Scale is "Quantifying the risk posed by potential Earth impacts" by Chesley et al., Icarus 159, 423-432 (2002).