It combines two types of data—probability of impact and estimated kinetic yield—into a single "hazard" value.
[1] A rating of +2 would indicate the hazard is 100 times as great as a random background event.
Palermo scale rating of the individual potential impacts:[1] For NASA, a unit of the Jet Propulsion Laboratory (JPL), the Center for Near-Earth Object Studies (CNEOS) calculates impact risks and assigns ratings in its Sentry Risk Table,[3] while another unit of JPL, Solar System Dynamics (SSD) provides orbital and close approach data.
[4] For ESA, similar services are provided by its Near-Earth Object Coordination Centre (NEOCC), which maintains its own Risk List[5] and Close Approaches List[6] The basis for the risk evaluation is the most recent orbit calculation based on all known reliable observations.
Due to measurement and model imprecision, the orbit calculation has an uncertainty, which can be quantified for the close approach distance.
Assuming a two-dimensional Gaussian probability distribution in the plane perpendicular to the asteroid's orbit (the B-plane), the uncertainty can be characterised by the standard deviation (sigma) the close approach point in the directions along the asteroid's orbit and perpendicular to it, where the former is usually much larger.
The one-sigma margin, which is used by ESA NEOCC one-sigma[7], means that the close approach point is within those bounds with a 68.3% probability, while the 3-sigma margin, used by NASA JPL SSD, corresponds to 99.7% probability.
[9] In 2002 the near-Earth asteroid (89959) 2002 NT7 reached a positive rating of 0.18 on the Palermo scale,[10] indicating a higher-than-background threat.
[11] 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.
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 value, with a value of 1.10 for a possible collision in the year 2029.
[15] The 1.10 value indicated that a collision with this object was considered to be almost 12.6[16] times as likely as a random background event: 1 in 37[17] instead of 1 in 472.
As of 12 February 2025[update], on NASA's Sentry Risk Table,[18] three asteroids have a cumulative Palermo scale value above −2: 2024 YR4 (−0.35), (29075) 1950 DA (−0.92) and 101955 Bennu (−1.40).
As of 12 February 2025[update], on the Risk List maintained by the Near-Earth Object Coordination Centre of the European Space Agency (ESA),[19] two asteroids have a cumulative Palermo scale value above −2: 2024 YR4 (−0.34) and 101955 Bennu (−1.41).