It combines two types of data—probability of impact and estimated kinetic yield—into a single "hazard" value.
The Palermo scale was devised for astronomers to compare impact hazards at a technical level, rather than for the general public.
[2] 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 June 11–16, 2001.
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.
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,[5] while another unit of JPL, Solar System Dynamics (SSD) provides orbital and close approach data.
[8] 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.
[11] In 2002 the near-Earth asteroid (89959) 2002 NT7 reached a positive rating of 0.18 on the Palermo scale,[12] indicating a higher-than-background threat.
[13] 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.
[15][16] In May 2024, a study that incorporated observations by the astrometry space observatory Gaia increased the impact risk, consequently, the rating was raised above −1 again.
[18] The 1.10 value indicated that a collision with this object was considered to be almost 12.6[19] times as likely as a random background event: 1 in 37[20] instead of 1 in 472.
[13] As of 22 February 2025[update], on NASA's Sentry Risk Table,[22] three asteroids have a cumulative Palermo scale value above −2: (29075) 1950 DA (−0.92), 2024 YR4 (−1.11) and 101955 Bennu (−1.40).
As of 22 February 2025[update], on the Risk List maintained by the Near-Earth Object Coordination Centre of the European Space Agency (ESA),[23] two asteroids have a cumulative Palermo scale value above −2: 2024 YR4 (−1.37) and 101955 Bennu (−1.41).