ThePalermo scale orPalermo technical impact hazard scale is alogarithmic scale used byastronomers to rate the potential hazard of impact of anear-Earth object (NEO). It combines two types ofdata—probability of impact and estimatedkinetic yield—into a single "hazard" value. A rating of 0 means the hazard is equivalent to thebackground 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).^[1] 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 theTorino scale, which is used for simpler descriptions in the non-scientific media.

As of 10 April 2025^[update], no asteroid has a cumulative rating for impacts above 0, and only two asteroids 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.

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 theUnited Nations Committee on the Peaceful Uses of Outer Space which was held inPalermo, Italy, on June 11–16, 2001.^[3]

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,${\displaystyle P}$, is defined by the equation:

${\displaystyle P\equiv {\log }_{10}\frac{p_i}{f_{B}T}}$

where

• ${\displaystyle p_i}$ is the impact probability
• ${\displaystyle T}$ is the time interval until the potential impact that is considered
• ${\displaystyle f_B}$ is the background impact frequency

The background impact frequency is defined for this purpose as:

${\displaystyle f_B=0.03E^{-\frac{4}{5}}{\text{ yr}}^{-1}}$

where the energy threshold${\displaystyle E}$ is measured inmegatons, and yr is the unit of${\displaystyle T}$ divided by one year.

For instance, this formula implies that theexpected 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${\displaystyle E}$.

However, another paper^[4] published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:

${\displaystyle f_B=0.00737E^{-0.9}}$

This formula gives considerably lower rates for a given${\displaystyle E}$. For instance, it gives the rate forbolides of 10 megatonnes or more (like theTunguska 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.

Palermo Background Risk Chart

Energy (MT)	Probability
	Once in this many years	At least once in one...
		decade	century	millennium	million years
0.1	5.28	87.73%	>99.99%	>99.99%	>99.99%
1	33.3	26.26%	95.24%	>99.99%	>99.99%
10	210	4.65%	37.91%	99.15%	>99.99%
100	1,327	0.75%	7.26%	52.94%	>99.99%
1,000	8,373	0.12%	1.19%	11.26%	>99.99%
10,000	52,830	0.019%	0.19%	1.88%	>99.99%
100,000	333,333	0.003%	0.03%	0.3%	95.02%
1,000,000	2,103,191	0.00048%	0.0048%	0.048%	37.84%
10,000,000	13,270,239	0.000075%	0.00075%	0.0075%	7.26%
100,000,000	83,729,548	0.000012%	0.00012%	0.0012%	1.19%
1,000,000,000	528,297,731	0.0000019%	0.000019%	0.00019%	0.19%
${\displaystyle E}$	${\displaystyle \frac{1}{f_B}=\frac{100}{3}{E}^{0.8}}$	${\displaystyle 1-{\left(1-f_B\right)}^{10}}$	${\displaystyle 1-{\left(1-f_B\right)}^{100}}$	${\displaystyle 1-{\left(1-f_B\right)}^{1,000}}$	${\displaystyle 1-{\left(1-f_B\right)}^{1,000,000}}$

For asteroids with multiple (${\displaystyle n}$) potential impacts, the cumulative Palermo scale rating,${\displaystyle P_{cum}}$, is the rating that can be calculated with the sum of the probability ratios of the individual potential impacts (each calculated with a${\displaystyle p_i}$ probability and a${\displaystyle T_i}$ time until potential impact), which can also be expressed as the logarithm of the sum of 10 raised to the${\displaystyle P_i}$ Palermo scale rating of the individual potential impacts:^[1]

${\displaystyle P_{cum}={\log }_{10}\sum _{i=1}^n\frac{p_i}{f_B{T}_i}=lo{g}_{10}\left(\sum _{i=1}^n{10}^{P_i}\right)}$

Top: example of the shrinking and changing position of the 3-sigma uncertainty region relative to Earth in the B-plane
Bottom: theoretical (blue and dashed red) and observational (crosses and triangles) evolution of impact probability as a function of uncertainty in the close approach distance for a bypass and an impact case, based on a planetary defense exercise using adapted real observations of99942 Apophis

ForNASA, a unit of theJet Propulsion Laboratory (JPL), theCenter for Near-Earth Object Studies (CNEOS) calculates impact risks and assigns ratings in itsSentry Risk Table,^[5] while another unit of JPL, Solar System Dynamics (SSD) provides orbital and close approach data.^[6] ForESA, similar services are provided by itsNear-Earth Object Coordination Centre (NEOCC), which maintains its own Risk List^[7] and Close Approaches List.^[8]

The basis for the risk evaluation is the most recent orbit calculation based on all known reliable observations. Along the calculated orbit, close approaches with Earth are determined. Due to measurement and model imprecision, the orbit calculation has an uncertainty, which can be quantified for the close approach distance. Assuming a two-dimensionalGaussian probability distribution in the plane perpendicular to the asteroid's orbit (the B-plane), the uncertainty can be characterised by thestandard 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,^[9] 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. The probability of an impact is the integral of the probability distribution over the cross section of Earth in the B-plane.

When the close approach of a newly discovered asteroid is first put on a risk list with a significant risk, it is normal for the risk to first increase, regardless whether the potential impact will eventually be ruled out or confirmed with the help of additional observations.^[10] After discovery, Earth will be close to the center of the probability distribution, that is, the 3-sigma uncertainty margin will be much bigger than the nominal close encounter distance. With additional observations, the uncertainty will decrease, thus the 3-sigma uncertainty region will shrink, thus Earth will initially cover an increasing part of the probability distribution, resulting in increased risk, and an increasing rating. If the real orbit bypasses Earth, with further observations, Earth will only intersect the tail of the probability distribution (the 3-sigma region will shrink to exclude the Earth) and the impact risk will fall towards zero; while in case the asteroid will hit the Earth, the probability distribution will contract towards its intersection (the 3-sigma region will shrink into Earth's intersection in the B-plane) and the risk will rise towards 100%.^[11]

It has been suggested that this section bemerged withList of objects with non-zero Torino ratings toList of near-Earth objects with significant impact hazard ratings. (Discuss) Proposed since February 2025.

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. The value was subsequently lowered after more measurements were taken.2002 NT₇ is no longer considered to pose any risk and was removed from theSentry Risk Table on 1 August 2002.^[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.^[14] By March 2022, the rating had been reduced to −2.0.^[15][16] In May 2024, a study that incorporated observations by theastrometry space observatoryGaia increased the impact risk, consequently, the rating was raised above −1 again.^[17]

For a brief period in late December 2004, with anobservation arc of 190 days, asteroid99942 Apophis (then known only by itsprovisional designation2004 MN₄) held the record for the highest Palermo scale value, with a value of 1.10 for a possible collision in the year 2029.^[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. With further observations, the risk of impact during later close approaches was completely eliminated^[21] and Apophis was removed from the Sentry Risk Table in February 2021.^[13]

As of 14 October 2025^[update], on NASA's Sentry Risk Table,^[22] two asteroids have a cumulative Palermo scale value above −2:(29075) 1950 DA (−0.92) and101955 Bennu (−1.40). Five have cumulative Palermo scale values between −2 and −3:2025 QF85 (−2.00),2008 JL3 (−2.68),1979 XB (−2.70),2000 SG344 (−2.77) and2010 RF12 (−2.97). Of the 32 that have a cumulative Palermo scale value between −3 and −4, two were discovered in 2024 and five in 2025:2025 LK (−3.48),2025 SC5 (−3.51),2024 JW16 (−3.63),2024 TK5 (−3.63),2025 RM1 (−3.74),2025 DT2 (−3.79) and2025 SQ4 (−3.87).

As of 14 October 2025^[update], on the Risk List maintained by theNear-Earth Object Coordination Centre of theEuropean Space Agency (ESA),^[23] one asteroid has a cumulative Palermo scale value above −2: 101955 Bennu (−1.41). Five have cumulative Palermo scale values between −2 and −3: 1950 DA (−2.13),2025 QF85 (−2.54),2023 VD3 (−2.67), 1979 XB (−2.70),2008 JL₃ (−2.73) and2000 SG₃₄₄ (−2.77). Of the 27 that have a cumulative Palermo scale value between −3 and −4, three were discovered in 2024 and four in 2025:2025 LK (−3.36),2024 RF10 (−3.51),2024 JW16 (−3.53),2025 SC5 (−3.62),2025 SQ4 (−3.65),2024 TK5 (−3.74) and2025 BA1 (−3.82).

• Asteroid impact avoidance
• Asteroid impact prediction
• Earth-grazing fireball
• Impact event
• List of asteroid close approaches to Earth
• List of Earth-crossing asteroids
• Time-domain astronomy

• The primary reference for the Palermo scale is"Quantifying the risk posed by potential Earth impacts" by Chesley et al.,Icarus 159, 423-432 (2002).

• Palermo Technical Impact Hazard Scale at theSentry monitoring system byCNEOS atJPL fromNASA

• Alert measurement systems
• Hazard scales
• Planetary defense
• Logarithmic scales of measurement

• CS1: long volume value
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• Articles containing potentially dated statements from April 2025
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