Palermo Technical Impact Hazard Scale facts for kids

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 January 2025^[update], no asteroid has a cumulative rating for impacts above 0, and only three 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.

Asteroids with high ratings

Typical evolution of impact probability in three stages: as the uncertainty region gets progressively smaller with additional observations, the probability of impact can increase because Earth covers a larger percentage of the shrunk uncertainty region, but drops to 0 once Earth is no longer inside the uncertainty region

In 2002 the near-Earth object (89959) 2002 NT₇ reached a positive rating on the scale of 0.18, 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 March 2022, the rating had been reduced to −2.0. As of October 2024, it has a rating of −0.93.

For a brief period in late December 2004, with an observation arc of 190 days, asteroid [[99942 Apophis{{{2}}}|99942 Apophis_{{{2}}}]] (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. 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 2021 there is no risk from Apophis for the next 100+ years.

As of 31 January  2025 (2025 -01-31)^[update], three asteroids have a cumulative Palermo Scale value above −2: 2024 YR₄ (−0.43), (29075) 1950 DA (−0.92) and 101955 Bennu (−1.40). Four have cumulative Palermo Scale values between −2 and −3: 1979 XB (−2.70), 2000 SG₃₄₄ (−2.77), 2008 JL₃ (−2.86) and 2010 RF₁₂ (−2.97). Of the 26 that have a cumulative Palermo Scale value between −3 and −4, three were discovered in 2024: 2024 BY₁₅ (−3.30), 2024 JW₁₆ (−3.63) and 2024 TK₅ (−3.76).

See also

In Spanish: Escala de Palermopara niños

• 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