Palermo Technical Impact Hazard Scale

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).[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 the Torino Scale, which is used for simpler descriptions in the non-scientific media.

Historic Palermo Ratings
(these objects have since dropped below −2)
Asteroid Palermo
rating		 Background
risk
99942 Apophis1.1012.6x greater
(89959) 2002 NT70.181.51x greater
(29075) 1950 DA0.171.48x greater
background risk0equal
(144898) 2004 VD17−0.251.78x less
(410777) 2009 FD−0.442.75x less
2022 AE1−0.664.57x less
2023 GQ2−0.705.01x less
2013 TV135−0.735.37x less
(367789) 2011 AG5−1.0010x less

As of June 2023,[2] one asteroid has a cumulative Palermo Scale value above −2: 101955 Bennu (−1.41). Seven have cumulative Palermo Scale values between −2 and −3: (29075) 1950 DA (−2.05), 1979 XB (−2.72), 2021 EU (−2.74), 2000 SG344 (−2.79), 2007 FT3 (−2.83), 2008 JL3 (−2.98), and 2010 RF12 (−2.98). Of those that have a cumulative Palermo Scale value between −3 and −4, one was discovered in 2023: 2023 DO (−3.60).

Scale

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:

where

  • pi is the impact probability
  • T is the time interval over which pi is considered
  • fB is the background impact frequency

The background impact frequency is defined for this purpose as:

where the energy threshold E is measured in megatons, and yr is the unit of T divided by one year.

Positive rating

In 2002 the near-Earth object (89959) 2002 NT7 reached a positive rating on the scale of 0.18,[3] 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.[4]

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.[5] By March 2022, the rating had been reduced to −2.0.[6][7]

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.[8] The 1.10 value indicated that a collision with this object was considered to be almost 12.6[9] times as likely as a random background event: 1 in 37[10] instead of 1 in 472. With further observation through 2021 there is no risk from Apophis for the next 100+ years.

See also

References
  1. "THE PALERMO TECHNICAL IMPACT HAZARD SCALE". NASA/JPL Near-Earth Object Program Office. 31 August 2005. Retrieved 9 March 2021.
  2. "Sentry: Earth Impact Monitoring - Impact Risk Data". Jet Propulsion Laboratory. Retrieved 27 June 2023. Use Unconstrained Settings, sort by Palermo Scale (cum.)
  3. "How A/CC broke the 2002 NT7 story". hohmanntransfer. 29 March 2003. Archived from the original on 6 November 2020. Retrieved 25 April 2019.
  4. "Sentry Risk Table - Removed Objects". NASA/JPL Near-Earth Object Program Office. Retrieved 9 March 2021.
  5. "Asteroid 1950 DA". NASA/JPL Near-Earth Object Program Office. Archived from the original on 1 October 2002. Retrieved 14 October 2011.
  6. "Sentry: Earth Impact Monitoring: 29075". NASA/JPL Near-Earth Object Program Office. Archived from the original on 2 April 2017. Retrieved 20 June 2017.
  7. "Updated Calculations Refine the Impact Probability for (29075) 1950 DA". Center for NEO Studies (CNEOS). JPL (NASA). Retrieved 19 August 2022.
  8. Daniel Fischer (27 December 2004). "2004 MN4 Earth Impact Risk Summary (computed 27 December 2004)". The Cosmic Mirror. Archived from the original on 14 March 2005. Retrieved 4 November 2011.
  9. Math: 101.10 = 12.589
  10. "Predicting Apophis' Earth Encounters in 2029 and 2036". NASA/JPL Near-Earth Object Program Office. Archived from the original on 18 November 2007. Retrieved 28 December 2007.

Further reading
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