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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 November 2024, two asteroids have a cumulative Palermo Scale value above −2: (29075) 1950 DA (−0.92) and 101955 Bennu (−1.40). Three have cumulative Palermo Scale values between −2 and −3: 2000 SG344 (−2.77), 2008 JL3 (−2.86) and 2010 RF12 (−2.97). Of those 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).

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:

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

where

• p_i is the impact probability
• T is the time interval over which p_i is considered
• f_B is the background impact frequency

The background impact frequency is defined for this purpose as:

${\displaystyle f_{B}=0.03\,E^{-{\frac {4}{5}}}{\text{ yr}}^{-1}\;}$

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

For instance, this formula implies that the expected value of the time from now until the next impact greater than 1 megatonne 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 E.

However, another paper 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 E. For instance, it gives the rate for bolides of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years 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.

Positive rating

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 (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.

See also

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

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 2 May 2024. Use Unconstrained Settings, sort by Palermo Scale (cum.)
3. P. Brown; et al. (November 2002). "The flux of small near-Earth objects colliding with the Earth". Nature. 420 (6913): 294–296. Bibcode:2002Natur.420..294B. doi:10.1038/nature01238. PMID 12447433. S2CID 4380864.
4. "How A/CC broke the 2002 NT7 story". hohmanntransfer. 29 March 2003. Archived from the original on 6 November 2020. Retrieved 25 April 2019.
5. "Sentry Risk Table - Removed Objects". NASA/JPL Near-Earth Object Program Office. Retrieved 9 March 2021.
6. "Asteroid 1950 DA". NASA/JPL Near-Earth Object Program Office. Archived from the original on 1 October 2002. Retrieved 14 October 2011.
7. "Sentry: Earth Impact Monitoring: 29075". NASA/JPL Near-Earth Object Program Office. Archived from the original on 2 April 2017. Retrieved 20 June 2017.
8. "Updated Calculations Refine the Impact Probability for (29075) 1950 DA". Center for NEO Studies (CNEOS). JPL (NASA). Retrieved 19 August 2022.
9. 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.
10. Math: 10 = 12.589
11. "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

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

External links

• Palermo Technical Impact Hazard Scale at the Sentry monitoring system by CNEOS at JPL from NASA

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