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Palermo Technical Impact Hazard Scale

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Title: Palermo Technical Impact Hazard Scale  
Author: World Heritage Encyclopedia
Language: English
Subject: (410777) 2009 FD, Torino scale, 101955 Bennu, Potentially hazardous object, Near-Earth object
Collection: Alert Measurement Systems, Hazard Scales, Logarithmic Scales of Measurement, Planetary Defense
Publisher: World Heritage Encyclopedia

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 dataprobability of impact, and estimated kinetic yield—into a single "hazard" value. A rating of 0 means the hazard is as likely as 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 more likely than 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.


  • Scale 1
  • Positive rating 2
  • References 3
  • External links 4


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:

P \equiv \log_{10} \frac {p_i} {f_B T}


  • 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:

f_B = \frac{3}{100} E^{-4/5} yr^{-1}\;

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

Positive rating

The near-Earth object (89959) 2002 NT7 was the first near-Earth object detected by NASA's latest NEO program to be given a positive rating on the scale of 0.06,[2] indicating a higher-than-background threat. The value was subsequently lowered after more measurements were taken and 2002 NT7 is no longer considered to pose any risk, and was removed from the Sentry Risk Table on 1 August 2002.[3]

For a brief period in late December 2004, asteroid (99942) Apophis (then known only by its provisional designation 2004 MN4) held the record for Palermo scale values, with a value of 1.10[4] 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 more likely than a random background event: 1 in 37[5] instead of 1 in 472. With further observations, the possibility of a 2029 impact was eliminated. As of 2013 a maximum Palermo rating of −3.32 applies, due to a possible event in 2068.[6]

Since September 2002, the highest Palermo rating maintained has been that of asteroid (29075) 1950 DA, with a value of 0.17 for a possible collision in the year 2880.[7] 1950 DA is the only known asteroid whose hazard could be above the background level.[7]


  1. ^ "THE PALERMO TECHNICAL IMPACT HAZARD SCALE". NASA/JPL Near-Earth Object Program Office. 31 Aug 2005. Retrieved 2011-10-14. 
  2. ^ Dr. David Whitehouse (24 July 2002). "'"Space rock 'on collision course. BBC News. Retrieved 2007-12-28. 
  3. ^ "Sentry Risk Table - Removed Objects". NASA/JPL Near-Earth Object Program Office. Retrieved 2006-03-07. 
  4. ^ Daniel Fischer (Dec 27, 2004). "2004 MN4 Earth Impact Risk Summary (computed on Dec 27, 2004)". The Cosmic Mirror. Retrieved 2011-11-04. 
  5. ^ "Predicting Apophis' Earth Encounters in 2029 and 2036". NASA/JPL Near-Earth Object Program Office. Retrieved 2007-12-28. 
  6. ^ "99942 Apophis (2004 MN4) Earth Impact Risk Summary".  
  7. ^ a b "Asteroid 1950 DA". NASA/JPL Near-Earth Object Program Office. Retrieved 2011-10-14. 

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

  • Description of the scale
  • NASA list of potential impactors
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