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 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). 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.
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 as the base 10 logarithm of the ratio of the impact probability pi to the background impact probability over the time in years T to the event:
The annual background impact frequency is defined for this purpose as:
where the energy threshold E is measured in megatons.
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, 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.
As of September 2006, the record for Palermo scale values is held by asteroid (29075) 1950 DA, with a value of 0.17 for a possible collision in the year 2880.
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 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 instead of 1 in 472. With further observations, the possibility of a 2029 impact was eliminated, but as of October 2006 a cumulative Palermo rating of −2.52 applies, largely due to a possible event in 2036.
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).
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