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 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, yr is the unit of T divided by one year.
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, and was removed from the Sentry Risk Table on 1 August 2002.
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 August 2011 a maximum Palermo rating of −3.08 applies, due to a possible event in 2036.
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. 1950 DA is the only known asteroid whose hazard could be above the background level.
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