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| The '''Palermo Technical Impact Hazard Scale''' is a [[logarithmic scale]] used by [[astronomer]]s to rate the potential hazard of impact of a [[Near-Earth object|near-earth object]] (NEO). It combines two types of [[data]]—[[probability]] of impact, and estimated [[kinetic energy|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).<ref name="palermo"/> 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.
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| ==Scale==
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| 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:
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| :<math>P \equiv \log_{10} \frac {p_i} {f_B T}</math>
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| where
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| :*''p<sub>i</sub>'' is the impact probability
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| :*''T'' is the time interval over which p<sub>i</sub> is considered
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| :*''f<sub>B</sub>'' is the background impact frequency
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| The background impact frequency is defined for this purpose as:
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| :<math>f_B = \frac{3}{100} E^{-4/5} yr^{-1}\;</math>
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| where the energy threshold E is measured in [[TNT equivalent|megatons]], yr is the unit of T divided by one year.
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| ==Positive rating==
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| The near-Earth object {{mpl|(89959) 2002 NT|7}} 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,<ref name="BBC-NT7"/> 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 (monitoring system)|Sentry Risk Table]] on 1 August 2002.<ref name="removed"/>
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| For a brief period in late December 2004, asteroid {{mpl|(99942) Apophis}} (then known only by its [[provisional designation]] {{mp|2004 MN|4}}) held the record for Palermo scale values, with a value of 1.10<ref name="mirror"/> 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<ref name="predicting-Apophis"/> 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.<ref name=Apophis/>
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| 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.<ref name="NEO-1950DA"/> 1950 DA is the only known asteroid whose hazard could be above the background level.<ref name="NEO-1950DA"/>
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| ==See also==
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| *[[Near-Earth object]]
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| *[[Torino Scale]]
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| ==References==
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| {{reflist
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| | refs =
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| <ref name="palermo">{{cite web
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| |date=31 Aug 2005
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| |title=THE PALERMO TECHNICAL IMPACT HAZARD SCALE
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| |publisher=NASA/JPL Near-Earth Object Program Office
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| |url=http://neo.jpl.nasa.gov/risk/doc/palermo.html
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| |accessdate=2011-10-14}}</ref>
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| <ref name="removed">{{cite web
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| |title=Sentry Risk Table - Removed Objects
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| |publisher=NASA/JPL Near-Earth Object Program Office
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| |url=http://neo.jpl.nasa.gov/risk/removed.html
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| |accessdate=2006-03-07}}</ref>
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| <ref name="BBC-NT7">{{cite web
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| |author=Dr. David Whitehouse
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| |date=24 July 2002
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| |title=Space rock 'on collision course'
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| |publisher=BBC News
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| |url=http://news.bbc.co.uk/2/hi/science/nature/2147879.stm
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| |accessdate=2007-12-28}}</ref>
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| <ref name=Apophis>{{cite web
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| |title=99942 Apophis (2004 MN4) Earth Impact Risk Summary
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| |url=http://neo.jpl.nasa.gov/risk/a99942.html
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| |publisher=[[NASA]]
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| |accessdate=2011-11-04}}</ref>
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| <ref name="mirror">{{cite web
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| |date=Dec 27, 2004
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| |title=2004 MN4 Earth Impact Risk Summary (computed on Dec 27, 2004)
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| |publisher=The Cosmic Mirror
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| |author=Daniel Fischer
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| |url=http://www.astro.uni-bonn.de/~dfischer/mirror/285neo041227.html
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| |accessdate=2011-11-04}}</ref>
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| <ref name="predicting-Apophis">{{cite web
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| |title=Predicting Apophis' Earth Encounters in 2029 and 2036
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| |publisher=NASA/JPL Near-Earth Object Program Office
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| |url=http://neo.jpl.nasa.gov/apophis/
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| |accessdate=2007-12-28}}</ref>
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| <ref name="NEO-1950DA">{{cite web | |
| |title=Asteroid 1950 DA
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| |publisher=NASA/JPL Near-Earth Object Program Office
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| |url=http://neo.jpl.nasa.gov/1950da/
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| |accessdate=2011-10-14}}</ref>
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| }}
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| The primary reference for the Palermo Technical Scale is [http://neo.jpl.nasa.gov/risk/doc/palermo.pdf "Quantifying the risk posed by potential Earth impacts"] by Chesley et al., [[Icarus (journal)|Icarus]] 159, 423-432 (2002).
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| ==External links==
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| * [http://neo.jpl.nasa.gov/risk/doc/palermo.html Description of the scale]
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| * [http://neo.jpl.nasa.gov/risk/ NASA list of potential impactors]
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| {{Planetary defense}}
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| [[Category:Alert measurement systems]]
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| [[Category:Hazard scales]]
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| [[Category:Planetary defense]]
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