The Palermo scale is used by NASA specialists to quantify the necessary level of alarm over the probability that a certain object might hit the Earth. The scale compares the likelihood of the detected potential impact with the average risk posed by unknown random 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 number of a near-Earth object (NEO) is determined by the following formula:

PN = log10 [PI / (fB × DT)]

where PI is the impact probability of the object in question and DT is the time until the potential event, measured in years. The annual background impact frequency,

fB = 0.03 × E-4/5

is the annual probability of an impact event with energy of E (in megatons of TNT) at least as large as the event in question.

The scale is logarithmic, so for example, a Palermo Scale value of -2 indicates that the detected potential impact event is only 1% as likely as a random background event occurring in the intervening years, a value of zero indicates that the single event is just as threatening as the background hazard, and a value of +2 indicates an event that is 100 times more likely than a background impact by an object at least as large.

This differs from the Torino scale in that Torino takes into consideration the predicted impact energy of the event as well as its likelihood of actually happening (i.e., the event's impact probability), while the Palermo number measures the level of study needed for a future impact probability. Much of the usefulness of the Palermo Scale lies in its ability to carefully assess the risk posed by events that would otherwise score a zero on the Torino scale.

http://neo.jpl.nasa.gov/risk/doc/palermo.html

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