The Palermo Technical Impact Hazard Scale was developed to enable NEO specialists to categorize and prioritize potential impact risks spanning a wide range of impact dates, energies and probabilities. Actual 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. Potential impacts with positive Palermo Scale values will generally indicate situations that merit some level of concern.

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*. For
convenience the scale is logarithmic, so, for examples, 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 before the date of the potential
impact in question.

The primary reference for the Palermo Technical Scale is a scientific paper entitled
“Quantifying the risk posed by potential Earth impacts”
by Chesley et al. (*Icarus* **159**, 423-432 (2002)).

The Torino Scale is designed to communicate to the public the risk associated with a future Earth approach by an asteroid or comet. This scale, which has integer values from 0 to 10, 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). The Palermo Scale is used by specialists in the field to quantify in more detail the level of concern warranted for a future potential impact possibility. Much of the utility of the Palermo Scale lies in its ability to carefully assess the risk posed by less threatening Torino Scale 0 events, which comprise nearly all of the potential impacts detected to date. Objects are prioritized according to their Palermo Scale values in order to assess the degree to which they should receive additional attention (i.e., observations and analysis). This scale is continuous (both positive and negative values are allowed) and does incorporate the time between the current epoch and the predicted potential impact, as well as the object’s predicted impact energy and likelihood of occurrence.

By estimating the so-called background hazard level of Earth impacts, we define a value for the threat from the entire asteroid and comet population averaged over very long periods of time. Because there are vastly more small asteroids than there are large ones in space, the background impact rate will depend upon the size of the near-Earth asteroid. The background level can be thought of as the usual state of affairs or status quo, and so when the close Earth approach of a large NEA rises above the background level (the Palermo Scale value is then greater than zero) we know this event is out of the ordinary and hence of some concern.

Since the Palermo Scale is continuous and it depends upon the number of years until the potential impact, there is no convenient conversion between these two scales. In general, however, if an event rises above the background level, it will achieve both a Palermo and Torino Scale value greater than zero.

The Palermo Scale is the base-10 logarithm of the *relative risk*.

PS = log

_{10}R.

The relative risk R is given by

R = P

_{I}/ (f_{B}× DT),

where P_{I} is the impact probability of the event in question
and DT is the time until the potential event, measured in years. The
annual background impact frequency,

f

_{B}= 0.03 × E^{-4/5}

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

The cumulative Palermo Scale value reflects the seriousness of the entirety of detected potential collision solutions. It is the base-10 logarithm of the sum of the individual relative risk values.

PS

_{cum}= log_{10}(10^{PS1}+ 10^{PS2}+ 10^{PS3}+ …)

A similar summation can be done to rate the relative hazard posed by a collection of objects.