
THE PALERMO TECHNICAL IMPACT HAZARD SCALE
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, 423432 (2002)).
PALERMO SCALE FREQUENTLY ASKED QUESTIONS
How is the Palermo Scale different from the Torino
Scale?
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.
Why is it important to compare each threat with the
background?
By estimating the socalled 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
nearEarth 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.
How does one convert from one scale to the other?
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.
How exactly is the Palermo Scale value computed?
The Palermo Scale is the base10 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.
And what about the cumulative Palermo Scale value?
The cumulative Palermo Scale value reflects the seriousness of
the entirety of detected potential collision solutions. It is the
base10 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.

