Special: from
the Torino to the Palermo scale Quantifying the risk by Livia Giacomini  copyright Tumbling Stone 2002 
version 
Quantifying a risk is not an easy problem, in any field. An example over all: how dangerous can a car, running on a certain road, be? Obviously, the answer depends on many factors (its speed, the road, the pilot skillness...). Even if we were able to formalise the connections between these parameters, it would still be necessary to define a statistical tool to quantify the specific danger (since a risk is always a statistical probability!) .
Imagine applying this easy example to the field of NEOs (dict.), to the attempt of setting the impact probability of a certain asteroid with Earth. This case is much more complicated than the example of our car, first because the orbit of a NEO is highly chaotic over long periods of time (see what chaos is in this issue of T.S.), second because this orbit can only be known under a statistical point of view (this is the concept of region of uncertainty, see dict). Sounds like trying to measure the risk that our car will have an accident, without knowing exactly who is driving it, at what speed, on which road etc.! 
All these problems make scientists' task of evaluating the risk associated to each new NEO very complicated. Of course, evaluating this risk is fundamental to alert the scientific world  and the media  of a possible danger! Today, just to make this difficult evaluation more precise, a little revolution is taking place in the astronomical community. In fact, in order to evaluate whether an IAU Technical Review of a prediction of possible NEO impact is warranted, the IAU Working Group on NearEarth Objects recommends the use of the new Palermo Technical scale, presented for the first time at the international meeting held in Palermo last year (see Tumbling Stone number 5: special issue) by Chesley and coworkers (click here for the preprint).
One thing at a time. You don't need to be a
specialist to know that a method of evaluation of impact danger
already exists since the year 1999: the very famous Torino Scale.
How does this scale work and why a new tool has become necessary?
The Torino Scale uses two parameters to classify the danger of an
impact, its kinetic energy and its impact probability (see box about the Torino Scale). Being initially established for public communication,
this tool avoids complexity presenting a very clear and simple
measure of the hazard by a ten point integer scale.
Avoiding complexity is at the same time the reason why the Torino scale presents several aspects that are considered problematic for scientific purposes, and that can be resumed in three main points:
 the length of time before the impact itself, is not taken in account. In other words, similar scenarios get the same score whether they should occur 90 days or 90 years from now! On the other hand, to defend Earth, the period of time before the impact is very, very important! (see Tumbling Stone number 9: "Methods of mitigation" by Germano D'Abramo)
 using an integer parameter, the Torino scale makes it impossible to recognize when events that have the same scale value are actually far apart.
 the Torino scale assigns a zero value to all impacts with energy below 1 MT (dict.), no matter their probability. For this reason, the scale is not useful for events of great scientific interest but low public outreach (such as fireballs!)
It's to solve these problems, that the new
Palermo Scale has been introduced. But how does this new tool
work exactly?
First of all, the new scale provides a different measure of the
hazard posed by the event itself, without any refernece to the
background risk of the collision. To achieve this goal, a new
parameter of classification of an impact, called expected
energy , can be introduced. is in a real
sense a probabilistic energy, being the average energy that would
be expected given a large statistically consistent sample of the
entire range of potential orbits consistent with the impact.
This energy can be defined using two very intuitive parameters, the impact energy E and the impact probability P:
E is simply the energy released by the collision, which can be defined as the kinetic energy of the body (). Although this parameter is conceptually simple, its value cannot be perfectly known for a NEO, due to the uncertainty in the object's mass. With the absence of more precise physical observations, the mass is usually evaluated from the volume, the shape and the mean density deduced from the spectral type of the asteroid.
P is the impact probability and is calculated with numerical simulations, taking into account how the orbits that belong to the region of uncertainty meet the target plane.
But, differently from the Torino scale, the new
tool also deals with the time at which the impact should take
place, measuring the danger of a certain event relative to the
statistical threat coming from the entire asteroid and comet
population averaged over long periods of time.
This background hazard is quantified by the parameter R,
called normalized risk, related to the impact
probability P previously defined with the equation:
the product is normally what is called background hazard and it corresponds to the probability that a body of equal or bigger dimension than the considered asteroid should impact Earth over a period of time f is the impact frequency and can be deduced from the observation of craters.
Without going into further details, it is obvious how the Palermo scale is a more technical and precise tool than the first Torino scale, which will continue absolving the task of giving an immediate and simple description of the threat of a certain impact. On the other hand, thanks to the Palermo Scale, monitoring NEOs programs will be able to run automatically and in a very precise way, taking into account the mere observation data to create an observational priority for the various objects discovered.
