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 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 . 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.