Palermo scale

   a scale used by astronomers to assess the risk of an impact on the earth by a comet or asteroid. The scale value is a logarithmic measure of the risk of impact compared to the average risk of an impact by objects of the same size or larger over the years until the date of the potential impact by the newly discovered object. If the object in question has probability p of impact at a time T years in the future, the Palermo scale value is PS = log¹⁰ (p/(0.03TE^-4/5)), where E is the projected energy of the impact in megatons of TNT. A possible impact is considered to be of concern if the Palermo scale value exceeds -2, that is, if the impact is more than 1% as likely as a random impact. The scale is used to prioritize the need for further observations of an object. See also Torino number.