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Palermo Technical Impact Hazard Scale

A continuous scale used to describe the urgency of an anticipated impact of an asteroid with Earth. Most approaches have a negative value on the scale. Values above 0 are emergencies.

Unlike the Torino Scale, the Palermo Scale takes into account how much time remains before the asteroid is likely to hit Earth, and it compares the impact with the status quo background. Thus the unusual and the impending get higher values.

an equation

where

P_I is probability of an impact
T is time in years until impact
f_B is the background impact frequency for asteroids of that size, defined by

where E is the energy in tons of TNT.

For which near-earth asteroids might hit us and when, see:

https://newton.spacedys.com/neodys/
or
https://neo.jpl.nasa.gov/risk/index.html

resources

Steven R. Chesley, Paul W. Chodas, Andrea Milani, Giovanni B. Valsecchi and Donald K. Yeomans.
Quantifying the risk posed by potential Earth impacts.
Icarus vol. 159, pages 423-432 (2002).

https://neo.jpl.nasa.gov/risk/doc/palermo.html