Torino Scale

Scale on which astronomers rate the threat to the Earth posed by a near-Earth object (NEO). The scale runs from 0 to 9 in whole numbers and depends on how likely the object is to collide with the Earth and the damage it would do in such a collision.

The position of an object on the scale varies as knowledge of its orbit and nature develop. Most NEOs are soon assigned to Torino value 0 when their orbits are accurately determined and it is found that they will not collide with the Earth. The highest value ever assigned was 4, briefly given to asteroid Apophis on its discovery in December 2004. Apophis will pass near the Earth in 2029, and the first calculations of the probability of impact rose to 2.7%. Continuous observations thereafter lowered the Torino rating; within a week the rating for the 2029 encounter was at 0. A subsequent encounter for 2036 is also at 0. An asteroid that was only the second to receive a Torino rating greater than 1 was observed in February 2006, and likewise has subsequently been downrated to 0.

Ranges of Torino values are also colour-coded to aid understanding in public presentations: white (0) – no threat; yellow (2–4) – meriting attention by astronomers; orange (5–7) – threatening; red (8–10) – certain collision, with a level of damage ranging from ‘regional devastation’ to ‘global catastrophe’.

The scale was devised by Richard P Binzel, Professor of Planetary Sciences at the Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. It received its name after it was presented at a conference held in the Italian city of Turin (Italian name Torino) in 1999.