Palermo scale

If you were paying close attention recently, you may have heard a report flash past that the Earth was going to be hit by an asteroid named 2002 NT7 on 1 February 2019. Within days, this had been put back to possibly sometime in 2060, or possibly never. The orbits of newly-discovered asteroids need time to be worked out in detail, but the press latched on to early reports without waiting for more accurate later figures.

For some years, there has been a rating scheme, the Torino scale, that estimates the risk of a body like this knocking us back into the Stone Age: it runs from 0 (no risk) to 10 (global catastrophe). That isn’t helpful for the great mass of asteroids, for whom the Torino figure is zero, but for which there may be risk of impact. So astronomers have just invented the Palermo scale, a more complex and subtle measure, which rates the impact risk of a cosmic body against the average risk of an impact by a body of the same size over a long period of time.

Unfortunately, it gives newspapers yet another incomprehensible number to quote, especially as it can become negative, somehow implying a less than zero chance of impact. Values less than -2 reflect events for which there are no likely consequences; values between -2 and 0 indicate situations that merit careful monitoring. Positive values suggest that some level of concern is merited.

2002 CU11 has been rated as less threatening than the general, or background, risk of any other impact; as of last week, it had a value of minus 1.28 on the Palermo scale.

Dallas Morning News, Apr. 2002

That gave 2002 NT7 the highest ever score on the Palermo scale, a rating system developed to help astronomers categorise impact risks.

New Scientist, Aug. 2002