The Palermo Technical Impact Hazard Scale is a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-earth object. It combines two types of data—probability of impact, and estimated kinetic yield—into a single "hazard" value. A rating of 0 means the hazard is as likely as the background hazard (defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact). A rating of +2 would indicate the hazard is 100 times more likely than a random background event. A similar but less complex scale is the Torino scale, which is used for simpler descriptions in the non-scientific media.

The Palermo Scale value, P, is defined as the base 10 logarithm of the ratio of the impact probability p_i to the background impact probability over the time T to the event:

The annual background impact frequency is defined for this purpose as:

where the energy threshold E is measured in megatons.

The near-Earth object (89959) 2002 NT₇ was the first near-Earth object detected by NASA's latest NEO programme to be given a positive rating on the scale of 0.06, indicating a higher than background threat. The value was subsequently lowered to -0.25 after more measurements were taken.

Asteroid 2004 MN₄ briefly (on December 27, 2004) held the record for Palermo scale values, with a value of 1.10 for a possible collision in the year 2029. The 1.10 value indicated that a collision with this object was considered to be almost 12.6 times more likely than a random background event: 1 in 37 instead of 1 in 472. The asteroid 2004 MN₄ was subsequently demoted to have a Palermo rating of -1.59. Before 2004 MN₄, asteroid (29075) 1950 DA held the record for Palermo scale values, with a value of 0.17 for a possible collision in the year 2880.

See also

• Near-Earth object
• Torino scale

External links

• NASA NEO Programme http://neo.jpl.nasa.gov/index.html
• Description of the scale http://neo.jpl.nasa.gov/risk/doc/palermo.html
• (89959) 2002 NT₇ is the first object discovered with a positive measurement on the Palermo scale