巴勒莫撞擊危險指數

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巴勒莫撞击危险指数 是一个被天文学家用来评判一个近地物体对地球的威胁的对数标准。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:

$P = \log_{10} \frac {p_i} {f_B T}$

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

$f_B = 0.03 E^{-0.8} \;$

where the energy threshold E is measured in megatons.

The near-Earth object Template:Mpl 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.

On December 27, 2004, 小行星99942 Apophis (then known only by its provisional designation 2004 MN₄) briefly 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. With further observations, the possibility of a 2029 impact was eliminated, but as of 2005 a cumulative Palermo rating of about −1.3 applies, largely due to possible events in 2035 and 2036. Before Apophis, 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.