Palermo Technical Impact Hazard Scale
The Palermo Scale is a measure used by astronomers to sort the identified numerous potential impacts near-Earth objects by any appropriate attention. Combining the probability of impact, the estimated kinetic energy ( as a measure of the potential damage ), and the time remaining until the potential impact time to a single value. The mathematical expression for this was proposed by Professor Richard P. Binzel (MIT) in 2000 - in a paper on the Turin scale whose father he is also - and 2002 by Chesley et al. justified. The latter suggested for this measure Palermo Technical Impact Hazard the name Scale ago - in memory of the local discovery of the first asteroid, and the venue of the " Asteroids 2001 ".
Definition
The measure is defined as the logarithm of the ratio R of the impact probability PI for the expected number of impacts with at least the same energy E to the instant considered:
This applies to the so-called background rate fB impact both on undiscovered as well as known objects. Most of the larger objects and their orbits are known. For smaller sizes towards the energy dependence of the impact rate from the size distribution of lunar craters was estimated. For use in the Palermo scale was established:
Calculation of the Palermo - number
The kinetic energy E (the unit MT means megatons of TNT ) arises from the speed of the object when it enters the Earth's atmosphere v and its mass m.
The collision velocity v is determined (ie, without the earth at this point ) ve of the relative velocity at the crossing point v ∞, as follows from the orbital elements, and that calculated from the mass and the radius of the Earth escape velocity:
The mass m of the object is generally less well known exactly. It is estimated from the absolute magnitude, but depends not only on the diameter, but also by the albedo. A fortunate circumstance is that objects with a dark surface that are so judged less because of their brightness as they are, most also have a below-average density, whereby the mass even then only about a factor of 2 is uncertain, if not from spectroscopic measurements ( can be including radar ) are closed to the material of the object ( such an effort is not worth it for most objects).
Application Examples
In July 2002, attracted by ( 89959 ) 2002 NT7 the first time an object with a positive number Palermo some public attention. However, by following more precise measurements, the risk of impact could be ruled out for the foreseeable future.
Currently (December 26 2012) is the near-Earth asteroid ( 29075 ) 1950 DA, the only known celestial body with a positive Palermo - number, namely 0.17 for a possible strike in 2880th
The second highest value, the asteroid 2007 VK184 on: -1.83 for a possible strike on June 3, 2048 with a probability of 1:3030.
End of December 2004, for a few days, the asteroid ( 99942 ) Apophis (then under the provisional designation 2004 MN4 known ) the list of, with a Palermo - number of up to 1.1 for a possible strike in 2029, according to the 12, 6 times the background risk. The highest determined for Apophis risk of impact was about 1:37 or 2.7%. However, subsequent observations and more accurate knowledge of the orbital elements, however, the impact could be excluded in 2029. However, there is a very low probability of 1:45455 for a strike, at a Palermo Number of -2.42 for 13 April 2036.