Palermo scale
The Palermo scale is a measure used by astronomers to sort the numerous identified possible impacts from near-Earth objects according to the appropriate level of attention .
It combines the probability of the impact, the estimated kinetic energy (as a measure of the potential damage) and the time remaining until the possible impact into a single value. The mathematical expression for this was proposed in 2000 by Richard P. Binzel ( MIT ) - in a work on the Turin scale , of which he is also the father - and founded in 2002 by Chesley et al. The latter suggested the name Palermo Technical Impact Hazard Scale for this measure - in memory of the discovery of the first asteroid there and the location of the "Asteroids 2001" conference.
definition
The measure is defined as the logarithm of the ratio R of the impact probability P I to the expected number of impacts with at least the same energy E up to the point in time considered:
The so-called background impact rate f B relates to both undiscovered and known objects. Most of the larger objects and their orbits are known. Towards smaller sizes, the energy dependence of the impact rate was estimated from the size distribution of lunar craters . For use in the Palermo scale, it was determined:
- (the unit MT means megatons TNT ; yr −1 per year)
Calculating the Palermo Number
The kinetic energy E results from the speed of the object when it enters the earth's atmosphere v and its mass m .
The collision speed v is determined from the relative speed at the crossing point v ∞ , as it results from the path elements (i.e. without the earth at this point), and the escape speed v e calculated from the mass and the radius of the earth :
The mass m of the object is usually less precisely known. It is estimated from its absolute brightness , which not only depends on the diameter, but also on the albedo . It is fortunate that objects with a dark surface, which are estimated to be smaller than they are due to their brightness, usually also have a below-average density , which means that the mass is only about a factor of 2 uncertain, if not from spectroscopic measurements including radar ) the material of the object can be deduced (such an effort is not worthwhile for most objects).
Application examples
In July 2002, with (89959) 2002 NT 7 , an object with a positive Palermo number caused a certain amount of public attention. However, through the following more precise measurements, the impact risk could be excluded for the foreseeable future.
On the day of observation December 26, 2012, the near-Earth asteroid (29075) 1950 DA was the only known celestial body with a positive Palermo number, namely 0.17 for a possible impact in the year 2880; As of June 30, 2015, however, it is only listed with a value of −2.32 under the Objects Not Recently Observed .
The asteroid 2007 VK 184 has the second highest value : −1.83 for a possible impact on June 3, 2048 with a probability of 1: 3030.
At the end of December 2004, the asteroid (99942) Apophis (then known under the provisional name 2004 MN 4 ) headed the list for a few days , with a Palermo number of up to 1.1 for a possible impact in 2029, corresponding to the 12th , 6 times the background risk. The highest impact risk determined for Apophis was about 1:37 or 2.7%. However, due to later observations and more precise knowledge of the orbital elements , the impact in 2029 could be ruled out. However, there is a very low probability of 1: 45,455 for an impact on April 13, 2036, with a Palermo number of −2.42. (On June 30, 2015 only −2.93.)
literature
- Richard P. Binzel: The Torino Impact Hazard Scale . In: Planet. Space Sci. 48, 2000, pp. 297-303 doi : 10.1016 / S0032-0633 (00) 00006-4 .
- Steven R. Chesley et al .: Quantifying the Risk Posed by Potential Earth Impacts . In: Icarus. 159, 2002, pp. 423-432, doi : 10.1006 / icar.2002.6910 , online (PDF; 176 kB).