Jiri Borovicka1, Pavel Spurny1, Peter Brown2,3, Paul Wiegert2,3, Pavel Kalenda4, David Clark2,3, Lukas Shrbeny1
1Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-251 65 Ondrejov, Czech Republic.
2Dept. of Physics and Astronomy, The University of Western Ontario, London Ontario N6A 3K7, Canada
3Centre for Planetary Science and Exploration, University of Western Ontario, London Ontario N6A 5B7, Canada
4Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holesovickach 41, CZ-18209 Praha 8, Czech Republic.
In the Nov 14 2013 issue of the journal Nature, astronomers present a definitive analysis of the asteroidal fragment that burned up so spectacularly over the Russian city of Chelyabinsk in February 2013.
Earth is continuously colliding with fragments of asteroids and comets of various sizes. The largest encounter in historical times occurred over the Tunguska river in Siberia in 1908, producing an airburst of energy equivalent to 5000-15000 kilotons of TNT (1 kiloton of TNT represents an energy of 4.185x1012joules). Until recently, the next most energetic airburst events occurred over Indonesia in 2009 and near the Marshall Islands in 1994, both with energies of several tens of kilotons. An analysis of selected video records of the Chelyabinsk meteor of 15 February 2013 revealed it had an energy equivalent to 500 kilotons of TNT.
An analysis of its trajectory from video records (such as these: 1 2 3 4 5) found that its orbit was similar to the orbit of the 2-km-diameter asteroid 86039 (1999 NC43), to a degree of statistical significance sufficient to suggest the Chelyabinsk meteoroid may once have been part of that larger body. The bulk strength of the Chelyabinsk asteroid, at about 1 megapascal, was similar to that of smaller meteoroids and corresponds to a heavily fractured single stone. The asteroid broke into small pieces between the altitudes of 45 and 30 km, saving the ground from more serious damage. The total mass of surviving fragments larger than 100 g was lower than expected.
Asteroid 86039 is unusually large for a near-Earth asteroid with an estimated 2.22 km diameter (Delbo et al. (2003). Icarus, v. 166, pp. 116-130). Surveys of the near-Earth asteroid population indicate that we expect that there are only 227 near-Earth asteroids as large or larger than this (Mainzer, A. et al. NEOWISE observations of near-Earth objects: Preliminary results. Astrophys J. 743, id. 156 (2011)). Given the rarity of asteroids this size we estimate that there is only about a 1 in 10,000 chance that the closeness of the Chelyabinsk asteroid to such a large near-Earth asteroid would have occurred by chance. There's no way to say for sure that the Chelyabinsk asteroid came to us from asteroid 86039 simply from the data at hand, but we can say that the orbits are close enough that a more careful investigation is warranted.
In fact, only about 500 of the estimated 20 million near-Earth asteroids the size of the Chelyabinsk impactor have been discovered to date. It is not easy, even with modern telescopes, to detect such small and faint objects.