The trajectory, structure and origin of the Chelyabinsk asteroidal impactor

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.

November 14 2013

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) 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.

The Chelyabinsk asteroid (red) and asteroid 86039 (green). (Animations: 5Mb MPEG)

Chelyabinsk and asteroid 86039

The orbits of the Chelyabinsk asteroid and near-Earth asteroid 86039 (1999 NC43) are similar, as illustrated in the animation on the left. But is this just a chance alignment or an indication of some deeper connection? Asteroid 86039 is the right colour (technically spectral type Q, Binzel et al. (2004) Icarus 170, 259-294) for a match with an LL5 chondrite, the type of rock dropped at the end of its flight as the Chelyabinsk meteorite. The orbits of the bodies also pass near each other, a condition that has persisted for at least the last thousand years. The 'kick' needed to move from the orbit of 86039 to that of Chelyabinsk corresponds to a speed of about 1 kilometer per second, a sizable kick (corresponding to 3600 km per hour or 2200 miles per hour) but one that is well in line with the collision speeds of asteroids which are typically a few km/s. Thus the Chelyabinsk object might have been ejected from 86039 by a collision with another asteroid

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.

Why wasn't the Chelyabinsk meteor detected beforehand?

As can be seen in some of the animations listed below, in the hours and days before contact, the Chelyabinsk meteoroid approached the Earth from the direction of the Sun. Even under ideal conditions, an approaching object the size of the Chelyabinsk meteor is visible for only a few hours before its arrival even in the very largest of telescopes. However, when approaching from the Sun's direction, objects become near-impossible to detect. The side of the meteoroid facing the Earth is buried in the object's own shadow and the glare from the Sun. Regardless, we did a check of nearly 6,000,000 images in the image catalogues of the major sky surveys on the off-chance that it had been imaged but not noticed for some reason. We checked for both the final approach of the meteoroid, and any possible near approaches in prior orbits. No images were found in which the object would be seen.

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.

Related links

  • Ondrejov Observatory, the Astronomical Institute of the Academy of Sciences of the Czech Republic
  • The Department of Physics and Astronomy at the University of Western Ontario
  • The Centre for Planetary Science and Exploration at the University of Western Ontario
  • The Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic

    Have a question or comment? Contact me ( Paul Wiegert at pwiegert[the @ sign]
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    Other Animations

    A combined animation of the orbit of the Chelyabinsk asteroid (white) prior to its arrival at Earth with a simulation of its final approach to Earth. (Animation: 79Mb Windows AVI)

    The orbit of the Chelyabinsk asteroid (red) prior to its arrival at Earth. (Animations: 18Mb Windows AVI, 5.7Mb MPEG, 29Mb HD Quicktime MOV, 70Mb HD Windows AVI)

    A simulated view from the Chelyabinsk asteroid as it approaches the Earth. The red dot in the final frames of the animation is the city of Chelyabinsk. (Animations: 4.4Mb MPEG, 21Mb Quicktime MOV, 42Mb HD Windows AVI, 22Mb HD Quicktime MOV )