THE CHELYABINSK AIRBURST: OBSERVATIONS AND MODELS

On Feb. 15, 2013, an asteroid exploded about 40 km SSW of the Russian city of Chelyabinsk. Its proximity led to many injuries and widespread blast damage, but also yielded a plethora of data from security and dashboard cameras. Combined with seismic, infrasound, and satellite records, this provides a rich and multi-faceted means to determine the projectile size and entry parameters, and develop a self-consistent model. We will present results of the first physics simulations to be initialized with accurate energy deposition derived from observations.

The best estimate of the explosive yield is 400-500 kilotons, making Chelyabinsk the most powerful such event observed since Tunguska (3-5 megatons). Analysis of video combined with subsequent on-site stellar calibrations enable precise estimates of entry velocity (19 km/s), angle (17° elevation) and altitude of peak brightness (29 km). This implies a pre-entry diameter of ~20 m and mass of ~1200 tonnes. Satellite sensors recorded the emission peak at 03:20:33 UT, with a total radiated energy of 3.75 ∙ 10¹⁴ J (~90 kilotons). A typical bolide luminous efficiency of 20% implies a total energy of ~450 kilotons, consistent with infrasound and other observations. The maximum radiant intensity was 2.7 ∙ 10¹³ W/ster, corresponding to a magnitude of -28.

The shallow entry angle led to a long bolide duration (16.5 s) and energy was deposited over hundreds of km leading to an extended, near-horizontal, linear explosion. The blast was distributed over a large area, and was much weaker than for a steep entry and a more concentrated explosion closer to the surface. The orientation also led to different phenomena than expected for a more vertical entry. There was no ballistic plume as observed from SL9 impacts (45°) or calculated for Tunguska (~35°). Instead, buoyant instabilities grew into mushroom clouds and bifurcated the trail into two contra-rotating vortices.

Chelyabinsk and Tunguska are “once-per-century” and “once-per-millennium” events, respectively. These outliers imply that the frequency of large airbursts is underestimated. Models also suggest that they are more damaging than nuclear explosions of the same yield (traditionally used to estimate impact risk). The risk from airbursts is therefore greater than previously thought.