RUSSIA The large fireball (technically, called a “superbolide”) observed on the morning of Feb. 15, 2013, in the skies near Chelyabinsk, Russia, was caused by a relatively small asteroid approximately 17 to 20 meters in size (about 18.6 to 21.9 yards) that entered Earth’s atmosphere at high speed and at a shallow angle. In doing so, it released a tremendous amount of energy, fragmented at high altitude, and produced a shower of pieces of various sizes that fell to the ground as meteorites.

The fireball was observed not only by video cameras and low-frequency infrasound detectors, but also by U.S. government sensors. Information on the composition of the meteor was also derived from meteorite fragments found in the Chelyabinsk area. With this new data incorporated, the details of the impact have become clearer.

At 9:20:20 a.m. local time (3:20:20 UTC) the meteor entered Earth’s atmosphere over the Kazakhstan/Russia border. As it descended through the upper atmosphere, it traveled northwest into Russia. The impactor’s trajectory approached Earth along a direction that remained within 15 degrees of the direction of the sun. Asteroid detection telescopes cannot scan regions of the sky this close to the sun.

During the atmospheric entry phase, an impacting object is both slowed and heated by atmospheric friction. In front of it, a bow shock develops where atmospheric gases are compressed and heated. Some of this energy is radiated to the object, causing it to ablate, and in most cases, break apart. Fragmentation increases the amount of atmosphere intercepted and so enhances ablation and atmospheric braking. The object disintegrates when the force from the unequal pressures on the front and back sides exceeds its tensile strength. This disruption, or disintegration, usually occurs around the time of maximum brightness.

Thirteen seconds after atmospheric entry, at 9:20:33 a.m. local time (03:20:33 UTC), the fireball, traveling at a velocity of 11.6 miles per second (18.6 kilometers per second), achieved its maximum brightness just south of Chelyabinsk, Russia, at an altitude of 14.5 miles (23.3 kilometers). The approximate effective diameter of the asteroid is estimated to be about 18 meters (about 19.7 yards), and its mass about 11,000 tons. Approximate total impact energy of the Chelyabinsk Fireball, in kilotons of TNT explosives (the energy parameter usually quoted for a fireball), is 440 kilotons. Note that these estimates of total energy, diameter and mass are very approximate. The Chelyabinsk event was an extraordinarily large fireball, the most energetic impact event recognized since the 1908 Tunguska blast in Russian Siberia.