Immediately following the meteoroid impact near Chelyabinsk on Feb. 15, 2013, social media was flooded by incredible video and firsthand accounts of this event. Scientists have now followed up on these reports and are giving a comprehensive overview in a paper published today in the journal Science. The report is led by Olga Popova of the Institute for Dynamics of Geospheres of the Russian Academy of Sciences in Moscow and by meteor astronomer Peter Jenniskens of NASA’s Ames Research Center and the SETI Institute. Jenniskens participated in a fact-finding mission to more than 50 villages around Chelyabinsk in the weeks after the fall.
"Our goal was to understand all circumstances that resulted in the damaging shock wave," said Jenniskens, who is best known for investigating the asteroid impact over Sudan in 2008 and last year's Sutter's Mill meteorite fall in California. "Based on infrasound data, the brightness of the fireball, and the extent of the glass damage area, we confirm that this event was 100 times bigger than Sutter's Mill.”
The team traveled to sites where citizens captured the event on video, and measured the viewing angles of the fireball in 10 of the best videos of the fireball's entry. They calculated that the meteoroid entered Earth's atmosphere at more than 42,500 mph (19 kilometers per second).
"Our meteoroid entry modeling showed that the impact was caused by a 20-meter sized single chunk of rock that efficiently fragmented during its passage through the atmosphere,” said team leader Popova. As the meteoroid reached an altitude of 19 miles (30 kilometers), the meteoroid appeared brighter than the sun and created so much ultraviolet emission that some people were severely sunburned.
At that moment, the meteoroid’s catastrophic fragmentation peaked, creating the debris that later would be recovered as meteorites on the ground just south of the trajectory in a long stretch from Aleksandrovka to Deputatskiy and Timiryazevsky. The team estimated that between 9,000 to 13,000 pounds (4,000 to 6,000 kilograms) of meteorites fell to the ground, which would only represent 0.03-0.05 percent of the initial mass of the meteoroid. An estimated 76 percent of meteoroid evaporated and the remainder became mostly dust.
"The reason so little survived," said Popova, "is that the radiation was so intense it contributed to evaporating the fragments before they could fall as meteorites out of this cloud." The cloud formed while the meteoroid fragmented was so hot it glowed orange. A small mushroom cloud formed by the rising hot air. The largest fragments survived the event one of which ultimately impacted Lake Chebarkul.
Ural Federal University researchers led by meteoriticist professor. Viktor Grokhovsky initially found small pieces of the meteorite around a 22-foot (7-meter) sized hole in the ice and set out to determine the position of more of the fallen rock using sophisticated metal detectors. After many months of searching, an approximately 1,400-pound (650-kilogram) meteorite was recovered from deep in the mud by professional divers on Oct. 16.
A shoreline security camera video recorded how this meteorite impacted the ice. A cloud of ice or snow seen rising in the air and then drifting away in the wind. This is the first time that an actual meteorite impact was recorded on video. The timing of the fall enabled Jenniskens and Popova to calculate an impact speed of about 500 mph (225 meters per second).
The surviving fragments added to the energy in the shockwave, most of which resulted from the approximate 18-mile (30-kilometer) fragmentation event. The team visited 50 villages in the area to map out the extent of the glass damage and interview eyewitnesses. Their map of broken windows has the form of a butterfly, stretching in a narrow swath out to 55 miles (90 kilometers) on either side of the trajectory. The city of Chelyabinsk, with over a million inhabitants, was right in the path of the wave. The team showed that the shape of the damaged area could be explained from the fact that the energy was deposited over a range of altitudes.
As part of an international 59-researcher-strong consortium from nine countries, other NASA researchers contributed to this investigation also by studying the recovered meteorites. During Jenniskens' visit in March, fresh snow had made meteorite searches impossible, but Chelyabinsk State University collaborator professor Alexandr Dudorov made some of the meteorites available for study that were recovered from the snow. Researchers were most interested in finding out why the meteoroid fragments did not penetrate deeper.
"We suspect it is the abundance of shock veins in this rock," said NASA Ames meteoriticist Derek Sears. "When we pressed on the rock, it broke along one of these shock veins." Cosmochemist Mike Zolensky from NASA’s Johnson Space Center in Houston may have found why these shock veins were so frail. They contained a layer of small iron grains at each rim, which had precipitated out of the glassy material when it cooled.
"Impacts on the parent body fractured the rock and pushed molten metal and metal sulfides through the cracks," says Zolensky. "There are cases where this increased a meteorite's mechanical strength, but Chelyabinsk was weakened by it."
The orbit of the Chelyabinsk meteoroid calculated by Jenniskens confirms it may have originated from the Flora asteroid family in the asteroid belt, the same source region as proposed for asteroid Itokawa, which is of the same meteorite type.
The team suspects that the chunk that hit the Chelyabinsk area was not broken up in the asteroid belt itself. A team of the University of Tokyo and Waseda University in Japan, led by Keisuke Nagao, which measured the rock estimated it had been exposed to cosmic rays for only 1.2 million years, an unusually short time for rocks originating in the Flora family. Jenniskens speculates that Chelyabinsk belonged to a bigger rubble pile asteroid before it broke apart 1.2 million years ago, possibly in an earlier close encounter with Earth.
"It was a privilege to participate in a field study organized by the Russian Academy of Sciences," says Jenniskens. "It is fantastic to see how much citizen science helped document this event. I very much look forward to see future events documented in the same way."