Chelyabinsk Eyewitnesses Help Scientists Resolve Meteor Mysteries
On February 15, 2013, people near Chelyabinsk, Russia felt the ground shake, smelled the sour stench of sulfur, heard windows shatter into sprays of glass and had to look away from a fireball in the sky so bright it hurt their eyes. The meteor that caused all this havoc largely dissolved into a cloud of dust during its passage through Earth’s atmosphere, so scientists are turning to clues on the ground and the memories of eyewitnesses to piece together what happened that day. Around 1,500 people were injured, although no one was killed. In the city of Chelyabinsk alone, more than 3,500 buildings were damaged, and the researchers found shockwave destruction as far as 100 kilometers away from the impact site.
Based on testimony from people near the impact zone as well as the copious video footage caught by residents’ dashboard cameras and security video feeds, scientists have calculated the precise trajectory of the inbound Chelyabinsk meteor, as well as the power of the atmospheric explosion and the dynamics of its shockwave. The findings are detailed in three papers published this week in Nature and Science. (Scientific American is part of Nature Publishing Group.)
A team led by Olga Popova of the Russian Academy of Sciences visited 50 villages surrounding the blast area in the month after the event to speak to residents and photograph the broken windows and other damage from the meteor. “Typically we’d go into a village and first find out where the local grocery market is, and we’d talk to the people behind the counter because they’d just listened for the past three weeks to what other people had experienced,” says research team member Peter Jenniskens of the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, Calif. “They’d summarize for us, and then we’d go into the streets and talk to people. Everybody had a story to tell.” The scientists met people who were blown off their feet by the meteor’s shockwave, and others who were sunburned by ultraviolet light from the fireball. “There was one person who said his skin even flaked afterward,” Jenniskens says. The team found that it was often the village schools, which tended to have the biggest windows, that suffered the most window damage. The scientists compiled the data from their visits and interviews, as well as from an online survey of residents, to calculate damage and injury patterns around the Chelyabinsk area.
Both Popova’s team and a second group, led by Jirí Borovicka of the Academy of Sciences of the Czech Republic and Peter Brown of the University of Western Ontario, used video footage to calculate the meteor’s trajectory. (Popova’s findings were reported in Science and Borovicka and Brown each led papers published in Nature). The researchers visited the locations where amateur videos had been filmed, and photographed the stars in the sky to calibrate the meteor’s precise location and the path it took through the atmosphere. Both calculations agree well with a trajectory computed from satellite images of the meteor by Colorado State University meteorologist Steven Miller and his colleagues, which was published October 21 in Proceedings of the National Academy of Sciences. “It was nice to see that confirmation,” Miller says.
Borovicka and Brown’s team found that the rock started out about 19 meters wide, and broke into small pieces as it descended from 45 to 30 kilometers over Earth. The meteor's airburst packed an energy equivalent to 500 kilotons of TNT, they calculated. The relatively small asteroid had escaped detection prior to impact, but by computing the meteor’s original velocity and direction of flight, the scientists were able to deduce the rock’s orbit around the sun, which proved to be markedly similar to the orbit of a known, much larger asteroid—a two-kilometer-wide object called 86039 (1999 NC43).
“After statistical analysis we found it’s very unlikely that the proximity of the orbits is only by chance,” Borovicka says. “So we cannot prove it, but we suggest this Chelyabinsk asteroid and this big asteroid were one time in the past part of the same body.” If an earlier collision broke the two apart, their orbits probably would have diverged over time, so such a break-up would have to have occurred relatively recently, says Nick Gorkavyi of NASA Goddard Space Flight Center, who was not involved in the new studies. “It’s an interesting possibility, but of course right now it’s just a hypothesis,” he says. NASA planetary scientist Don Yeomans of the Jet Propulsion Laboratory agrees that the idea is “quite possible” but notes that researchers must now look for compositional similarities between the two objects. “I would say the jury is still out … until the asteroid's spectral characteristics can be matched with the Chelyabinsk meteorites,” he says.
The new analyses also suggest that events like Chelyabinsk might be more common than had been assumed. Based on telescopic observations of asteroids, researchers had previously estimated that a Chelyabinsk-scale impact took place every 150 years, on average. But after analyzing various historic surveys and the new information about the February meteor, Brown’s team estimates that such objects might smack Earth as often as once every few decades. “There may be more of these airburst-type events, things like Chelyabinsk, than we previously thought,” Brown says. Even though most impacts of this size do not cause serious damage, and the vast majority will hit over ocean rather than land, the finding is nonetheless sobering. “We may well in our lifetime see another one like Chelyabinsk,” Brown warns.
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