A fireball streaking across the sky as seen from a driver's dashboard. via
It’s been a little over a week since a sizable rock fell from the skies and exploded less than 15 miles above the Chelyabinsk region of Russia, but astronomers are already working on reconstructing the meteorite’s path, orbit, and likely origin. And they’re doing it with some unlikely research materials: amateur videos.
The Chelyabinsk meteorite was unexpected, but the digital age means we’re always ready to capture something interesting. Case in point. News of the meteorite broke with a slew of amateur videos from cars, security cameras, and cell phones, offering immediate proof that the news wasn’t a hoax. The videos all also captured something really useful: the trail of a fireball streaking across the sky.
To researchers, that’s perfect trajectory data. Amateur videos caught enough of the fireball’s trajectory that Jorge Zuluaga and Ignacio Ferrin, two researchers at the University of Antioquia in Medellin, Colombia, were able to work backward to find the meteorite’s orbit and origin. Their results are now available on the arXiv pre-print server (PDF).
The Colombian researchers were inspired by Stefen Geens, a self-described technologist whose blog, Ogle Earth, “looks at the political, social and scientific impact of networked digital maps and geospatial imagery, with a special focus on Google Earth.” It was on his blog that Geens first published an estimate of the Chelyabinsk meteor’s path. Matching dashboard and security cameras footage with data from Google Earth, he was able to reconstruct the path of the rock as it entered the atmosphere. It matched the trajectory image taken by the geostationary weather satellite Meteosat-9.
The figure above from Zuluaga and Ferrin's paper shows how they triangulated the path of the meteorite via various dashcam perspectives. They include "the Revolution Square at Chelyabinsk (C), the Korkin metropolitan area (K) and Lake Chebarkul (L). The brightening point (BP) and the fragmentation point( FP) are the points of the trajectory seen from Chelyabinsk and measured using the shadow cast by the poles at the Revolutionary Square."
Zuluaga and Ferrin took the work a step further. Part of the difficulty with the array of amateur video is the difference in time and date stamped on each source – some videos of the event recorded times different by minutes, and that makes for very imprecise calculations. So the researchers settled on two videos they thought were most reliable, a camera in Revolutionary Square in Chelyabinsk and one video recorded in the a nearby city of Korkino. They calculated the rock’s trajectory based on these two videos. Then they added a third data point, the hole in the ice in Lake Chebarkul about 44 miles west of Chelyabinsk that’s thought to have been made from falling chunks of the rock.
Between these three sources, Zuluaga and Ferrin were able to get a pretty good idea of the rocks height, speed, and position before it impacted the Earth’s atmosphere. But going further to determine the meteor’s orbit around the Sun was harder and less exact.
Zuluaga and Ferrin’s reconstructed orbits for the Chelyabinsk meteoroid. via
This type of triangulation demands six critical factors, all of which Zuluaga and Ferrin had to estimate using the so-called Monte Carlo methods. They calculated the most probable orbital parameters based on the meteor’s brightening point, the point where the becomes bright enough to cast a noticeable shadow in the videos. From there they could determine its height, elevation, and azimuth, it’s the longitude and latitude on the Earth’s surface, as well as the rock’s velocity.
They found that the Chelyabinski meteor became noticeably bright in the videos when it was between 20 and 30 miles above the Earth’s surface traveling between 8 and 12 miles per second. This estimate puts them squarely in the ballpark others researchers found; most estimates have the rock falling at about 11 miles per second. The researchers then imported this data into the Naval Observatory Vector Astrometry, special software developed by the U.S. Naval Observatory, to calculate the likely orbit. What they found was a very strong likelihood that the Chelyabinsk meteorite was an Apollo asteroid, a well-known class of rocks that regularly cross Earth’s orbit.
Astronomers have seen over 240 Apollo asteroids that are larger than half a mile wide, though estimates suggest there are some 2,000 more of about the same size prowling the solar system. The meteor that fell over Chelyabinsk was much small, only about 50 feet across, but that’s a much more common size. Astronomers expect there are close to 80 million Chelyabinsk-sized Apollo asteroids out there, each weighing over 7,700 tons.
But this is all speculative. Going forward, the team has decided not to use the hole in Lake Chebarkul as one of their triangulation points since they can’t be certain it was caused by the meteorite. And of course, knowing the meteorite’s path in hindsight is an academic interest that can’t help predict the next one. That’s the eventual goal – knowing where these rocks are so we can so what we can to avoid a devastating impact.