Although the cosmic intruder that exploded over Russia on February 15th had an estimated mass of 10,000 tons, less than 200 kg of it has been recovered to date.
|Update, July 6th: A Russian meteorite scientist has finally responded to questions about the large hole that formed in Lake Chebarkul, ostensibly by a large (300-kg?) fragment of the Chelyabinsk impactor. According to Dmitry Sadilenko (Russian Academy of Sciences), the hole was definitely caused by the impact, as many small fragments were found on the ice nearby. But divers found nothing during an initial search in February, and officially there are no plans to search further. "I have been two times in the region, collecting fragments," Sadilenko says. "We had no plans for recovering the main mass as we had no experience in such things and no equipment. With the time passing, there is less and less chance to find anything."
Four months have passed since the early-morning bustle in and around Chelyabinsk, Russia, was shattered by a powerful cosmic explosion high in the sky. As reported here hours after the blast and in the days thereafter, the intruder was a stony object about 54 feet (17 m) across that had a mass of roughly 10,000 tons. Windows shattered in 7,000 buildings as a powerful blast wave engulfed the area, and some 1,700 people were injured (most by flying shards of glass).
Now the warmth of early summer and normalcy have returned to the Chelyabinsk region. Injuries have healed, and most of the damage has been repaired. The hordes of television crews and treasure hunters are long gone.
But the scientific analysis of this historic event continues. A report by Alexis Le Pichon and other researchers, appearing this month in Geophysical Research Letters, describes how the blast wave triggered infrasound (acoustic) monitors at 20 stations in the International Monitoring System (IMS), a global network maintained by Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). A station in Alaska even picked it up three days later, after reverberations had completely circled Earth twice.
The researchers conclude that the Chelyabinsk impactor delivered the explosive equivalent of 460 tons of TNT, making this event "the most energetic ever detected by the global IMS infrasound network."
Meanwhile, even simple Russian peasants quickly realized that rocks falling from outer space are worth plenty of rubles. Although deep snow covered the region at the time, winter's blanket actually made finding fragments relatively easy. Eager searchers simply looked for small, deep tunnels in the drifts. A few seconds digging by hand down into the snow frequently revealed a piece of stone. Some holes became partly filled with icy particles that adhered to the meteorite and sintered together into relatively hard cylinders, yielding the weird cosmic snow cones like those seen at right.
Within days, entrepreneurial Russians scavenged all the fragments they could find. Two searches organized by Russian meteorite specialists together turned up about 100 stones. It took Dmitry Nuzhnenko and his search party just five days to locate 160 samples. American and European meteorite hunters rushed to the fall site as well.
Classified as ordinary chondrites containing relatively little iron (LL5), the Chelyabinsk meteorites typically have distinctive light-gray interiors with dark (sometimes red-tinged) fusion crusts. They've been fetching about $20 per gram on eBay and other online outlets.
To fully appreciate the climatic and political challenges of looking for samples of the Chelyabinsk meteorite, you should read the "road-trip" reports posted by meteorite hunters Rob Wesel (Portland, Oregon) and Svend Buhl (Hamburg, Germany).
The easy pickings ended when fresh snow moved into the fall area on Feburary 25th. Then, once winter's snows finally melted, spring rains made much of the region a quagmire of mud. Then a knee-high carpet of dense vegetation hid the bare ground. Even so, Moscow-based Pjotr Muromov returned to the area in mid-April, and over three weeks his search party turned up 46 stones.
Buhl and his colleague Karl Wimmer have painstakingly documented 233 discovery reports, as supplied by Russian, Polish, and American searchers, that cover an elongated area 45 miles long and 12 wide (70 by 20 km). This pattern, called a strewnfield, is typical of meteorite falls involving many fragments. The smallest specimens (those most affected by air resistance as they fall) land near the beginning of the track and the most massive ones at the far end.
But the Chelyabinsk event is unique, because stones were shed by the main body in up to seven explosive outbursts along the flight path. This resulted in a series of overlapping fall patterns, as depicted schematically at right. Searchers often found small and large stones clustered together, while nothing turned up in huge swaths of the strewnfield.
"Strewnfields may assume many shapes, depending on wind speed and direction," notes Wimmer, "but they are virtually never ellipses. In our case they are more like slightly curved bones."
What's remarkable about the Russian mega-meteor is how little of it has been found. According to Buhl and Wimmer, the combined mass of the submitted finds
is just 117 pounds (53 kg). Surely thousands of others small bits will never turn up, but notably the largest stone found to date tips the scale at only 7.5 pounds (3.4 kg). "Our estimate is that the current total known weight is in a range between 100 and 200 kg," says Buhl.
There's been a lot of speculation about a large hole that appeared in the frozen slab covering Lake Chebarkul, at the far end of the strewnfield. Ice fishermen along its shore saw a large object plunge into the lake, creating a column of water, ice, and steam. But divers have found nothing on the murky lake bottom, even though magnetic soundings indicate the presence of several massive blobs. The lake bottom is about 30 feet (9 m) deep at this spot is site and covered by thick layer of slit.
Based on Buhl and Wimmer's analysis of the object's trajectory and wind factors, roughly 700 pounds (300 kg) should be down there. Last week RIA Novosti announced that a huge chunk had been found, but that report cites only existing, circumstantial evidence and has not been confirmed.
So what happened to the other 99.997%? It was vaporized and pulverized to dust, say meteor specialists.
We've certainly not heard the last word about the Chelyabinsk impact. See Sky & Telescope's roundup of links to scientific assessments, news reports, blogs, and slide shows about this once-in-a-century event.