The asteroidal fragment that exploded over Chelyabinsk, Russia, last month delivered the kinetic-energy equivalent of at least 400 kilotons of TNT. Damage was limited because it disrupted so high up — but the situation could have been much worse.
It's been nearly a month since a big chunk of asteroid plunged into Earth's atmosphere on the morning of February 15th and put on a spectacular show in the skies over central Russia. Since then impact and meteorite specialists have raced not only to figure out where it came from but also to gather up and analyze as many fragments of the impactor as possible. From reports and interviews gathered by Sky & Telescope, they've had great success on both fronts.
First, let's recap where this interloper came from. Thanks to ample video recordings (many from the dashboard-mounted cameras of security-wary Russian drivers), it's been a snap to reconstruct the entry circumstances and, from those, the pre-impact orbit. But different teams come up with different values, as the table below reveals. (Uncertainties aren't shown; see the linked sources for those.)
|Orbit of the Russian Mega-Meteor|
|Borovicka & others||1.55 a.u.||0.50||3.6°||0.77 a.u.||2.33 a.u.|
|Zuluaga & Ferrin||1.73 a.u.||0.51||3.5°||0.82 a.u.||2.64 a.u.|
|Chodas & Chesley||1.73 a.u.||0.57||4.2°||0.75 a.u.||2.78 a.u.|
The spread of values arises from how each team interprets the videos, derives the trajectory and speed as the bolide passed through the atmosphere, and then derives an orbit.
Still, there's consensus that the object came from a well-populated section of the inner asteroid belt. How it got bumped inward toward Earth isn't known — yet. For example, the path's aphelion might overlap a location, 2½ a.u. from the Sun, at which there's a strong orbital resonance with Jupiter. In that case, gravitational perturbations by the giant planet could have yanked the object out of a nearly circular orbit and onto its eventual collision course with Earth.
A refined atmospheric trajectory and pre-crash orbit might eventually emerge from a team led by Peter Brown (University of Western Ontario). They're now analyzing star positions in nighttime images taken at the exact locations where several bolide videos were recorded.
Here's a "top five" list of video recordings compiled by Brown, along with the exact coordinates of the camera that took each one:
- Kichigino (54.50056°N, 61.27165°E)
- Yemanzhelinsk (54.756579°N, 61.304044°E):
- Kurchatovskiy (55.220774°N, 61.296265°E)
- Korkino (54.89092°N, 61.39958°E)
- Central Chelyabinsk (55.16632°N, 61.44478°E)
The intruder entered the atmosphere at roughly 12 miles (19 km) per second along an east-to-west track, more or less. A European weather satellite glimpsed it skirting Earth's limb (here's a reprojected view of that image). According to Juri Borovicka's team, the bolide's flight path had a slope of 16½°, and it started breaking apart relatively high up, 20 miles (32 km) above the ground.
The dazzling light, brighter than the Sun, certainly got the attention of anyone looking its way — though some videos show apparently pedestrians unfazed by the bright-as-the-Sun spectacle. Purdue impact specialist H. Jay Melosh notes that what many are calling a contrail (as in "condensed water vapor") is actually a smoke trail. "Probably most of the mass ended up as fine dust that blew away," he says.
What certainly did get everyone's attention was the bolide's powerful shock wave, which reached the ground about 88 seconds later. This blast blew out countless windows in and near the city of Chelyabinsk. Although many were injured, no one was killed.
However, the damage could have been much worse. "All that blast energy was distributed over a large area," explains Mark Boslough (Sandia National Laboratories). If this half-megaton blast had been directed straight downward, he says, its fireball might have engulfed the ground. That's what happened during the 1908 Tunguska event, which involved a blast at least 10 times more powerful and much closer to the ground. But such a fate didn't befall the residents of Chelyabinsk or in the surrounding countryside, Boslough concludes. "The villages are still there."
Meanwhile, Russian meteorite specialists have been scrambling to collect as many pieces of the meteoritic shrapnel as they can. These fell over a wide area near the town of Chebarkul. In most cases, searchers simply looked for small holes in the ubiquitous snow cover and dug down to claim their cosmic prizes. Most of the pieces are small, no more than an inch across, and the largest fragment found to date weighs only 4 pounds (1.8 kg).
According to a team of analysts led by Viktor Grokhovsky (Ural Federal University), the fragments are a common stony meteorite type, called an ordinary chondrite, containing relatively little metallic iron. Given the power of the blast, the entry velocity, and the typical density of chondrites (about 3.6 g/cm3), Brown estimates the original object had a diameter of about 54 feet (17 m) and a mass of roughly 10,000 metric tons.
Melosh, who's studying how meteoroids break up in the much-thinner atmosphere of Mars, thinks the small-fry stones imply that the precursor object was loosely bound together, what's often termed a "rubble pile." he explains that objects slamming into the thin Martian air "break up multiple times — like a cosmic-ray cascade — so you don't get any large fragments."
But what about that big, mysterious 50-foot (16-m) hole created in the ice covering Lake Chebarkul? Grokhovsky believes it was created when a large chunk of meteorite, 2 feet (60 cm) long and weighing roughly 200 pounds (100 kg) fell into the lake. That's what divers expected to find lying on the lake bottom, about 35 feet (10 m) down, but after exploring the murky, silty conditions they came up empty-handed.
However, earlier today researcher Evgeny Narkhov (also at Ural Federal University), released a preliminary map of magnetometer readings taken over an area the size of a football field. The lakebed map shows several hot spots, suggesting that the meteorite likely broke apart on impact. "There are not small pieces and a large one, as you might think, but several large fragments," Narkhov notes in a university news statement. Analysis is continuing, but I'm guessing the divers will be heading back down very soon.
Meanwhile, after ignoring some initial offers that were clearly fraudulent, meteorite collectors report that genuine bits of the Chebarkul fall are finally making their way into the marketplace. The going rate is $30 to $40 per gram. But nationalistic pride and protectionism are involved, and the Russian Duma is considering a proposal to prohibit the exporting of meteorites — a practice that is permitted for now.