[meteorite-list] Update on Russia's Mega-Meteor

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 6 Mar 2013 11:13:46 -0800 (PST)
Message-ID: <201303061913.r26JDkj4022379_at_zagami.jpl.nasa.gov>

http://www.skyandtelescope.com/news/Update-on-the-Russian-Mega-Meteor-195553631.html

Update on Russia's Mega-Meteor
Kelly Beatty
Sky & Telescope
March 6, 2013

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
*Authors* *Semimajor axis* *Eccentricity* *Inclination* *Perihelion* *Aphelion*
Borovicka & Other 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.
NASA/JPL /(to come)/ /(to come)/ /(to come)/ /(to come)/ /(to come)/

The spread of values arises from how each team interprets the videos,
derives the trajectory and speed as it passed through the atmosphere,
and then derives an orbit.

Orbit of the Cherbakul meteoroid

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 and might never be - for example, the path's aphelion
doesn't correspond to any strong orbital resonances with Jupiter.

A refinement 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 compiled by Brown, along
with the coordinates of the camera:


    * Kichigino <http://www.youtube.com/watch?v=0CoP7WB8Gew>
      (54.50056??N, 61.27165??E)

    * Yemanzhelinsk <http://www.youtube.com/watch?v=ELPLV3Rkslg>
      (54.756579??N, 61.304044??E):

    * Kurchatovskiy <http://www.youtube.com/watch?v=4ZxXYscmgRg>
      (55.220774??N, 61.296265??E)

    * Korkino <http://www.youtube.com/watch?v=odKjwrjIM-k> (54.89092??N,
      61.39958??E)

    * Central Chelyabinsk <http://www.youtube.com/watch?v=XqZhMClRHpM>
      (55.16632??N, 61.44478??E)


In any case, everyone agrees that the object entered the atmosphere at
roughly 12 miles (19 km) per second along an east-to-west track, more or
less. According to Juri Borovicka's team, the bolide's flight path had a
slope of 16????, 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 got 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 Cherbakul. 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/cm^3 ), 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 Cherbakul? 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
<http://urfu.ru/en/home/press/news/article/new-details-about-chelyabinsk-meteorite-research/>.
Analysis is continuing, but I'm guessing the divers will be heading back
down very soon.
Received on Wed 06 Mar 2013 02:13:46 PM PST