[meteorite-list] Additional Details on the Large Fireball Event over Russia on Feb 15, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 1 Mar 2013 15:47:05 -0800 (PST)
Message-ID: <201303012347.r21Nl5gl019591_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/news/fireball_130301.html

Additional Details on the Large Fireball Event over Russia on Feb. 15, 2013
Don Yeomans & Paul Chodas
NASA/JPL Near-Earth Object Program Office
March 1, 2013

The large fireball (technically, a "superbolide") observed on the
morning of February 15, 2013 in the skies near Chelyabinsk, Russia, was
caused by a relatively small asteroid approximately 17 to 20 meters in
size, entering the Earth's atmosphere at high speed and a shallow angle.
In doing so it released a tremendous amount of energy, fragmented at
high altitude, and produced a shower of pieces of various sizes that
fell to the ground as meteorites. The fireball was observed not only by
video cameras and low frequency infrasound detectors, but also by U.S.
Government sensors. As a result, the details of the impact have become
clearer. There is no connection between the Russian fireball event and
the close approach of asteroid 2012 DA14, which occurred just over 16
hours later.

New Fireball Data

U.S. Government sensor data on fireballs are now reported on the NASA
Near-Earth Object Program Office website at

http://neo.jpl.nasa.gov/fireballs

The February 15th event is the first entry on this new site, and it
provides the following information about the fireball:

    * Date and time of maximum brightness: 15 Feb. 2013/03:20:33 GMT
    * Geographic location of maximum brightness:
      Latitude: 54.8 deg. N
      Longitude: 61.1 deg. E
    * Altitude of maximum brightness: 23.3 km (14.5 miles)
    * Velocity at peak brightness: 18.6 km/s (11.6 miles/s)
    * Approximate total radiated energy of fireball: 3.75 x 10^14
      Joules. This is the equivalent of about 90 kilotons (kt) of TNT
      explosives, but it does not represent the total impact energy (see
      note below).
    * Approximate total impact energy of the fireball in kilotons of TNT
      explosives (the energy parameter usually quoted for a fireball):
      440 kt.

Note that the total energy of a fireball event is several times larger
than the observed total radiated energy. The JPL fireballs website uses
the following empirical formula derived by Peter Brown and colleagues to
convert the optical radiant energy Eo into an estimate of the total
impact energy E (see: Brown et al., The flux of small near-Earth objects
colliding with the Earth. Nature, vol. 420, 21 Nov. 2002, pp. 294-296):.

E = 8.2508 x E_o ^0.885

During the atmospheric entry phase, an impacting object is both slowed
and heated by atmospheric friction. In front of it, a bow shock develops
where atmospheric gases are compressed and heated. Some of this energy
is radiated to the object causing it to ablate, and in most cases, to
break apart. Fragmentation increases the amount of atmosphere
intercepted and so enhances ablation and atmospheric braking. The object
catastrophically disrupts when the force from the unequal pressures on
the front and back sides exceeds its tensile strength.

This was an extraordinarily large fireball, the most energetic impact
event recognized since the 1908 Tunguska blast in Russian Siberia.

The meteorites recovered from the Chelyabinsk fireball are reported to
be ordinary chondrites, which have a typical density of about 3.6 g/cm^3.
Given the total energy of about 440 kt, the approximate effective
diameter of the asteroid would be about 18 meters, and its mass would be
roughly 11,000 tons. Note that these estimates of total energy, diameter
and mass are very approximate.

Where Did the Chelyabinsk Impactor Come From?

An approximate path for the Chelyabinsk impactor can be calculated from
the newly released fireball data. (A similar calculation can be made
from analysis of video records of the event; both methods yield similar
results.) The first diagram shows the ground track of the impactor over
the last minute or so before impact. The altitudes along this ground
track have been called out and the asterisk on the path indicates the
point of peak brightness, just south of Chelyabinsk.

[Diagram 1: Ground track of impactor showing altitude values along the track]

The second diagram shows the impactor's final trajectory over the last
several hours, as it approached the Earth along a direction that
remained within 15 degrees of the direction of the Sun. Asteroid
detection telescopes cannot scan regions of the sky this close to the Sun.

[Diagram 2: Approximate final trajectory of impactor]

The third diagram shows the orbit of the impactor about the Sun. The
orbit reaches from the asteroid belt at its farthest from the Sun to
near the orbit of Venus at its closest to the Sun. The impactor had
likely been following this orbit for many thousands of years, crossing
the Earth's orbit every time on its outbound leg.

[Diagram 3: Heliocentric orbit of asteroid that impacted near Chelyabinsk
Russia]

Was the Chelyabinsk Fireball Related to the Close Approach of Asteroid
2012 DA14?

Asteroid 2012 DA14 made a very close flyby of the Earth just over 16
hours after the Russian fireball event, passing within 27,700 km (17,200
miles) of the Earth's surface, but there is no connection whatever
between these two events. First of all, the two objects approached the
Earth from completely different directions, and had entirely different
orbits about the Sun. A custom version of the JPL orbit display applet
has been created to show the orbits of the Chelyabinsk impactor and 2012
DA14 at the same time:

http://neo.jpl.nasa.gov/orbits/2012da14.html

A second reason we know the two asteroids approaching Earth on Feb. 15
were unrelated is their disparate compositions. Telescopic spectral data
do not support any physical connection between asteroid 2012 DA14 and
Chelyabinsk meteorites. Nicholas Moskovitz and Richard Binzel (MIT)
report 2012 DA14 displays spectral colors which suggest a carbon
dominated composition similar to CO or CV carbonaceous chondrite
meteorites with abundant calcium- and aluminum-rich inclusions. On the
other hand, meteorite fragments being recovered from the fireball event
are reported as silicate-rich ordinary chondrites; a completely
different and unrelated class of meteorites. About 80% of all meteorite
falls are in the ordinary chondrite category.

Acknowledgements

Peter Brown, University of Western Ontario and William Cooke at the
Marshall Space Flight Center provided impactor details. Paul Chodas and
Steve Chesley (JPL) provided orbital computations and diagrams.
Ron Baalke (JPL) provided the custom interactive applet showing the
heliocentric orbits of both 2012 DA14 and the asteroid impacting the
atmosphere over Russia. Richard Binzel (MIT) provided information on the
nature of the atmospheric impactor and near-Earth asteroid 2012 DA14.
Received on Fri 01 Mar 2013 06:47:05 PM PST