[meteorite-list] Chelyabinsk trajectory modeling -- how well did we do? Part 1 of 2

From: Matson, Robert D. <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Wed, 16 Oct 2013 11:10:19 -0700
Message-ID: <7C640E28081AEE4B952F008D1E913F170859CB44_at_0461-its-exmb04.us.saic.com>

Hi Jim,

> What I would like to know, after they can confirm the weight of this
last
> recovery, is if path calculations proved accurate by those that were
doing
> the calculations????? Was anyone able to actually calculate the
trajectory
> accurately (different mass calculations) with the information that was
able
> to be gathered over the first month or so after the fall. Has anyone
> calculated any dark flight data based on any of the finds that have
been
> recovered?

The answers are yes, yes and yes. :-)

Esko Lyytinen was one of the first to generate a preliminary orbit and
track on
February 15th -- less than 24 hours after the event. Esko and I worked
with Bill
Gray to generate an ephemeris (just on the off-chance that the asteroid
had any
chance of having been serendipitously imaged in the days/hours prior to
impact),
which Bill posted at the following link:

http://projectpluto.com/temp/chelyab.htm

Unfortunately, the approach direction had a solar elongation of less
than
20 degrees, so ground-based telescopic observation was not possible.

One week later, I triangulated two space-based images of the meteor,
yielding
an estimate of the following terminal burst time/location:

ECF position (in km) at 15 Feb 2013 03:20:33 UT: 1787.29, 3237.68,
5207.62
ECF velocity vector (in km/sec) at same time: +12.8, -13.3, -2.4.

This vector's bearing is 279.8 degrees -- within only a few degrees of
what Esko
had computed a week earlier from ground-based videos.

With the vector in hand, the next task was to locate upper atmospheric
wind data
so that I could run my dark flight model, and I quickly found radiosonde
data
for the two nearest stations: Orenburg and Kurgan. Since Kurgan was
about half
the distance from the meteor as Orenburg, I weighted the Kurgan data
correspondingly higher to come up with my profile. I ran my
ablation/dark-flight
model assuming a typical chondritic density, generating preliminary
impact
points for masses of various sizes from 10 grams to 100 kg. Here's the
message
I sent to Bill, Esko and Marco Langbroek on 22 February (and a similar
message
to Peter Brown):

> Hi Esko/Bill/Marco,
>
> Using radiosonde upper atmospheric wind data from Kurgan launched a
> little over 3 hours prior to the fall, my dark flight code computes
> the following impact coordinates for a variety of post-ablation masses

> (assuming bulk density of 3.3 g/cm^3):
>
> 100 kg: 54.850 N, 60.448 E
> 10 kg: 54.821 N, 60.665 E
> 1 kg: 54.794 N, 60.844 E
> 100 g: 54.765 N, 60.983 E
> 30 g: 54.749 N, 61.041 E
> 10 g: 54.728 N, 61.122 E
>
> My heaviest mass does not quite reach Chebarkul Lake, but it is within

> the uncertainty of the terminal burst coordinates that I was provided.
> Overall, I think my impact coordinates may be a little south of the
> actual impact track, suggesting my wind speed estimate is a little on
> the high side.
>
> --Rob

Interestingly (in light of the mass just recovered from Lake Chebarkul),
Esko suggested I extend the mass range to cover 1000 kg, which I did.

The very next morning (Saturday, Feb. 23rd) a Central Bureau Electronic
Telegram was released for the trajectory and orbit of the Chelyabinsk
superbolide:

- - - - - -
                                               Electronic Telegram No.
3423
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director: Daniel W. E. Green; Hoffman Lab 209; Harvard University;
20 Oxford St.; Cambridge, MA 02138; U.S.A.
e-mail: cbatiau at eps.harvard.edu (alternate cbat at iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


TRAJECTORY AND ORBIT OF THE CHELYABINSK SUPERBOLIDE
    Jiri Borovicka, Pavel Spurny, and Lukas Shrbeny, Astronomical
Institute
of the Academy of Sciences, Ondrejov, Czech Republic, report that they
have
computed the atmospheric trajectory and velocity of the superbolide of
2013
Feb. 15.139 UT (3h20m UT), which caused some damage in the city of
Chelyabinsk, Russia. They used seven casual video records provisionally
calibrated with Google Maps tools. The trajectory was assumed to be
linear.
The geographical coordinates of selected points along the trajectory are
as
tabulated below:

Relative Longitude Latitude Height Velocity Notes
Time (s) (deg E) (deg N) (km) (km/s)

 0.00 64.266 54.508 91.83 17.5 beginning
of
 
registration
 9.18 61.913 54.788 41.02 17.5 minor
flare
11.20 61.455 54.836 31.73 17.5 major
flare
12.36 61.159 54.867 25.81 17.5 flare
13.20 60.920 54.891 21.05 12.5 minor
flare
16.20 60.606 54.922 14.94 4.3 end of
 
registration

The observed trajectory was 254 km long. The azimuth of the trajectory
was
279.5 degrees, and the slope was 16.5 degrees to the horizontal (for the
end
point). The uncertainty of the radiant is about one degree. The
uncertainty
of the position of the trajectory is about 1 km (at the beginning, up to
4
km).
    The pre-entry object that caused the superbolide was relatively
fragile.
Severe fragmentation started at a height of 32 km under dynamic pressure
of 4
MPa. The mass of the largest fragment, which landed in the lake
Chebarkul,
was estimated to be 200-500 kg. One or two meteorites of the mass of
several
tens of kg can be expected not far from the village Travniki. One piece
of
mass approximately 1 kg may have landed to the northwest of Shchapino.
Numerous small fragments can be expected in the wide band located about
5 km
south of the trajectory, mostly between longitudes 60.9 and 61.35
degrees.
    The blast wave, which strongly affected Chelyabinsk, was generated
between heights of 25 and 30 km. The radiant and heliocentric orbit
were
calculated to be as follows:

Apparent radiant: Right ascension 328.6 +/- 1.0 deg
(equinox 2000.0) Declination +8.0 +/- 1.0 deg
                          Velocity 17.5 +/- 0.5 km/s

Geocentric radiant: Right ascension 334.7 +/- 1.2 deg
                          Declination -1.0 +/- 1.4 deg
                          Velocity 13.2 +/- 0.7 km/s

Orbit: a = 1.55 +/- 0.07 AU e = 0.50 +/- 0.02
                       q = 0.768 +/- 0.011 AU Q = 2.33 +/- 0.14
AU
(equinox 2000.0) Peri. = 109.7 +/- 1.8 deg Node = 326.41 deg
                       i = 3.6 +/- 0.7 deg

The data do not allow determination of the initial mass of the object
prior
to entering the atmosphere. The trajectory will be further refined in
the
future, provided that proper in situ calibrations of the videos are
made.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
     superseded by text appearing later in the printed IAU Circulars.

                        (C) Copyright 2013 CBAT
2013 February 23 (CBET 3423) Daniel W. E.
Green

- - - - - -

This trajectory confirmed my suspicion that the true track was slightly
north of my Friday estimate, so I reran the dark flight model using the
new
CBAT-based trajectory. Midday Saturday, I sent this to Esko, Marco and
Marc
Fries (and a similar message to Peter Brown):

> Hi Esko/Marco,

> Here are the predicted wind-drifted impact coordinates using the
trajectory
> from the CBAT:

> Mass Lat Long
> ------- ------- -------
> 1000 kg 54.9559 60.1786
> 100 kg 54.9268 60.4247
> 10 kg 54.8975 60.6452
> 1 kg 54.8676 60.8369
> 100 g 54.8359 60.9954
> 30 g 54.8186 61.0637
> 10 g 54.8013 61.1201

> The heaviest masses look like a very nice match to Lake Chebarkul. ...

- - - - -

A line connecting the 100-kg and 1000-kg coordinates above intersects
the
southern half of Lake Chebarkul -- meaning that a multi-hundred kilo
mass
would be required to "hit the bull's eye". Esko addressed this point
specifically:

"Ok Rob, this would mean a hit to the lake with a weight of about 300 kg
quite well consistent with the CBAT value - range. This is also quite
well
consistent to our value of around 400 to 800 kg."

To be continued in part 2, hopefully later today... --Rob
Received on Wed 16 Oct 2013 02:10:19 PM PDT