[meteorite-list] Trajectory for Earth-grazing UK bolide of 9/21/2012

From: Matson, Robert D. <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Tue, 25 Sep 2012 15:54:00 -0700
Message-ID: <7C640E28081AEE4B952F008D1E913F17061FCBE3_at_0461-its-exmb04.us.saic.com>

Hi All,

Been a little slow to report this to the Meteorite List -- got a little
side-tracked with comet C/2012 S1, and was waiting to do a little more
analysis to confirm the solution.

Let me start with the analysis result, since it's pretty exciting:
the UK bolide of 21 September 2012 was an earth-grazer: it's pre-
earth-encounter trajectory did NOT intersect the earth! It came
very close -- a minimum altitude of about 57 km over western Ireland.
Coincidentally, this is the same minimum altitude that was
achieved by the Grand Teton Daytime Fireball of 1972, although
that encounter lacked the significant fragmentation seen last
Friday.) Thanks to that fragmentation coupled with the low altitude,
some meteorites may have actually made it to the ground (or more
likely the ocean). But a significant fraction of the original
meteoroid went right back into space. Depending on the velocity
(which I would need a good video to estimate), the original
asteroid's orbit may have been sufficiently aerobraked to have
been captured by earth's gravity. If so, then the remaining fragments
would have reentered for good one orbit later in the middle of
the North Atlantic.

I know this is a bit of bad news as far as meteorite recovery, but
it's nevertheless an important result since it is one of the
extremely rare instances of an earth-grazing asteroid being not
only witnessed by hundreds if not thousands of people, but also
imaged by multiple cameras, both still and video.

Three key images showing excellent star background references were
what allowed me to compute the trajectory:

1. Damien Stenson's beautiful image taken just south of O'Brien's
Tower on the Cliffs of Moher in County Clare, Ireland. At least
four bright fragments pass through the bowl of the Big Dipper,
behind the central tower and then disappear behind clouds low
in the west-northwest.

<http://www.worldirish.com/story/12297-photographer-captures-stunning-image-of-fireball-fragments-at-cliffs-of-moher>

2. Craig Usher's shot from Greenock, Scotland, facing southwest
shows the tracks of at least five individual fragments. Four of
these appear to be the same ones captured by Damien Stenson.

<http://www.bbc.co.uk/news/uk-19683687>

3. Truls Gabrielsen's time-lapse photography from Skjern?ya,
Mandal, Norge (southern Norway) includes two frames showing the
bolide track very low in the southwest sky:

<http://www.aftenposten.no/nyheter/iriks/--Meteoritten-lyste-intenst-og-knallgront-6999353.html>

Since the first two images show the meteor tracks from closer range
than the third (and thus at higher elevation angles), these were my
main sources for astrometric measurements. When I triangulate these
two, the point of closest approach to the earth was near 54.18 N,
8.25 W, altitude 57 km -- a sub-meteoroid point about 10 miles
southeast of Sligo, Ireland. The bearing at that point was toward
azimuth 263 (7 degrees south of west). When the meteoroid was over
England, the local bearing was nearly due west.

As a cross-check, I triangulated using images 1 and 3. The solution
there is nearly parallel to the first solution, just shifted
slightly to the south. (Due to the very low elevation angles from
Norway, a very small change in the measured angles leads to a
significant shift in the north-south position of the track).

Some towns underneath my computed trajectory in order from east
to west are:

In England (UK):

Whitby, Danby, Stokesley, Crathorne, Dalton-on-Tees, Melsonby,
Kirby Steven, Tebay, Ambleside, Loughrigg and Gosforth.

Isle of Man: Cranstal and The Lhen.

In Northern Ireland (UK):

Strangford, Saul, Downpatrick, Loughinisland, Katesbridge,
Loughbrickland, Acton, Poyntzpass, Markethill, Lisnadill

In Ireland: Silverstream, Willowbridge, Monaghan, Ballinode

Back into Northern Ireland: Lisnaskea, Kinawley

Back into Ireland: Coppanaghbane, Corrard, Dowra, Drumkeeran,
Collooney, Coolaney, Ballina, Crossmolina, Owenglass, Lagduff
More, and finally Doona.


I'm very much indebted to David Entwistle of the British and Irish
Meteorite Society for contacting me with links to these three images.
I would also like to thank list member Martin Goff for posting his
early report to the list last Friday, and George Herbert at MPML
for forwarding David Jordan's Seesat-L post in which he reported
seeing the fragmenting fireball (which at the time he thought
might be a satellite reentry) from south Dublin, Ireland.

Best wishes,
Rob
Received on Tue 25 Sep 2012 06:54:00 PM PDT