[meteorite-list] Carancas meteorite

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sat, 6 Oct 2007 13:44:04 -0700
Message-ID: <GOEDJOCBMMEHLEFDHGMMAEPIDFAA.mojave_meteorites_at_cox.net>

Hi All,

I've been following the Carnacas story from the beginning (mostly on
the Minor Planet Mailing List and through Babelfish translations of
news stories and scientific initial reports), but couldn't resist
paying a visit to the Meteorite-List archives to see what kind of
excitement had transpired here. I'm happy to see Sterling and
Mexico Doug have been engaged in theoretical vollies on the survival
odds for the body that produced the impact pit. I haven't modeled
it yet (since there is little dynamical information to go on), so
I can't say with any certainty whether a significant (greater than
1 metric ton) mass could have avoided pulverization. Given the
pre-impact size (probably greater than 1 cubic meter), composition
(non-iron), and comparatively high altitude of the impact site, we
really have no past analogs that we can use to predict expected
outcomes.

But the size of the impact pit and the chondritic composition do
place a reasonably narrow restriction on the size of the body (e.g.
larger than a 32" TV but smaller than a VW bug). One question for
any bolide is whether there is an initial exoatmospheric velocity
and angle of attack that permits sufficiently graceful deceleration
and ablation for a big chunk of it to survive to the ground. If the
answer is "yes", then the second question is whether it can survive
largely intact after impacting the ground. My intuition suggests
that since the first question involves greater forces than the
second, if the meteoroid can avoid atmospheric vaporization or
pulverization, it should certainly survive hitting the ground. For
meteoroids in this size range, maximum dynamic pressure should have
occurred miles above the ground -- even the high altitude ground
of the Peruvian-Bolivian border.

That the pit was created at all pretty much answers the first question,
since a shower of small meteorites could hardly have done the job.
Now, let's take a look at the possibilities for the initial velocity.

According to the INGEMMET initial report, the time of the fall was
11:45 local, though the report *also* says that the "World Time" of
the fall was 19:45, which is not consistent with Peru's UTC-5hr
timezone. Since I find it less likely that scientists screwed up
their local time than made a mistake converting it to GMT, I'll
assume the time was 16:45 UT. At this time, the sun was just past
the meridian in Carancas so astronomically speaking it was in the
"afternoon". This is good for meteorite survival chances.

The slowest possible meteoroid at this time of year and time of day
would have approached Carancas from the east-southeast. The best
information so far suggests that this meteoroid travelled from SSW
to NNE, indicating that the meteoroid was on an ascending node
crossing of the ecliptic and had a low to moderate encounter
velocity. If Marco Langbroek is still a list member, he has a tool
which can probably be used iteratively to find the range of
reasonable earth-encounter velocities for meteoroids with aphelions
in the main belt. The radiant was probably somewhere in the
constellation of Centaurus, Lupus or Circinus, unless the entry
angle was very steep (Hydra) or very shallow (Norma, Triangulum
Australe, Ara or Pavo).

--Rob
Received on Sat 06 Oct 2007 04:44:04 PM PDT