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meteorites hitting vehicles

In reply to recent email about whether planes might get struck by a
meteorite:

I'm sure some of you will improve on my very crude calculations, but
here's a start.  The following is my off-hand personal estimate, and
assumes a random meteorite flux over the entire surface of the Earth
(which is not strictly justified).  I visited an NBAA web site and got
the following statistics for business aircraft:
   
   16,922 turbine-powered aircraft
      422 average hours flown annually
     15.1 yrs average life
      9.2 percent of general aviation traffic into top 20 hubs

I didn't track down average area projected toward the sky by a typical
airplane, but let's start with an assumption that it is 50 m^2 for
typical smaller aviation aircraft; e.g., business planes.  Larger
passenger aircraft could be measured in hundreds of square meters.

The Earth has an average radius R of 6378 km.  The formula for a sphere
is 4 pi R^2, so there is about 5 x 10^14 m^2 area for aircraft to fly
through.  In a year there is 365 days x 24 hrs/day = 8760 hrs.  Thus,
the fraction of a year that any one plane is in flight is 422/8760 =
.05, so over the course of a year it is as if 16,922 x .05 = 846 such
planes are in the sky at all times, projecting a surface area of 846 x
50 = 42,000 m^2.  The fraction of the total area that could be flown is
then 42,000/5x10^14 = 8.4 x 10^-11.  If we assume there are 10,000
meteors (greater than a tens of grams) per year penetrating to flight
altitudes, then the random chance of a meteorite intercepting a general
aviation plan in flight appears to be approximately one in a million
over a year (about one in 60,000 over the  typical "fleet" life).  The
number of meteorites capable of punching holes may be several times
larger, since even a few grams (if it reaches down to the plane's
altitude) could penetrate something depending on residual speed.  Thus,
in a year there may be one chance in 100,000 per year or so of a hole in
one general aviation turbine plane somewhere.  If we factor in that
these turbine planes are only a part of the traffic, then the chance may
be more like a range of one in ten thousand to one in one hundred
thousand per year of a small hole in some plane somewhere, which in the
lower limit could exceed one in a thousand over a typical lifetime of a
"fleet" versus one in ten thousand for the upper bound.  Calculations
that include larger area aircraft might adjust this estimate upward.
However, it appears doubtful that you would see a number of holes from
various aircraft that were attributable to meteorites.  The chances of a
hit on one aircraft at some time over the entire history of flight
appears very small but not entirely negligible.

Assuming the above reasoning gives us the right order of magnitude, lets
think about automobiles.  There may be more like a billion m^2 at any
time presented by cars, trucks etc. compared to a few hundred thousand
m^2 for aircraft in flight, so crudely the estimate suggests that the
liklihood of a car being hit is several thousand times greater. (The
ocean is irrelevant to this model, since placing the cars all on land
merely increases the cross section on land and decreases the cross
section at sea, but the total is invariant for a random meteorite
strike.) Thus, over a few decades instead of getting the extremely low
chances we expect for aircraft, we may see one to a few automobiles get
struck. It still seems slightly incredible that something exceeding ten
kilograms struck a car in our lifetime, but these are random events.
Follow-Ups: