It's nice and passes the smell test, and I now realize we are looking for a threshold size limit for survival. I would support your ballpark figure and combining some of the ideas I mentioned in the prior comment on the subject, add that the slowing frictional force taking the initial velocity down to the impact velocity must be integrated over distance...it is really a work question in an exponentially increasing pressure situation, so I wouldn't put much faith in the 0.004 multiple linear assumption you did, already shaky based on limited Earth limiting size data points...and I see you acknowledge this by varied it by 1000% (to 10 cm), so I guess that's directionally a good guess as any can be. Also add that a 45 degree angle of incidence vs. vertical will add about 41% (sqrt(2)) more frictional damping, so angle of incidence is quite important to consider important, will help reach the thresholds you discuss. And I think the potential terminal velocity will be limited proportional to the sqrt(mass), as mentioned previously, so stone will be 2-3 times iron upper size limit for similar shapes.
Now does that sound right to you? It's quite late here...
Saludos,
Doug Dawn
Mexico
En un mensaje con fecha 01/08/2004 4:10:31 AM Mexico Standard Time, [EMAIL PROTECTED] escribe:
Dear List,
I was pondering what Ron had to say about hypersonic
impacts and other comments.
From the Wilemette, Alnighito and Hoba meteorites,
it's safe to say the largest non-hypervelocity
impactors on Earth are about ~10 meters, as an order
of magnitude.
To avoid hypervelocity impact, the object must be
slowed by some dynamic pressure, which is proportional
to the density of the atmosphere x velocity squared.
The entry velocity is escape velocity for the planet;
For no hypervelocity impact the final velocity must be
less than the speed of sound in rock, which, in
comparison to the planet escape velocity, and for the
purposes of this crude proportionality calculation, is
close to zero.
The escape velocity squared is proportional to g
for the planet. This is 0.4 for Mars, approximately,
compared to Earth.
The desnity of the Mars atmosphere is about a
hundredth of Earth's, i.e., .01.
So the max dynamic pressure available on Mars is
about 0.4 x 0.01 = ` .004 compared to Earth. So a
MASS only .004 of the largest meteorites on Earth
could be brought below hypervelocity on Mars.
Since mass is proportional to radius cubed, the
largest meteorites on Mars to survive hypervelocity
impacts are therefore in the order of about 1 meter in
size. Since that is an approximate upper limit, we
would expect to find centimeter-size to 10 cm. size
meteorites in the Gusev strewnfield.
Is my thinking right on this? I admit I made a
great many handwaving assumptions and used a very tiny
envelope to write on the back of. Am I in the ball
park?
Francis Graham
