Why are meteor craters so circular?

Is gravity so strong that a meteor swinging by the earth, moon or planet at
25,000 miles per hour on a tangent trajectory is pulled in for a direct
impact? Viewing pictures of planets I don't think I ever saw an oblong
meteor crater that would come from a glancing blow.

In article ,
"Gary Helfert" wrote:

Is gravity so strong that a meteor swinging by the earth, moon or planet at
25,000 miles per hour on a tangent trajectory is pulled in for a direct
impact? Viewing pictures of planets I don't think I ever saw an oblong
meteor crater that would come from a glancing blow.

There are no glancing blows. A meteor has a huge amount of kinetic
energy; when it hits something solid like a planet, this kinetic energy
is converted to heat. Converted *very quickly* to heat. In other
words, there is an explosion. It's the explosion that leaves a crater,
just as if you'd detonated a nuclear bomb.

So you won't find any oblong craters because explosions are always
round. You will, however, sometimes find chains of craters, where the
incoming meteor broke up a bit and caused several detonations along the
meteor's path.

Best,
- Joe

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Gary Helfert wrote:
Is gravity so strong that a meteor swinging by the earth, moon or planet at
25,000 miles per hour on a tangent trajectory is pulled in for a direct
impact? Viewing pictures of planets I don't think I ever saw an oblong
meteor crater that would come from a glancing blow.


Actually, what happens is that the impact speed is so high that the
impactor vaporizes. The explosion excavates the crater, which is many
times larger than the size of the impactor. Only at very shallow
impact angles does the crater become substantially asymmetrical.

Paul

In article , Gary Helfert
wrote:

Is gravity so strong that a meteor swinging by the earth, moon or planet at
25,000 miles per hour on a tangent trajectory is pulled in for a direct
impact? Viewing pictures of planets I don't think I ever saw an oblong
meteor crater that would come from a glancing blow.

It takes an extremely shallow angle of impact to make a crater that is
anything other than circular.

This is because the impact speed is so high. The higher the speed, the
greater the ratio of kinetic energy (which causes the cratering
explosion) to momentum (which would give the explosion a preferred
direction).

Imagine the difference between being hitting e.g. a vase with a
baseball bat or with a BB-gun BB with the same energy. In both cases
they shatter equally due to the energy, but the slow moving baseball
bat sends all the shards flying in the direction it was hit, while the
fast BB vase just collapses into fragments where it stood. A meteor is
like the BB only more so.

Another way to look at it is rock is not as strong in shear as in
compression, vaporized rock even more so. The oblique impact of a
meteor can press down hard to excavate a crater, but when it presses to
the side, the upper material it is pressing just slips across the
underlying material without affecting it much, more so if it is melted.
You get a debris fan from the surface material pointing along the
meteor's direction of travel, but the deeper rock feels only the down
blast when it is deciding how to crater.

--
David M. Palmer (formerly @clark.net, @ematic.com)

Joe Strout wrote:
So you won't find any oblong craters because explosions are always
round.

Incorrect:

http://www.boulder.swri.edu/~bottke/...s/oblique.html

Highly oblique impacts do sometimes generate elliptical craters.

Mike Miller