On Tue, 15 Jun 2004 14:20:55 -0500, "Tim Williams" wrote:
wrote in message
.. .
and remember- velocity is relative. Earth is moving, as is the chunk
of space rock. if both are moving at about the same speed in about the
same direction it will have an impact speed about what gravity gives
it as it falls... still enough to bust through your roof.

Even so, you have the gravitational potential energy (wow that's an
incredibly long couple of words for something so simple) of falling to Earth
a few thousand miles... that's a generous acceleration and will have it
streaking hypersonically through the atmosphere anyway.

Neglecting air friction (which you can't) an object falling from rest at
infinity will acquire a final velocity of 7 miles a second at impact with
the Earth's surface (same velocity as escape velocity for something
going the other way).

Relative orbital motions can boost that into the 20 to 40 miles per second
range, or slow it to only a few miles per second, depending on the exact
geometry. But even an object at relative rest at 200 miles above the surface
would acquire a final velocity of around 5 miles a second by the time it
reached the surface.

However, you can't ignore air friction. An object with a fairly poor drag
coefficient will be slowed greatly by atmospheric friction. Most relatively
small meteors are moving relatively slowly by the time they get deep into
the atmosphere, often subsonic.

They also often break apart as they plow deep into the atmosphere.
That's why you can often pick pieces of one up and find them *cold*
rather than hot (these pieces came from the interior of a larger meteor
which broke up after spending most of its energy against the atmosphere).

I'd note that the large amount of relatively intact debris which reached
the ground from the Shuttle Columbia is another example of this.

Gary