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[log in to unmask] writes:
>I have two follow-up questions that, admittedly, I ask out of my own
>curiosity and for my own professional development. If an object of this
>size were to have a trajectory that defined a collision course with the
>Earth: 1) Would it actually reach the surface of the Earth (or would it
>burn as it passed through the atmosphere), and 2) What kind of an impact
>would it have?


As promised, below is additional information regarding Bill's questions
above.

The traditional rule of thumb is that stony objects smaller than about
100m in diameter are totally disrupted in the Earth's atmosphere, and do
not create significant impact craters. This size cutoff, though, depends
on impactor composition and strength.  If an asteroid broke apart, the
devastation from the impact of the fragments would be relatively localized
- objects in the direct path of the fragments would be hit, but other
places in the immediate vicinity would not be affected by the impacts
themselves.  A slightly wider area would be affected by the sonic boom
from the incoming object & fragments.  This would be a local impact.

What happens, though if the object is made of iron?  Iron objects would
stay together and would impact as one solid object.  These would be more
devastating, indeed, due to the force of the impact and the ejecta that
would be thrown out from the impact site.  There are calculations that can
be done to estimate the size of the crater formed.  It depends on the
composition of the impactor, the composition of the impacted area, the
angle of attack, & the speed of the object, mostly.  There is a website
done by the world's foremost expert in craters & the formation of craters.
 The website is:

http://www.lpl.arizona.edu/tekton/crater.html

The first listed name at the top of the page is the expert I spoke of, H.
Jay Melosh.  One thing to remember regarding the crater sizes presented
here, is that the code does NOT take into account the deceleration or
fragmentation of the impactor due to the atmosphere.  You can also do
calculations on this website to estimate crater sizes on different planets.

We did a quick calculation using the website & found that, if the object
that is passing Earth in about 2 hours actually hit the Earth & stayed
together as one chunk (unlikely, as mostly iron asteroids are somewhat
rare), it would form a crater from about 200 meters to about 2 kilometers
in diameter.  We assumed an iron object 50 meters in size, Earth gravity,
a speed of about 17 km/second, an impact angle of 45 degrees, and a  dense
rock target.  AT WORST, the force of the impact would be up to about 2
megatons in size (Hiroshima atomic blast was about 15 kilotons).  To give
some perspective: we had about a day's notice that this object was passing
by.  If it was going to impact, say, Chicago, and was 50 meters in
diameter, we'd have to evacuate not only Chicago but the entire
metropolitan area (7 million people+) in about a day in order to avoid the
object, the crater, and the subsequent ejecta.

If you are dealing with the public, please don't quote my numbers above as
gospel truth.  Keep in mind that this object was probably rocky and would
have broken apart had it entered the Earth's atmosphere.  What I listed
above is worst-case, in case you get questions from the media or the
public and they insist on asking that question.  One other thing is that
it is rare that objects hit populated areas - most of the Earth is covered
by water!

Some recent work suggests that the Earth's atmosphere is even more
effective at disrupting stony objects than previously thought (Bland &
Artemieva, Nature 7/2003, Vol 424, Issue 6946, ppp 288-291).

This is, I'm sure, more than you ever wanted to know about this subject.
Hope you enjoyed it!

Michelle

Michelle Nichols, Senior Educator
Adler Planetarium & Astronomy Museum
1300 S. Lake Shore Dr.
Chicago, IL  60605
312-322-0520
312-322-2257 (fax)
[log in to unmask]
http://www.adlerplanetarium.org

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