Expert: Tom Whiting - 10/5/2006

Question
Has it ever been worked out what angle of impact the Meteor that made Crater Tycho on the Moon? Or is this not possible? If it is possible how close to the Earth did this meteor pass?

Answer
Hi Dei,
I had no idea, previous to your question, that anyone had
any interest in "impact angle" of lunar craters.  But I guess
that is a specific study of some people.
After Googling it, I found a site that you might be interested
in, if this is your specific line of study....it's in a PDF file of
over 5 megabyes, so I couldn't download the entire study,
(I still run an AMDK6 at 233 mega-somethings...very slow).
But I'll pass it on to you....cut and pasted...
http://www.springerlink.com/content/xtk122p2348661nw/#search=%22tycho%20crater%2...

I did notice that most of the work in this field only states impact angles as either low, medium, or high angles.
And judging by Tycho's very circular appearance, it would
appear that on the surface (no pun intended ;-)... the parent
asteroid came in at a very high angle of impact, if not
perfectly vertical....a near-direct head on collison.
(A low angle of attack pre-supposes a very oblong formed crater, whereas a high angle of attack means a near-circular
crater with a central peak forms (due to land rebound) which
Tycho has.  I think that is the best one can do as far as the
impact angle.

As far as the Earth's involvement, I think we would have to know the Earth's exact position relative to the moon at the EXACT time of impact, therefore we would have to know an exact date and time of the impact itself.  This is virtually impossible to know....it's hard enough that we just know that Tycho is only 100 million years old, one of the freshest impacts on the moon....but the exact date and hour is unknowable, therefore there is no way to know the Earth's position relative to the moon, or the exact date of collision.....so you are correct in assuming that that
information (How close was the Earth?) is just unknowable.

Of course, if something like that happened today, we could
be right on top of it.  There is one asteroid we are keeping
close track of, as it appears to be on a possible collision course with the earth on April 13, 2036.....check out newly
discovered asteroid 99942 Apophis.

Hope all this helps,
Clear Skies,
Tom Whiting, Pres...ECMOG
Erie PA



FOLLOW UP:
Oh, one thing that we can infer about Tycho, it's location, located at about 43 degrees south latitude on the surface of the moon, and assuming a near-head-on collison...the parent asteroid would have been in that kind of orbit, tilted some 45
degrees to the ecliptic before impact, crossing the ecliptic
from south to north at the lunar distance at the time of impact.  Thus the earth was probably in no danger of impact, since the original orbit of the parent asteroid crossed the ecliptic at the lunar distance of...at that time.....100 million years ago...about 200,000 miles distant (as compared to today's value of average 238,000 miles to the moon).   So as far as the Earth is concerned, it (the parent asteroid of Tycho) would have been one of many.....near misses.

EXTRA...Realize that the Earth is a very small, fast moving
target....we move our own diameter (about 8000 miles)
every 7.2 minutes as we revolve around the sun at 18.5
miles per second.   (8000 divided by 18.5 = 432 seconds).
So any asteroid body taking a pot-shot at the earth would not
only have to be exactly on target azimuth-wise
(otherwise it would go either over, or under...the Earth,) but
IT would also have to be ON TIME during that 7.2 minute
period, otherwise it would pass in front of, or behind the
Earth).  That's why large hits on the Earth's surface are extremely rare, about once every 50-100 million years, or so.