[meteorite-list] Nahkla's entry angle

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:13 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86901B4E2C4_at_US-Torrance.mail.saic.com>

Hi Ron and List,

One comment on the angle of atmospheric entry for Nakhla. You wrote:

> Another consideration for the Nakhla strewnfield is the angle
> of trajectory through the atmosphere. Nahkla had a very inclined
> path, just 30 degrees from the horizontal.

If you exclude the effect of earth's gravitional field for the
moment and consider only the scope of linear trajectories that
are possible, you may be surprised to learn that 30 degrees is
exactly the AVERAGE entry angle relative to the horizon. Half
of all possible trajectories are between 30 and 90 degrees, half
are between 0 and 30 degrees. So in that respect, Nahkla's
path was not unusual. Of course, the earth's gravity well
will bend the trajectories of approaching meteoroids toward the
earth's center, which will steepen the slope of entry. The
amount of gravitional bending of the trajectory depends on
the relative velocities between the earth and the meteoroid,
but this focusing effect will skew the statistics toward higher
average entry angles -- perhaps 40 degrees.

However, the probability that a meteoroid will survive atmospheric
passage is a function of that entry angle. The closer to zenith
that angle is, the worse the chances that meteorites will make it to
the ground. This tips the scales back in favor of shallower
trajectories when talking about meteorite-producing bolides.
So we come back to 30 degrees not being at all unusually shallow;
indeed it may be steeper than average.

I would be curious to know what the estimated angles were for
Peekskill, Tagish Lake and the recent Bavarian fall. I suspect
they are all 25 degrees or less.

Cheers,
Rob
Received on Tue 13 Aug 2002 02:00:21 PM PDT