[meteorite-list] Meteoroid entry angle

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Sat, 18 Sep 2010 20:47:22 -0400
Message-ID: <AANLkTi=GgNOh2ewW874qTwSL=c0+nmLZS0902DaRATrR_at_mail.gmail.com>

Hi Rob,

Thanks for the easily-digested explanation of entry angles. That
gives me a great picture in my head of the angles these objects
current enter our atmosphere (and progress through).

Best regards,

MikeG

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Mike Gilmer - Galactic Stone & Ironworks Meteorites

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On 9/18/10, Rob Matson <mojave_meteorites at cox.net> wrote:
> Hi All,
>
> Playing message catch-up after 10 days' vacation/biz trip in Maui.
> Saw John's message below, and thought I'd make a small correction
> to a common misconception about meteoroid entry angles.
>
> "Gilbert and Barringer both realized that 45 degree impacts are
> the most probable trajectories for meteoritic material."
>
> I find many people make this mistake in solving all kinds of
> problems that depend on spherical geometry. In *two* dimensions,
> if the range of possible angles is from 0 to 90 degrees, and
> the angle is uniformly random, then the mean angle is of course
> 45 degrees: the midpoint of the uniform distribution. In three
> dimensions, this is no longer true.
>
> If you look at a globe, it is easy to see why. Compare the
> surface area of the earth from the equator up to 45 degrees
> north latitude with the surface area from 45 N up to the North
> Pole. The latter is a much smaller area. In fact, for a sphere,
> the surface area from the equator up to *30* degrees north
> latitude is the same as the area from 30N to 90N. Similarly,
> the average entry angle for a meteoroid coming from a uniformly
> random direction is 30 degrees from horizontal.
>
> That said, the entry angle distribution is not quite uniformly
> random because earth's gravity bends all trajectories slightly
> toward the center of the earth. This has the effect of making
> all entry angles a little bit steeper than they would be if
> gravity didn't play a part.
>
> Bottom line is that the average entry angle for a meteoroid is
> a little bit more than 30 degrees, but definitely not as much
> as 45 degrees. To answer Eric's question below about what's
> considered shallow vs. steep, I consider any entry angle less
> than 20 deg to be "shallow", anything greater than 40 deg to
> be "steep", and anything from 20-40 (which is about 30% of all
> meteoroids) to be average.
>
> --Rob
>
> -----Original Message-----
> From: meteorite-list-bounces at meteoritecentral.com
> [mailto:meteorite-list-bounces at meteoritecentral.com]On Behalf Of Kashuba
> Sent: Friday, September 10, 2010 8:06 AM
> To: bernd.pauli at paulinet.de; Meteorite-list at meteoritecentral.com
> Subject: Re: [meteorite-list] Meteor Crater Shape and Entry Angle
>
>
> Eric, Bernd, Sterling, List,
>
> David Kring of LPL put together a great guidebook for the 2007 MetSoc tour
> of the crater (150 pages). He is Gene Shoemakers successor as advisor to
> the Barringer family. He and family members lead the tour. Carolyn
> Shoemaker was there too.
>
> Chapter 9. "Trajectory" begins and ends thusly:
>
> The trajectory of the impacting asteroid is another issue of considerable
> debate and still unresolved.
> Historically, circular plan views of impact craters confounded many
> investigators who assumed a circular
> crater requires a vertical impact. They wondered why more craters are not
> elliptical. Gilbert and
> Barringer both realized that 45 degree impacts are the most probable
> trajectories for meteoritic material.
> Yet Gilbert, like many of his contemporaries, mistakenly thought a 45 degree
> impact produces an oval
> crater (Hoyt, 1987). Barringer, on the other hand, realized that a 45 degree
> impact will produce a round
> crater (Hoyt, 1987). Despite this insight, Barringer, like Gilbert,
> initially assumed that the northern
> Arizona impact had been vertical or nearly vertical and that the asteroid
> was buried beneath the center of
> the crater floor.
>
> When extensive drilling did not locate a main mass beneath the crater floor
> and instead only
> produced traces of the projectile, Barringer began to consider other
> options. He had already noted several
> features that seem to have a directional symmetry.
>
> - snip -
>
> More recently, techniques similar to those of Sutton were applied by
> Holliday et al. (2005) to the
> Odessa impact site. They estimated the Odessa craters were produced
> approximately 63,000 years ago.
> Although the ages of Barringer and Odessa craters are still not precisely
> known, these approximate ages
> suggest Odessa formed earlier, with the caveat that the Barringer crater may
> be older than 49,000 yrs.
> (See discussion in Chapter 11). Thus, the two impact events may not be
> directly related and may not have
> any bearing on the issue of trajectory.
>
> Nonetheless, several other potential indicators of trajectory survive (and
> even the Odessa connection
> might be revived). Unfortunately, these indicators cannot be reconciled at
> the present time and I think it
> fair to conclude that the trajectory of the impacting asteroid that produced
> Barringer Crater remains
> uncertain.
>
> Chapter 9:
> http://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/chapte
> r_9.pdf
>
> Whole "guidebook":
> http://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/index.
> shtml
>
>
> Regards,
>
> - John
>
> Ontario, California
>
>
> -----Original Message-----
> From: meteorite-list-bounces at meteoritecentral.com
> [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of
> bernd.pauli at paulinet.de
> Sent: Friday, September 10, 2010 3:26 AM
> To: Meteorite-list at meteoritecentral.com
> Subject: [meteorite-list] Meteor Crater Shape and Entry Angle
>
> Eric wrote:
>
> "The crater is not perfectly round as would be expected from an impactor
> coming in at a sharper angle. In fact the crater is more elliptical in
> shape."
>
> SHOEMAKER E.M. and KIEFFER S.W. (1974, 1979) Guidebook to the
> Geology of Meteor Crater, Arizona (Publ. No. 17, Center for Meteorite
> Studies, Arizona State University, Tempe, Arizona):
>
> "Regional jointing has controlled the shape of the crater, which is somewhat
> squarish in outline; the diagonals of the "square" coincide with the trend
> of the
> two main sets of joints. The largest tears occur in the "corners" of the
> crater."
>
> Eric also inquired:
>
> "What would a "relatively low" impact angle be? 10 degrees, 20 degrees?"
>
> I tried to find more precise information on that but was unable to find
> something
> that might be of help here. Maybe someone else can shed more light on this!
>
> Regards,
>
> Bernd
>
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--

Received on Sat 18 Sep 2010 08:47:22 PM PDT