[meteorite-list] Fall Dates, Orbits, and the twins of Innisfree

From: Herbert Raab <herbert.raab_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:30 2004
Message-ID: <3CCBD068.58C4E578_at_ris.at>

Hello List,

regarding the recent discussion about fall dates, orbits, and meteorite streams,
I wanted to note a few things.

First, it is important to recognize that an orbit is defined by it's size (usually
this is specified by the semimajor axis of the orbit), by it's shape (described
by the numerical excentricity), and it's orientation in space (described by the
angles: the inclination to the ecliptic plane, the longitude of perihelion, and
the longitude of the ascending node).

As the meteorid and the Earth meet either in the ascending or descending node
(i.e., where the meteorid crosses the orbital plane of the Earth), the longitude
of the node is determined very precisely by the date and time (in fact, by the
longitude of the Earth on it's orbit around the sun) of the impact. But it's
important to note that this is only one of five orbital elements. All other
elements could still have any possible value.

On the orther hand, the longitude of the node (and thus, the date of impact)
can vary quite a lot even for very similar orbits. Just think of the well
known Perseid meteor shower, which peaks around August 11/12 each year.
Perseids are opbserved at least 10 days before and after the peak.

Finally, I want to draw your attention to the paper "Evidence for the existence
of groups of meteorite-producing asteroidal fragemnts" by Ian Halliday et al.,
published in "Meteoritics", Volume 24, No.2 (June 1990), pp 93. They write:
"One unexpected discovery in the MORP data was the Ridgedale, Saskatchewan event
of 1980 February 6, that is believed to have survived as a kg-sized meteorite.
It appeared exactly 3 years after the fall of the Innisfree meteorite with
similar astmospheric entry parameters and the two meteorids shared almost
precisely the sane orbit." Sounds familiar, doesn't it?

Halliday et al. linked a further three meteorite dropping fireballs to the
Innisfree group (including on that appeared on 1977 02 17, that is, only
11 days after Innisfree, dropping a ~6kg meteroite that was, unfortunately,
not found.) The fall dates of the Innisfree group, by the way, vary between
January 31 and February 17, and still the orbits of all five members are
nearly identical.

Halliday et al. found three more groups with three of four members, each
of which dropped a meteorite of at least 0.25kg (though none of these has
been found), but there is no group related to Pribram in their data. They
conclude: "If 16 objects from a population of 89 appear to belong to related
groups, then such groups account for a significant fraction of all meteorites
falling at at particular time."

  Best greetings,
   Herbert Raab
Received on Sun 28 Apr 2002 06:35:20 AM PDT