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 ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
