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METEOR SHOWERS FROM BROKEN COMETS
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Short Description: in all likelihood are unrelated to the Taurid shower. In 1983, Fred Whipple recognized that minor .... past fragmentation history. The comet does appear ...
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METEOR SHOWERS FROM BROKEN COMETS
Peter Jenniskens
(1)
(1)
The SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, U.S.A., Email: pjenniskens@mail.arc.nasa.gov
ABSTRACT Even though comets have been observed to break, there was no strong evidence until now that the meteoroids generated in these discrete and relatively rare events accounted for meteoroid streams detected as meteor showers on Earth. That has changed now minor planet 2003 EH1 and other such inactive (dormant) and weakly active comet nuclei are found still intimately associated with known meteoroid streams. More and more of such minor bodies are now identified. The meteor showers that are likely from the fragmentation of comets rather than from Whipple-type ejection by water vapor drag now include the Quadrantids, Daytime Arietids, delta-Aquariids, Capricornids, Taurids, Andromedids, Phoenicids, and Geminids, representing most of our strongest annual showers. This makes the breakup of dormant comet nuclei an important mechanism to replenish the zodiacal cloud. 1. ASSOCIATION OF MINOR BODIES WITH METEOROID STREAMS
In 1983, Fred Whipple recognized that minor planet 3200 Phaethon orbits among the very short-period (P ~ 1.59 yr.) and unusually small perihelion distance (q = 0.14 AU) meteoroid stream responsible for the Geminid shower [3]. Due to the uncommon orbit, the probability of this good an association by chance is only about 1 in 2 million, depending on the actual number of objects in this still sparsely sampled population. However, the reflectance properties of the minor planet (taxonomic type B) made the nature of this object as an extinct comet nucleus uncertain. The small perihelion distance heated the surface to above 700 K and sintered the Geminid meteoroids enough to change their morphology. It has since been shown that the Geminids appear to have been created close to perihelion, more typical of comet ejection than asteroidal collisions [4].
When Whipple [1] discovered a mechanism to accelerate meteoroids by the drag of water vapor in 1951, the old idea of meteor showers originating from comet breakup went into remission. Meteoroid streams were thought created by the gradual ejection of meteoroids during the comet's sublimation of ice. Our meteor showers tell a different story. Until recently, most showers had no known active parent comets, even those that have short orbital periods and frequently should have their parent body in the Earth's neighborhood. It was surmised, that those parent bodies had now evolved into orbits quite different from the meteoroids that we now see on Earth as a meteor shower. Traditionally, minor bodies have been associated with meteoroid streams using the D-criterion, a measure of the similarity of orbits based on the integrals of motion. Especially when the orbit of the stream was similar to that of asteroids, many potential parents were thus identified in orbits different from that of the meteoroids. The best example being the Taurid Complex of minor bodies associated with comet 2P/Encke [2]. All of the originally proposed objects investigated since have asteroidal colors and, therefore, in all likelihood are unrelated to the Taurid shower.
Fig. 1: 2003 EH1 and the Quadrantid meteoroid stream, in a model by Jerémié Vaubaillon. For long, this object was our "Pluto" in the context of the long-undiscovered Kuiper Belt. Then, in 2003, I identified a minor planet 2003 EH1 in the highinclination orbit of the Quadrantids [5]. Due to the rare high inclination of the orbit (i ~ 72º), the probability of this good a match by chance alone was again only about 1 in 2 million. This time, the cause of the stream must have been a breakup. The Quadrantids are a massive stream (Fig. 1), containing a thousand times more mass than typically ejected by an active Jupiter-family comet over one orbit. The minor planet passed outside of Earth orbit, but the stream evolved rapidly due to
Proc. `Dust in Planetary Systems', Kauai, Hawaii, USA. 26 -- 30 September 2005 (ESA SP-643, January 2007)
