Central Bureau for Astronomical Telegrams
IAUC 3647: 1979 XI
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Circular No. 3647
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
TWX 710-320-6842 ASTROGRAM CAM Telephone 617-864-5758
COMET HOWARD-KOOMEN-MICHELS (1979 XI)
Further to IAUC 3640, Z. Sekanina, Jet Propulsion Laboratory,
California Institute of Technology, reports that he has been able
to derive orbits for this object that have their perihelion directions
precisely coincident with those of comets of the Kreutz sungrazing
group. There seems little doubt that the comet was a member
of the group and that it collided with the sun. The following
representative elements are based on the assumption that the perihelion
direction coincides with that of comets 1882 II and 1965 VIII:
T = 1979 Aug. 30.919 ET Peri. = 72.065
Node = 350.103 1950.0
q = 0.00164 AU (= 0.35 Rs) Incl. = 142.676
A very similar result is obtained by forcing the perihelion direction
of comet 1963 V. Six of the observations are satisfied with a
mean residual of +/- 0'.8 (= 0.7 pixel size); residuals of 3' remain in
R.A. on Aug. 30.802 and 30.885 UT, on the latter occasion in the sense
that the comet was already partly hidden by the coronagraph's occulting
disk. Sekanina adds: "Preliminary study of the dust tail on
the images obtained between 10.79 Aug. 31.0 and 31.5 UT (Michels et
al., submitted to Science) shows the predominance of particles subjected
to radiation-pressure accelerations exceeding up to 2 to 2.5
times solar gravity. The only exception is the tail's sharp boundary
to the west-southwest of the sun, where most of the particles
must have been ejected at heliocentric distances r > 10 Rs and be
still approaching perihelion. The extent of the tail in the
north-western sector is determined by the peak radiation pressure on
particles emitted between r ~ 2 and 10 Rs whereas the tail boundary in
the northeastern sector depends on the degree of particle evaporation.
Tentative evidence suggests that a significant fraction of
particles with perihelion distances down to ~ 1.4 Rs survived,
indicating a material more refractory than iron. A noticeable
brightening of the tail during Aug. 31.0-31.5 confirms that the comet must
have approached the sun from the near side and thus in a retrograde
orbit. The particulate matter, moving in hyperbolic trajectories
convex to the sun, was then deflected in the general direction of
the earth, producing a strong forward-scattering effect. If the
comet had a direct orbit, back scattering would have resulted, and
there would have been no perceptible increase in tail intensity.
The general situation contrasts sharply with that of comet 1887 I,
whose straight, narrow, headless 'tail' can be interpreted as a
synchrcone corresponding to an ejection time 0.23 day after
perihelion."
1981 November 20 (3647) Brian G. Marsden
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