International Astronomical Union Circular No.
7471
Sunday, 30 Jul 2000 22:02:17 -0400 (EDT)
COMET C/1999 S4 (LINEAR)
Z. Sekanina, Jet Propulsion Laboratory, reports: "The unusually
large nongravitational forces found by B. G. Marsden (MPEC 2000-O07)
suggest that comet C/1999 S4 was a trailing fragment of a more
massive comet that has been moving in the same orbit, arrived
at perihelion long (centuries?) ago but (not surprisingly) was
missed. Trailing fragments of known comet pairs have a tendency
to sudden disintegration (e.g., Sekanina 1997, A.Ap. 318,
L5). If much of the comet's mass did indeed dissipate into a cloud
of dust in the recent event, as suggested by M. R. Kidger (IAUC
7467) and others, the total mass involved could be estimated by
further monitoring the tail. Experience with the past initially
bright comets that later became headless and disappeared shows
that a narrow, bandlike tail--a developing synchronic formation--should
survive the head by several weeks or even longer (Sekanina 1984,
Icarus 58, 81). A very preliminary analysis suggests that the
event may have begun as early as July 23.6 UT and involved submillimeter-sized
and larger dust (repulsive accelerations up to 0.024 of the solar
attraction). The position angle and approximate length of this
tail feature are then predicted to reach: July 30.0 UT, 90 deg,
2'; Aug. 4.0, 98 deg, 4'; 9.0, 102 deg, 7'; 14.0, 104 deg, 10';
19.0, 105 deg, 12'; 24.0, 106 deg, 15'; 29.0, 106 deg, 17'. Especially
toward the end of this period, the predicted length probably is
a crude upper bound. If no such tail persists, the comet's upper
mass limit should be tightly constrained, or the amount of dust
lost in the event did not represent a substantial fraction of
the total mass."