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IAUC 8330: C/2002 T7

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                                                  Circular No. 8330
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
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     Z. Sekanina, Jet Propulsion Laboratory, writes: "The sudden
appearance of a prominent antitail is the result of unusual
coincidences.  Due to projection effects the antitail could not
develop before the end of March 2004, at which time the comet was
too close to the sun to be observed from the ground (and not close
enough to show up in the SOHO's C3 coronagraphic images).  From the
orientation in the available images of Apr. 17-20, the dust
contained in the antitail is found to have been ejected most
probably between discovery (mid-October 2002) and about 300 days
before perihelion (the end of June 2003).  Because the comet was
not brightening dramatically between discovery (except perhaps in
the first post-discovery days) and the 2003 conjunction with the
sun, it is likely that the material in the antitail dates back to a
fairly sudden onset of persistent dust production at or shortly
before discovery (i.e., the comet was detected because it 'turned
on' at about that time), or the comet's dust-emission activity
picked up rapidly in mid-2003, when it was behind the sun.  The
steeper light curve beginning in August 2003 can support this
scenario.  In either case, there was no antitail around 2003 Dec.
27 and none is expected to appear around 2004 June 26, the times of
the earth's transit across the comet's orbital plane.  In the
intervening period of time, the position angle of the antitail and
the angle it subtends with the prolonged radius vector (i.e.,
approximately with the plasma tail) are predicted to vary as
follows (the first number in each interval refers to an onset time
in late June 2003, the second to onset in mid-October 2002):  Apr.
15, p.a. 42-45 deg, angle with respect to the radius vector 142-145
deg; 20, 32-37 deg, 137-142 deg; 25, 16-24 deg, 125-133 deg; May 5,
323-334 deg, 80-91 deg; 15, 291-295 deg, 82-86 deg; 20, 274-278 deg,
120-124 deg; 25, 255-262 deg, 130-137 deg; 30, 236-251 deg, 117-132
deg; June 4, 197-232 deg, 80-115 deg; 9, 154-186 deg, 37-69 deg.
The angular deviation from the radius vector dropping below 90 deg
implies the antitail's disappearance.  There are two maxima in this
angle, one on Apr. 13, the second on May 21.  Unless the dust
production terminated early, more recent ejecta fill the entire
sector between the antitail and the plasma tail.  The visibility of
this sector is affected by projection geometry and, to a degree, is
anticorrelated with the antitail's prominence.  Also, in long-
exposure images, which show the antitail to extend further out, its
slight curvature may be noticed toward greater position angles
(counterclockwise) farther from the comet, although this effect may
be concealed by decreasing brightness."
     Visual total magnitudes and tail lengths by A. Pearce, Noble
Falls, W. Australia (8x40 binoculars): Apr. 15.89 UT, 4.6, 2.5 deg;
22.88, 4.2, 6.5 deg.

                      (C) Copyright 2004 CBAT
2004 April 23                  (8330)            Daniel W. E. Green

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