Organization: Washington University, St. Louis
Date:         Fri, 16 Oct 92 09:42:43 CST
From:         Gary Kronk <C09630GK@WUVMD>
To:           NETNEWS@WUVMD
Subject:      Swift-Tuttle and Earth in 2126
Newsgroups:   sci.astro


Now here is an intriguing bit of information from our friends at the
Smithsonian Astrophysical Observatory. This Circular was issued
yesterday evening.

                                                  Circular No. 5636
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM     EASYLINK 62794505
MARSDEN@CFA or GREEN@CFA (.SPAN, .BITNET or .HARVARD.EDU)


PERIODIC COMET SWIFT-TUTTLE (1992t)
     Orbital computations by the undersigned, and also by S. Nakano,
Sumoto, Japan, have so far failed to link all the observations, even
when allowance is made for nongravitational forces.  Although a
reasonable fit can be made to the 1862 (except October) and 1992
observations, the resulting transverse nongravitational component is
so large that the resulting eighteenth-century perihelion time is 15
months too late.  Alternatively, although the three perihelion times
can be well represented without any consideration of nongravitational
forces at all, there are strong systematic errors, amounting to more
than 1', in 1862 and 1992.  The gravitational orbital elements below
satisfy the observations in 1992 and in Oct. 1862 very well, and they
also represent the presumed 1737 perihelion time within 1 day.
Backward computation of this solution reveals few candidates for
earlier appearances of the comet, although the one of -68 fits within
1 year (there being 15 revolutions between then and 1862), and the
comet of +60 may also belong.  Future extrapolation gives the next
return to perihelion as 2126 July 11, although the problem with the
computation of the nongravitational forces must introduce some
uncertainty; a change by +15 days could cause the comet to hit the
earth on 2126 Aug. 14.  It therefore seems prudent to attempt to
follow P/Swift-Tuttle for as long as possible after the present
perihelion passage, in the hope that an adequate independent orbit
determination, uncontaminated by nongravitational effects, can be made
from mid-1993 (at r = 3 AU and far to the south) to, say, 1998 (when r
= 15 AU and an assumed nuclear absolute magnitude of 14 yields an
apparent magnitude of 26).

                    Epoch = 1992 Dec. 4.0 TT
     T = 1992 Dec. 12.323 TT          Peri. = 153.013
     e = 0.96359                      Node  = 139.456   2000.0
     q = 0.95812 AU                   Incl. = 113.430
       a = 26.31666 AU     n = 0.007301     P = 135.00 years

     Further total visual magnitude estimates: Oct. 13.01 UT, 7.9 (H.
Luthen, Hamburg, Germany, 0.10-m reflector); 13.76, 7.6 (S. Canonaco,
Genk, Belgium, 0.20-m refractor); 14.74, 7.8 (H. Mikuz, Ljubljana,
Slovenia, 7 x 50 binoculars); 14.86, 7.6 (M. Tombelli, Montelupo,
Italy, 0.20-m reflector).


1992 October 15                (5636)              Brian G. Marsden
Newsgroups: sci.astro