Central Bureau for Astronomical Telegrams
IAUC 5840: PERSEID METEORS
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Circular No. 5840
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)
PERSEID METEORS
With reference to the remark on IAUC 5839 concerning the possible
observation of a glow arising from sunlight scattered by meteoroidal dust,
D. I. Steel, Anglo-Australian Observatory, draws attention to a discussion
of this topic by Baggaley (1977, Obs. 97, 123), whose prediction
in the case of the Perseids is very different from that given by Rao
(1993, Sky Tel. 86, No. 2, 48). The point to watch must be the true
radiant (and anti-radiant), which has been computed by the undersigned
to be at R.A. = 33.5 deg, Decl. = +84.7 deg (and its antipodal point); this
differs by 27 deg from the apparent (geocentric) radiant (i.e., that
modified by the earth's orbital velocity), which the current orbit of
P/Swift-Tuttle indicates to be at R.A. = 46.4 deg, Decl. = +57.8 deg
(equinox 2000.0). This computation is confirmed by Steel, and also by
D. Kessler and H. Zook, NASA Johnson Space Center, who remark that,
if there should be a strong concentration of material exactly such that
it will collide with the earth, then the glow will be at the apparent
radiant. They therefore suggest that, before the earth crosses the comet's
orbital plane, the arc between the apparent and true radiants should be
monitored; as the orbital plane is crossed, any glow should converge on
the true radiant---to jump to the true anti-radiant and then to the arc
joining the true and apparent anti-radiants afterward. It seems that
the glow points indicated by Rao considered the adjustment from apparent
to true radiant with the wrong sign.
J. G. Hills and R. Hoffman, Los Alamos National Laboratory, discuss
the possibility of observing larger individual pieces of cometary debris
outside the earth's atmosphere. For the purpose of searching for these
bodies, one needs to concentrate on the apparent radiant and anti-radiant,
as well as that half of the great circle joining them that crosses the
equator near R.A. 136.4 deg.
Further details regarding the ejection simulation by Williams
and Wu (IAUC 5839) show that the basic assumption was that 1-mm particles
were ejected from the comet in various directions from the sunward side
at a speed of 50 r**(-9/8) m/s, 99 percent of them when the comet was
within r = 2 AU of the sun in 1862, forces due to planetary gravitational
attractions and radiation pressure being considered. The suggestion that
the Perseids might peak in 1994 was based on a cometary orbit that was
too small. Use of the correct orbit shows the current activity starting
around 1989 and suggests that the peak might not be until 1995 or even 1997.
1993 August 10 (5840) Brian G. Marsden
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