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THE ASTRONOMER Electronic Circular No 3709      2022 May 36 10.08UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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TAU HERCULID METEOR SHOWER 2022

P. Jenniskens, SETI Institute et.al. report that a meteor outburst of tau
Herculid meteors is anticipated for May 31 around 4h-5h UT this year.  This
outburst is unusual in that the debris was generated during the 1995 breakup
of comet 73P/Schwassmann-Wachmann.  Independent calculations at I.M.C.C.E.
and the University of Maryland, now put the 1995 dust trail in the earth's
path with an expected peak on May 31d05h01m and 03h52m UT, respectively.  In
both models, meteors will radiate from R.A. =
209 deg, Decl. = +28 degrees with an apparent velocity of 16.4 km/s
(geocentric velocity 12.1 km/s).  The outburst is expected to last about two
hours.  Observers in the continental USA and Mexico are most favourably
located to see this event in new-moon conditions, with a radiant high in the
northwest.

The authors note that there are two caveats.  First, an unusual meteor
shower will only happen if the meteoroid ejection velocities during the
breakup and decay of fragments were a factor of 2.5 higher than under normal
cometary ejection conditions.  Because the comet itself is not near the
earth, normal ejection does not have the meteoroids disperse far enough
ahead of the comet to intersect the earth's path.  The higher gas-production
rate of comet 73P in 1995 suggests that ejection velocities may have been
higher by up to a factor of 2.7.  However, the ejection velocities of cm-
and mm-sized meteoroids were not measured in 1995.  Second, if the outburst
happens, the meteors will be mostly faint.  Rare tau Herculids are also
known from regular cometary activity during normal returns.  The Cameras for
Allsky Meteor Surveillance (CAMS) project, which triangulates video-detected
meteors visible to the naked eye, measured the orbit of 18 tau Herculids
during dust-trail crossings in 2011, 2017/ and 2019; from this, the authors
calculate a steep magnitude-distribution index of 5.4 +/- 1.1, meaning that
there were five times more meteors of magnitude +4 than +3, five times more
meteors of mag +3 than +2, etc. -- but with the camera sensitivity such that
more +meteors of magnitude +3 were detected.  In practice, a few meteors
were of magnitude +1, but most were near the +4 magnitude detection limit of
the video cameras.

Finally, two dust trails from the normal 1892 and 1897 returns of 73P will
also be in the earth's path around 16h UT on May 30 and 10h UT on May 31,
respectively (cf., Wiegert et al. 2005, MNRAS 361, 638).  Because the comet
has been known only since 1930, these trail crossings could shed new light
onto the past activity of the comet, if detected.

(Dan Green, CBET 5125) 

Guy M Hurst


