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THE ASTRONOMER Electronic Circular No 1552      2000 Aug 12 20.40UT
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             GMH at AST.STAR.RL.AC.UK
WORLD WIDE WEB                    http://www.demon.co.uk/astronomer
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Comet C/1999 S4 (LINEAR)

H.Weaver,Johns Hopkins University,and R.West,European Southern
Observatory,on behalf of a large group of collaborators,report the
following results:"Hubble Space Telescope (HST) images taken during
Aug.5.167-5.396 UT and Very Large Telescope (VLT) images (image quality
about 0".6) taken during Aug.6.978-6.999 revealed about a dozen active
fragments,most of them located within about 20" of the western tip of
the dust tail (cf.IAUC 7474).The correspondence between fragments in the
HST and VLT images is generally very good, but the brightest fragment in
the HST image is not seen in the VLT image,indicating rapid variability
in the activity levels.The dynamic nature of the fragments was further
highlighted by a dramatic change in the appearance of the fragments in
VLT images taken during Aug.9.976-9.996,when they were barely
detectable.Although the later images were taken under mediocre
observational conditions (image quality about 1"-1".3,thin cirrus,and
nearly full moon),that alone seems unlikely to account for the observed
changes.A very preliminary estimate for the R magnitude within a
circular aperture of radius 0".23, of well-isolated fragment is about
24.A completely inactive fragment with a diameter of 100 meters observed
under these conditions (r=0.79 AU,Delta=0.69 AU,Phase=86o) would have R
about 25.9 (assuming a 4-percent albedo and 0.04 mag/deg phase law).We
urge ground-based observers to continue monitoring the comet and to
report any unusual changes near the "tip",both in brightness and
morphology."

IAUC 7476    

-- 
Denis Buczynski