Comet Hale-Bopp Update
COMET HALE-BOPP UPDATE
Sky & Telescope News Bulletin
April 25, 1997
HALE-BOPP's THIRD TAIL
Comet Hale-Bopp (C/1995 O1) has a tail of sodium atoms that is
distinct from its dust and gas tails. This third tail was first seen on
April 16th in images filtered to isolate the yellow light (D line)
emitted by neutral sodium atoms. The sodium tail is nearly 7 degrees long
but only 10 arcminutes wide. Too faint to be seen in normal photographs, it
lies a few degrees east of the anti-Sun direction. Spectra show that the
sodium atoms are accelerating along the tail and reach 95 km/second at
a point 11 million km from the nucleus. Apparently this acceleration
is linked to the process that gives rise to sodium's fluorescent glow.
Its atoms absorb photons from the Sun but reradiate them in random
directions, an imbalance that provides the accelerating force.
COMET-TAIL WATCH
Space scientists from NASA and the European Space Agency predict that
the ion tail of Comet Hale-Bopp will be particularly susceptible to
the solar wind's influence between April 28th and May 15th. They say
the blue-colored tail could show brightness changes, kinks, and
perhaps moving structures. Also possible are "disconnection events,"
when the ion tail breaks off from the comet's coma and reforms a few
hours later. Scientists have issued this "tail watch" because on the
28th the comet will be within 10 degrees of the plane of the solar
equator. In this region the outflowing solar wind is gusty and the
magnetic field irregular. When the plane is crossed on May 3rd, the
direction of the interplanetary magnetic field will change. Amateur
astronomers are encouraged to monitor the tail's appearance carefully
during this period. Interested observers can access the comet watch
home page at http://www-istp.gsfc.nasa.gov/istp/halebopp/
HALE-BOPP SEEN WORLDWIDE
Comet Hale-Bopp has faded to about magnitude 0, but it is still
obvious and impressive in the evening sky. Observers in the Southern
Hemisphere are getting their first glimpses of it too, spotting it low
on the horizon during twilight. Strong moonlight ceases to be a
problem this week. So look for the comet after sunset; it should be
obvious about 20 degrees above the west-northwestern horizon.
APRIL 27 -- SUNDAY
Comet Hale-Bopp is still in view this week, though it has faded and
shrunk considerably now that it has been traveling away from both the Earth
and Sun for about a month. It's getting lower each evening too. Look low in
the west-northwest right at the end of twilight. Bring binoculars!
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