- Here's a 2 x 1 composite showing the main
part of 17P with a V filter as of 2008 Jan 06.0.
In dark skies, the dust coma of this comet is readily visible to the naked
eye. However in 2 months or so its solar elongation will grow small and so
it will prove a difficult object after March of this year.
With CCD cameras, my calculations show that it should be possible to record
the dust coma at its next opposition in Dec 2008 / Jan 2009.
The increase in heliocentric distance and delta should result in a further
1.6 magnitudes fading in the surface brightness of the coma (including the
effect of phase angle) notwithstanding the further fading due to its
Currently (Jan 6) the width of the coma at it's photocentre is about 1.13
degrees. In 1 year's time, this will have grown to an apparent width of 3.8
degrees or so (allowing for the increased viewing distance of 3.3 AU as
opposed to the current 2.1 AU). So the fading due to expansion (presuming it
does not slow down) comes to 2.6 mags. So the overall decrease in surface
brightness would be 4.2 mag. The dynamics of the expansion should
result in an apparent deceleration in the growth of the width of the coma as
material is gradually swept downstream whilst some of the central
condensation will expand to fill the outer coma and I therefore suspect the
overall fading in the surface brightness of the outer coma should be about
Presently a 30-second exposure at f/6 (+ V filter) will show the extent of
the outer coma. In one year's time, the same signal-to-noise should be
achievable using a stacked 6000 second exposure at f/2.8. Now does anyone
out there have a 200mm f.l. f/2.8 lens?
- By late January 2009, the large almost
circular coma of 17P/Holmes will be located within the region of the
Zodiacal Light known as the Gegenschein, and whilst at this location the
coma will serve to enhance its brightness making this elusive phenomenon
more visible than normal
- Richard Miles