  Comet C/1995 O1 (Hale-Bopp) has provided an unprecedented opportunity to
observe a bright comet over a wide range of heliocentric distances. We present
here Spitzer Space Telescope observations of Hale-Bopp from 2005 and 2008 that
show a distinct coma and tail, the presence of which is uncommon given its
large heliocentric distance (21.6 AU and 27.2 AU, respectively). The morphology
of the dust is compared to dynamical models to understand the activity of the
comet. Our analysis shows that the shape of Hale-Bopp's dust tail in these
images cannot be explained using the usual Finson-Probstein (solar gravity +
solar radiation pressure) dynamical model. Several alternative explanations are
explored. The analysis suggests that the most likely cause of the discrepancy
is that the dust is being charged by the solar wind, then being affected by the
interplanetary magnetic field via the Lorentz force. Though this effect has
been explored previously, if correct, this seems to be the first time that the
Lorentz force has been required to model a cometary dust tail. The analysis
also suggests that Hale-Bopp was actively emitting particles when these images
were taken, and the tail characteristics changed between observations.

Earth and Planetary Astrophysics