  We observed comet 2P/Encke with the Infrared Space Observatory ISOCAM on July
14, 1997 from a particularly favorable viewing geometry above the comet's
orbital plane and at a distance of 0.25 AU. A structured coma was observed,
along with a long, straight dust trail. For the first time, we are able to
observe the path of particles as they evolve from the nucleus to the trail. The
particles that produce the infrared coma are large, with a radiation to
gravitational force ratio beta<0.001 (corresponding to >mm-sized particles).
The dust trail follows the orbit of the comet across our image, with a central
core that is 20,000 km wide, composed of particles with beta<1e-5 (size $\sim
5$ cm) from previous apparitions. The abundant large particles near the comet
pose a significant hazard to spacecraft. There is no evidence of a classical
cometary dust tail due to small particles with beta>0.001, in marked contrast
to other comets like P/Halley or C/Hale-Bopp. The structure of the coma
requires anisotropic emission and that the spin axis of the nucleus to be
nearly parallel to the orbital plane, resulting in strong seasonal variations
of the particle emission. While most of the infrared coma emission is due to
dust produced during the 1997 apparition, the core of the dust trail requires
emissions from previous apparitions. The total mass lost during the 1997
apparition is estimated to be
  2-6e13 g. Comparing to the gas mass loss from ultraviolet observations, the
dust-to-gas mass ratio is 10-30, much higher than has ever been suggested from
visual light observations. Using the recently-measured nuclear diameter, we
find that Encke can only last 3000-10,000 rhoN yr (where rhoN is the nuclear
density in g/cc) at its present mass loss rate.
