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 < 10 ^ { -3 } ( corresponding to > mm-sized particles ) .
The dust trail follows the orbit of the comet across our image , with a central core that is 2 \times 10 ^ { 4 } km wide , composed of particles with \beta < 10 ^ { -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 > 10 ^ { -3 } , 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 2–6 \times 10 ^ { 13 } 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 \rho _ { N } yr ( where \rho _ { N } is the nuclear density in g cm ^ { -3 } ) at its present mass loss rate .