The formation of Encke meteoroids and dust trail

We observed Comet 2P/Encke with the Infrared Space Observatory ISOCAM on Ju ly 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 observ ed, 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 tr ail, The particles that produce the infrared coma are large, with a radiati on to gravitational force ratio beta < 10(-3) (corresponding to >mm-sized p articles). The dust trail follows the orbit of the comet across our image, with a central core that is 2 x 10(4) km wide, composed of particles with b eta < 10(-5) (size <similar to>5 cm) from previous apparitions. The abundan t large particles near the comet pose a significant hazard to spacecraft. T here is no evidence of a classical cometary dust tail due to small particle s with beta > 10(-3), in marked contrast to other comets like P/Halley and C/Hale-Bopp. The structure of the coma requires anisotropic emission and re quires that the spin axis of the nucleus be nearly parallel to the orbital plane, resulting in strong seasonal variations of the particle emission. Wh ile most of the infrared coma emission is due to dust produced during the 1 997 apparition, the core of the dust trail requires emissions from previous apparitions. The total mass lost during the 1997 apparition is estimated t o be 26 x 10(13) g, Compared to the gas mass loss from ultraviolet observat ions, the dust-to-gas mass ratio is 10-30, much higher than has ever been s uggested from visible light observations. Using the recently measured nucle ar diameter, we find that Encke can last only 3000-10,000 rho (N) yr (where rho (N) is the nuclear density in g cm(-3)) at its present mass loss rate. (C) 2000 Academic Press.