The nucleus of comet 22P/Kopff and its inner coma

We report the detection of the nucleus of Comet 22P/Kopff with the Planetary Camera of the Hubble Space Telescope (HST) and with the Infrared Camera of the Infrared Space Observatory (ISOCAM). The HST observations were performed on 18 July 1996, 16 days after its perihelion passage of 2 July 1996, when it was at [R.sub.h] = 1.59 AU from the Sun and [DELTA] = 0.57 AU from the Earth. A sequence of images taken with four broad-band filters was repeated eight times over a 12-h time interval. The ISOCAM observations were performed on 15 October 1996, 106 days after the perihelion passage, when the comet was at [R.sub.h] = 1.89 AU from the Sun and [DELTA] = 1.32 AU from the Earth. Seven images were obtained with a broad-band filter centered at 11.5 [micro]m. In both instances, the spatial resolution was appropriate to separate the signal of the nucleus from that of the coma. We determine the Johnson-Kron-Cousins BVRI magnitudes of the nucleus. The visible lightcurves constrain neither the rotation period nor the ratio of semiaxes. We favor the solution of a rather spherical nucleus, although the situation of a pole-on view of an irregular body cannot be excluded. The systematic decreasing trend of the lightcurves could suggest a period of several days. Combining the visible and infrared observations, we find that an ice-dust mixed model is ruled out, while the standard thermal model leads to a nuclear radius of [R.sub.n] = 1.67 [+ or -] 0.18 km of albedo [P.sub.v] = 0.042 [+ or -] 0.006. The red color of the nucleus is characterized by a nearly constant gradient of S' = 14 [+ or -] 5% per k[Angstrom], from 400 to 800 nm. We estimate a fractional active area of 0.35 which places 22P/Kopff in the class of highly active short-period comets. At [R.sub.h] = 1.59 AU, the dust coma is characterized by a red color with a reflectivity gradient S' = 17 [+ or-] 3% per k[Angstrom], compatible with that of the nucleus, and Af [rho] = 545 cm, yielding a dust production rate of [Q.sub.d] = 130 kg [sec.sup.-1].