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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].

The nucleus of Comet 9P/Tempel 1, the target of the Deep Impact mission, was detected during Hubble Space Telescope observations taken with the wide-field planetary camera 2 (WFPC2) on 31 December 1997 when the comet's solar phase angle was 3. [degrees] 8, its heliocentric distance was 4.48 AU, and its geocentric distance was 3.53 AU. Sixteen images were taken through the F675W filter, and all of them revealed a point-like source without any detectable coma. From these images, we derived the R magnitude of the nucleus in the Johnson-Kron-Cousins photometric system for the entire 11.5-h time span of the observations. Assuming a prolate spheroid whose spin axis lies close to the plane of the sky, the partial lightcurve indicates semi-axes a = 3.9 km and b = 2.8 km (assuming a geometric albedo of 0.04 for the R band and a phase coefficient of 0.04 mag [deg.sup.-1]) and a rotational period in the range of ~25-33 h. The upper limit of the parameter Af[rho], which characterizes the dust production rate, is 1 cm. We also derive a fractional active area at 1.78 AU of ~4%.

Comet P/Faye 1991 XXI (1991 n) was observed with the planetary camera (PC) of the Hubble Space Telescope (HST) between October 29 and November 21, 1991, when its geocentric distance was in the range 0.62-0.67 AU. The resulting high resolution - a single PC pixel projected to a distance of [appoximately]20 km at the comet - made it possible to clearly discriminate the nucleus and to study the dust coma within [approximately]100 km from the nucleus. The spherical aberration which affected the early operation of the HST has severely complicated the analysis and image restoration using the Richardson-Lucy method has failed to give satisfactory results. The coma is dominated by the point spread function (PSF) of the nucleus up to a radial distance of [approximately]100 km and cannot be recovered. Beyond, it is affected by the spherical aberration up to approximately 750 km and was analyzed by comparison with a grid of models convolved with appropriate PSFs. Beyond 750 km, it remains unaffected and can be studied directly. From the outer to the innermost regions, the coma presents an elongated shape which may be explained either by an active source on the nucleus or, more likely, by a projection effect of the dust tail. If this second interpretation is correct, then the temporal evolution of the comet is very slow and smooth, suggesting an extended source of dust on the nucleus. The dust production rate, corrected for the projection effect of the tail, amounts to 125 kg [sec.sup.-1].

The nucleus of Comet 45P/Honda-Mrkos-Pajdusakova was detected during Hubble Space Telescope observations taken with the Wide-Field Planetary Camera 2 (WFPC2) in February 1996 when the comet was 0.17 AU from the Earth. The high spatial resolution (1 pixel projected to 5.6 km at the distance of the comet) allowed us to separate the signal of the nucleus from that of the coma, and images taken through five broadband filters allowed us to determine the UBVRI magnitudes of the nucleus in the Johnson-Kron-Cousins photometric system (sometime known as the Landolt system). Assuming a spherical body whose geometric albedo is 0.04 for the V band and a phase coefficient of 0.04 mag/deg, we derive an effective radius of 0.34 km. The uncertainty is dominated by the unknown phase law; the radius may be as small as 0.22 km if the phase coefficient is 0.03 mag/deg. The correlated variations of the magnitudes over 1 day suggest an elongated body with an axis ratio of at least 1.3. The strong reddening of the nucleus in the ultraviolet progressively decreases, and its color tends to neutral in the near infrared. We also derive a fractional active area of 11% and a dust production rate of 1.0 kg [s.sup.-1] (86 metric tons [day.sup.-1]).