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P. L. Lamy (Laboratoire d'Astronomie Spatiale, France), I. Toth (Konkoly Observatory, Hungary), H. Weaver (Johns Hopkins University), L. Jorda (Laboratoire d'Astronomie Spatiale, France), M. Kaasalainen (Observatory, University of Helsinki)
We report the detection of the nucleus of comet 67P/Churyumov-Gerasimenko, the new target of the Rosetta mission, with the Planetary Camera (WFPC2) of the Hubble Space Telescope (HST). The observations were performed between 11.4 and 12.3 March 2003 (i.e., a time interval of 21 hr) when the comet was at heliocentric and geocentric distances of respectively 2.51 and 1.53 AU and at a phase angle of 4.8 deg. The high spatial resolution (a WFPC2 pixel projected to 50 km at the distance of the comet) insured a very large contrast of the nucleus over its surrounding coma, reaching a factor of typically 30 in the peak pixels. The 61 images allowed us to determine 49 R magnitudes and 12 V magnitudes of the nucleus. Assuming a spherical body with a geometric albedo of 0.04 and a phase coefficient of 0.04 mag/deg, we derived an effective radius of 1.98 ± 0.02 km. The lightcurve data were analyzed using both the PDM (Phase Dispersion Minimization) and the Fourier technics. Assuming a classical double peak lightcurve, the two technics give the same rotational period of P=12.3 ± 0.27 hr. This result is highly robust since the probability that the period detection with the PDM technics is due to random noise amounts to only 3E-11. The lightcurve which conspicuously deviates from a sinusoide with shallow minima and narrow, unequal maxima, was inverted in order to model the 3-dimensional shape of the nucleus using the method developed by Kaasalainen et al. (Icarus 153, 37, 2001). The rotational state is constrained to be close to principal axis rotation of maximum inertia to get physically consistent solutions. All plausible solutions share the same qualitative global shape characteristics. The model which best fits the observed lightcurve (rms deviation of 0.028 mag) has a rotation axis tilted by about 8 deg from the principal axis, a rotational period of 12.69 ±0.10 hr, an aspect angle of 90 ± 20 deg (i.e., equatorial view) and a slightly rugged shape. The color of the nucleus is moderately red with a color index V-R = 0.51, the most common value among cometary nuclei. The OH production rates impose a rather low active fraction at perihelion, 4% to 7%. The activity is thus limited to probably a few, very small areas, consistent with the detection of several jets in the coma. The dust production rate was characterized by Af\rho = 40 cm.
We thank S. Beckwith for granting us HST Director's Discretionary time on a very short notice. This work was supported by grants from CNRS and CNES, and from NASA to H. W.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.