Hubble Space Telescope observations of the nucleus of Comet 9P/Tempel 1

Abstract 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.°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 −1 ) and a rotational period in the range of ∼25–33 h. The upper limit of the parameter Af ρ, which characterizes the dust production rate, is 1 cm. We also derive a fractional active area at 1.78 AU of ∼4%.

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