Hubble Space Telescope observations of the nucleus of Comet 9P/Tempel 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 . ° 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 Afρ, which characterizes the dust production rate, is 1 cm. We also derive a fractional active area at 1.78 AU of ∼ 4 % .