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C.E. Delahodde, O.R. Hainaut (ESO/Chile), P.L. Lamy (Laboratoire d'Astrophysique de Marseille), K.J. Meech (Institute for Astronomy)
Very little is known on the surface properties of cometary nuclei -- with the notable exception of the in-situ measurements for comet Halley. A still unexplored method of investigation is the study of the solar phase function, which can give us direct information on the surface roughness as well as some independent constraints on the albedo. In order to estimate properly the Hapke parameters describing the phase function, one has to obtain measurements not only over a broad phase angle (\alpha) range, but also at very small phase angle, in order to sample the ``opposition surge'', a brightening of the phase curve occurring at \alpha < 2-3\circ.
In 2000, Comet 28P/Neujmin~1 was an ideal candidate for a phase function study: its April opposition was almost ideal (\alpha = 0.8\deg), and previous observations by Meech et al. at similar heliocentric distances (rh \geq 4 AU) indicated that the comet was likely not to be active. We observed this comet using ESO's NTT and 2.2m telescopes at La Silla, on 6 epochs from April to August 2000, covering \alpha = 0.8 - 8\circ , with a good sampling of the opposition surge.
In order to disentangle the rotation effects from the phase effects, we obtained a complete rotation coverage at opposition, providing us with a light-curve template. For each subsequent epoch, we have obtained enough rotation coverage to re-synchronize the light-curve fragments with the template, and determine the magnitude change caused by the phase function.
Using these data, we hope to present the first detailed comet nucleus phase function, and compare it with known C-type asteroids.