Aphelion Observations of the Nucleus of Comet 67P/Churyumov-Gerasimenko: Target of ESA's Rosetta Comet Orbiter Mission

Details Aphelion Observations of the Nucleus of Comet 67P/Churyumov-Gerasimenko: Target of ESA's Rosetta Comet Orbiter Mission Aphelion Observations of the Nucleus of Comet 67P/Churyumov-Gerasimenko: Target of ESA's Rosetta Comet Orbiter Mission

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.0801l&link_type=abstract

American Astronomical Society, DPS meeting #38, #08.01; Bulletin of the American Astronomical Society, Vol. 38, p.492

Astronomy and Astrophysics

Astronomy

6

Scientific paper

Rosetta is ESA's new comet orbiter mission, launched in March 2004 and currently en route to Jupiter-family comet 67P/Churyumov-Gerasimenko. The probe will rendezvous with the comet in 2014 and remain in orbit around the nucleus for on-going detailed physical and compositional analysis. Pre-encounter observations of the target are important for characterization of the heliocentric light-curve behaviour and the physical properties of the nucleus, information that is critical for mission planning. We present our latest results from detailed observations carried out when the comet was near aphelion at a heliocentric distance of 5.6 AU. The comet was also at opposition, with an average phase angle of just 0.46°. Three nights of optical imaging data were taken with ESO's 3.5m NTT at La Silla (Chile), between May 10-14, 2005. The data includes CCD VRI-filter imaging taken with the EMMI instrument. The rotational signature of the nucleus was clearly detected at all passbands with no sign of resolved coma or dust trail. Full rotational light-curves in all three passbands were obtained, allowing detailed inspection of surface colours at all rotational phases. A Fourier analysis of the R-filter light-curve gives a robust best-fit synodic rotation period of 12.72 ± 0.05 hours. The observed brightness range of 0.38 ± 0.04 magnitudes implies a projected nucleus axial ratio of 1.42 ± 0.05. The mean R-filter magnitude is 22.41 ± 0.03, which corresponds to mean nucleus effective radius of 2.26 ± 0.03 km (assuming an albedo and linear phase coefficient of 0.04). The projected semi-axial dimensions are therefore 2.94 ± 0.15 and 2.07 ± 0.04 km. Our size is slightly larger than a previous estimate [Lamy et al. 2003. BAAS 35, 970], which implies a steep phase-darkening law or a more complex phase function that includes an opposition-surge effect.

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