Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0804l&link_type=abstract
American Astronomical Society, DPS meeting #28, #08.04; Bulletin of the American Astronomical Society, Vol. 28, p.1083
Astronomy and Astrophysics
Astronomy
4
Scientific paper
Comet 45P/Honda-Mrkos-Pajdusakova (H-M-P) was observed with the Planetary Camera (WFPC2) of the Hubble Space Telescope on two consecutive days, 4 and 5 February 1996, at which time the geocentric distance was 0.17 AU (1 pixel projected to a distance of 5.6 km at the comet) and the phase angles were 93 and 88 deg, respectively. Although much fainter than expected, the nucleus was clearly detected superimposed on a very weak coma. The accuracy of the determination of the size of the nucleus is limited primarily by the unknown phase law for such large phase angles. Assuming a constant phase coefficient of 0.04 mag/deg and a geometric albedo of 0.04 near 675 nm, we derived an effective diameter of 0.70 km. Comet 22P/Kopff was observed on 18 July 1996 at a geocentric distance of 0.57 AU (1 pixel projected to a distance of 19 km at the comet) and a phase angle of 3 deg. Over the 11 hour time interval spanned by the observations, we see a variation in the nuclear magnitude that corresponds to changes in the effective nuclear diameter from 3.30 to 3.84 km, using the same assumptions as above for the albedo and phase law. The uncertainty in the nuclear diameter is determined primarily by the systematic error in estimating the contribution of the coma to the peak pixel intensity. The light curve data are being used to determine the rotation period for the nucleus. Comparison of our derived sizes with published gas production rates for comets H-M-P and Kopff indicate that the fractional active surface area is ~ 10% for each nucleus. Comet 46P/Wirtanen will be observed on 28 August 1996, and we expect to report preliminary results on its size.
A'Hearn Michael F.
Lamy Philippe L.
Toth Ignác
Weaver Harold A.
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