VLT observations of comet 46P/Wirtanen^*

¹ European Southern Observatory ESO, Alonso de Cordova 3107, PO Box 19001, Santiago 19, Chile e-mail: hboehnha@eso.org;cdelahod@eso.org;tsekiguc@eso.org;ohainaut@eso.org;ramestic@eso.org
² Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy e-mail: tozzi@arcetri.astro.it
³ European Southern Observatory ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: jspyromi@eso.org;mtarengh@eso.org;rwest@eso.org
⁴ European Space Agency ESA, ESTEC, Keplerlaan, 2200 AG Noordwijk, The Netherlands e-mail: rschulz@estec.esa.nl;gschwehm@estec.esa.nl

Corresponding author: H. Boehnhardt, hboehnha@eso.org

Received: 4 December 2001
Accepted: 28 March 2002

Abstract

Comet 46P/Wirtanen, prime target of ESA's ROSETTA mission, was successfully observed at the Very Large Telescope Observatory in Chile: on 17 May 1999 with the Test Camera at the Cassegrain focus of the 8.2-m VLT Kueyen telescope and on 8 December 2001 with FORS1 at Unit Telescope 4 Yepun. May 1999: no coma was detected at heliocentric distance  AU. From the measured brightness in the Bessell R-filter, a mean nucleus radius of  m is derived (for a geometric albedo of 0.04 and a phase darkening of 0.04 mag/deg). The nucleus signal varies during the 2.7 h observing interval and a peak-to-peak amplitude of ~0.38 mag is determined. The measured lightcurve is in agreement with a rotation period of 6–7.5 hours and a ratio of the main nucleus axes of at least 1.4. The non-detection of a coma allows one to put an approximate upper limit for Afρ of <0.45 cm (suggesting a dust production rate of 0.05 kg/s). December 2001: a weak and condensed coma seems to be present in the seeing disk of the comet at 2.9 AU inbound, causing a higher brightness than expected from the previous size estimates of the nucleus. The colour of the comet appears very red ( spectral gradient ~47%/100 nm). The Afρ value of the comet was  cm (equivalent to a dust production rate of about 1 kg/s).