First observation of Comet Wirtanen with ESO/VLT
21 September 1998
The European Southern Observatory's new giant telescope, has provided excellent new images of Rosetta's target comet, Comet Wirtanen.The images were taken with the VLT 8.2m telescope (UT1), which completed its science verification programme on 1 September.
The comet Wirtanen photo mosaic is based on a series of 3-minute exposures through a red filter, obtained with the VLT Test Camera in the evening of July 28, 1998. They were performed in a bright sky (5-day old Moon high in the sky) that resulted in some straylight due to internal reflections in the telescope. In the first three pictures (1 - 3), the very faint image of the comet (in the circles and somewhat elongated because of the motion) approaches a brighter background star from the right hand side. It is hardly visible in the next (4), since it is in front of this star, and in the last two images (5 - 6), it reappears on the left side of the star.
At the time of the observations, Comet Wirtanen was 605 million kilometres (4.05 AU) from the Earth and 630 million kilometres (4.20 AU) from the Sun. The estimated magnitude is approx. 23 or beyond. It is an impressive feat of the UT1, to observe such a faint object in such a short time and under these mediocre conditions.
Comet Wirtanen was discovered in 1948 by C. A. Wirtanen at the Lick Observatory (California, USA). With an orbital period of 5.5 years, it belongs to the so-called Jupiter family of comets, a class of short-period comets whose orbits are repeatedly modified by close encounters with Jupiter.
ESA's Rosetta mission, to be launched in 2003, will arrive at the comet in 2012.
The new VLT exposures contribute to the monitoring programme now underway with other ESO telescopes in preparation of the Rosetta mission. This programme has revealed that Comet Wirtanen has one of the smallest nuclei known (just over 1 km across), but at the same time one of the most active. Compared to observations with the ESO New Technology Telescope earlier this year, it appears that the comet is now much fainter and shows much less activity. The nucleus will now become frozen and "dormant" for the next two to three years until it is warmed up again during the next approach to the Sun.
