IMAGES OF COMET WIRTANEN

Comet Wirtanen

This comet has been selected as target for ESA's Rosetta mission. To characterize the comet and set constraints on the spacecraft and experiment design, observations were performed on Pik Terskol. Shown is a three color image of the faint comet, that clearly separates the three cometary components dust, neutral gas and ions. We observed the cometary H2O+, dust and neutral CN gas with the filters IF 614, RX and IF 390. In the above image they are represented in red, green and blue, respectively. For the first time Wirtanens ion tail (here H2O+) could be imaged, visible as straight red diffuse band to the left side (anti-sunward direction). The blue sphere is the very extended neutral CN Coma. In contrast to this, the dust is much more concentrated and dominates the near nucleus region, here seen as yellowish green color. So one result, illustrated with this three color image, is that 46P/Wirtanen is dust poor and about 2-3 times less dusty than Comet Halley.

Comet Wirtanen July 31, 1998

The 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, i.e. over 100 times fainter than that of Wild 2. 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 (as Comet Wild 2 also does) to the so-called Jupiter family of comets, a class of short-period comets whose orbits are repeatedly modified by close encounters with Jupiter.

The European Space Agency ESA has selected Comet Wirtanen as the prime target for its ROSETTA mission, a cornerstone project of the European HORIZON 2000 programme for the exploration of the solar system. The ROSETTA spacecraft will be launched in 2003 on an Ariane 5 rocket and will arrive at Comet Wirtanen in 2012.

ROSETTA will rendezvous with Comet Wirtanen and will go into orbit around its nucleus. During more than one year, remote sensing and in-situ experiments will explore this object and its atmosphere (the coma) from close distance. The highlight will be the landing of a science package that will perform measurements on the surface of the icy nucleus.

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.

Technical information for Photo 28b/98: Six 3-min and one 6-min R (red) exposures with the VLT Test Camera on July 28, 1998. Mediocre observing conditions in bright moonlight. Picture no. 1 is a combination of two 3-min exposures; nos. 2 - 5 are single 3-min exposures; no. 6 is a 6-min exposure. The individual frames were rebinned (4x4 pixels), sky subtracted, noise and cosmics filtered, and shifted in order to center the comet. The fields shown measure approx. 27 x 27 arcsec. North is to the upper right; East is to the upper left.