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.
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
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