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ESO Photos Of Comet Wild-2, Comet Wirtanen Available

 ESO Press Photos 28a-b/98
 31 July 1998                                                         
 For immediate release

VLT Quick Views of Spacecraft Targets

The first VLT 8.2-m telescope (UT1) is now undergoing a "Commissioning
Phase" during which all systems are thoroughly tested and further tuned.
Although priority is given to technical work, some astronomical images have
been obtained during the recent weeks.

Some days ago, during short periods when no technical tests were scheduled,
"quick shots" were obtained of two comets, both of which are designated
targets for space missions.

The first, Comet Wild 2, will be visited during NASA's STARDUST mission that
will start early next year. The other, Comet Wirtanen, will be explored by
ESA's ROSETTA spacecraft that is due to be launched in 2003.

The VLT will be able to provide important support to both of these space
missions by obtaining detailed observations of the comets and their
momentary behaviour. In particular, the great light collecting power of the
VLT Unit Telescopes will make it possible to study these comets exhaustively
when they are farthest from the Sun in their orbits and hence very faint,
shortly before the spacecraft encounters.

                           ESO Press Photo 28a/98 (Comet Wild 2)
                           [Preview - JPEG: 800 x 922 pix - 400k]
                           [High-Res - JPEG: 3000 x 3460 pix - 3.1Mb]

The first picture (ESO PR Photo 28a/98) is a composite of seven 2-minute
exposures of Comet Wild 2, obtained through a red filter with the VLT Test
Camera in the early morning of July 25, 1998. The observations were made
during rather windy conditions, about 15 m/sec, whereby substantial,
variable pressure was exerted on the upper part of the telescope structure.
Still, the guiding worked very well and the resulting image shows the fine
structure of the comet's "coma" of dust that has been released from the
two-kilometre "dirty snowball" nucleus. In this combined image, the stars in
the field are seen seven times each, reflecting the motion of the comet in
the sky.

At the time of the observations, the comet was about 400 million kilometres
from the Earth and 560 million kilometres from the Sun, moving outwards
between the orbits of Mars and Jupiter. No reports of observations of Comet
Wild 2 have been made since last year and the present image shows the comet
in unprecedented detail at the current large distance. The angular size of
the visible coma is about 20 arcseconds, or 40,000 kilometres (projected).
The magnitude is 17-18, about 50,000 times fainter than what can be
perceived with the unaided eye.

Comet Wild 2 moves around the Sun in an elliptical orbit with a period of
6.4 years. It was discovered by Swiss Astronomer Paul Wild in 1978.
According to the current plan, the STARDUST spacecraft will encounter Comet
Wild 2 in January 2004 and collect dust particles from the coma. At that
time, the comet will be somewhat closer to the Sun and Earth than now. The
STARDUST mission will bring the captured cometary dust particles back to
Earth in January 2006. This will be the first time that a detailed
laboratory analysis of such particles will become possible. It is assumed
that this is original and widely unaltered material from the formation
period of the Sun some 4.6 billion years ago.

Technical information for Photo 28a/98: Combination of seven 2-min R (red)
exposures with the VLT Test Camera on July 25, 1998. The individual frames
were flat-fielded, shifted in order to center the comet and then combined.
The field shown measures 1.3 x 1.3 arcmin. North is up; East is to the left.

                           ESO Press Photo 28b/98 (Comet Wirtanen)
                           [Preview - JPEG: 567 x 800 pix 296k]
                           [High-Res - JPEG: 2130 x 3000 pix - 2.4Mb]

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.

Contrary to the STARDUST mission that is a short fly-by, ROSETTA will
rendez-vous 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
upper left.


This is the caption to ESO PR Photo 28a/98 and ESO PR Photo 28b/98. They are
also available in high-resolution versions. They may be reproduced, if
credit is given to the European Southern Observatory.


Further images of astronomical objects from the VLT UT1 will be published at
irregular intervals.

                 ESO Education & Public Relations Department
           Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany

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