SETI bioastro: Fw: VLT-Wirtanen-Rosetta - Press Conference - ESO Outreach Network - Planetarium

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Date: Tue Feb 26 2002 - 07:39:23 PST

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Sent: Tuesday, February 26, 2002 10:04 AM
Subject: VLT-Wirtanen-Rosetta - Press Conference - ESO Outreach Network - Planetarium Show Project

Dear subscribers,

today's photo release is concerned with a small solar system object, the
1-km icy nucleus of Comet Wirtanen - the target of the Rosetta mission
of the European Space Agency (ESA). It has been observed with the ESO
VLT at Paranal to investigate the conditions to be expected when the
spacecraft arrives at the comet in 2011. Look at:

The following news items are of particular interest to representatives
of the media and the planetaria.

A Press Conference will be held in Garching (near Munich, Bavaria,
Germany) on Thursday, March 7, 2002, 12:15 - 13:00 CET, on the occasion
of the ESO-CERN-ESA Symposium on Astronomy, Cosmology and Fundamental
Physics. More information is available at:

The ESO EPR Dept. has established a network of persons in the ESO Member
Countries who serve as local contacts for the media in connection with
ESO developments, Press Releases, etc.. At the same time, they may help
to provide useful contacts between the media and the scientists in their
area. See:

Please be informed that ESO and the French Planetarium Association,
APLF, are currently preparing a multi-language planetarium show with the
working title "Secrets of the Southern Sky", to be launched in October
2002 in connection with the 40th Anniversary of ESO. For further
information about this ambitious project (and to learn how other
planetaria may join), please contact Prof. A. Acker, 11 Rue de
l'Universite, F-67000 Strasbourg, Tel.+33-3-90-242455, email:

Kind regards,

The ESO EPR Dept.


             Information from the European Southern Observatory

ESO Press Photos 06a-b/02

26 February 2002 [ESO]

For immediate release

ESO's VLT Helps ESA's Rosetta Spacecraft Prepare to Ride on a Cosmic Bullet

New Images of Comet Wirtanen's Nucleus [1]


New images of Comet Wirtanen's 1-km 'dirty snowball' nucleus have been
obtained with the ESO Very Large Telescope at Paranal (Chile). They show
this object at a distance of approx. 435 million km from the Sun, about
the same as when the Rosetta spacecraft of the European Space Agency (ESA)
arrives in 2011.

The new observations indicate that the comet has a very low degree of
activity at this point in its orbit - almost no material is seen around
the nucleus. This means that there will not be so much dust near the
nucleus as to make the planned landing dramatically difficult.

PR Photo 06a/02: The Nucleus of Comet Wirtanen (composite photo).
PR Photo 06b/02: Comet Wirtanen's motion in the sky (animated).

A distant target

  [ESO PR Photo 06a/02] ESO PR Photo [ESO PR Photo 06b/02] ESO PR Photo
                         06a/02 06b/02

[Preview - JPEG: 400 x 445 pix - [Animated GIF: 400 x 420 pix - 312k]
[Normal - JPEG: 800 x 890 pix -

Caption: PR Photo 06a/02 shows a (false-colour) composite image of the
nucleus of Comet Wirtanen (the point of light at the centre), recorded on
December 9, 2001, with the FORS2 multi-mode instrument at the 8.2-m VLT
YEPUN Unit Telescope. It is based on four exposures and since the
telescope was set to track the motion of the comet in the sky, the images
of stars in the field are seen as four consecutive trails. The measured
brightness and the fact that the image of the comet's 'dirty snowball'
nucleus is almost star-like indicates that it is surrounded by a very
small amount of gas or dust. The diameter of the nucleus is about 1 km and
the distance to the comet from the Earth was approx. 534 million km. In PR
Photo 06b/02, the four exposures have been combined to show the motion of
the comet during the four exposures. Technical information about the
photos is available below.

Chase a fast-moving comet, land on it and 'ride' it while it speeds up
towards the Sun: not the script of a science-fiction movie, but the very
real task of ESA's Rosetta spacecraft. New observations with the ESO Very
Large Telescope (VLT) provide vital information about Comet Wirtanen -
Rosetta's target - to help ESA reduce uncertainties in the mission, one of
the most difficult ever to be performed.

Every 5.5 years Comet Wirtanen completes an orbit around the Sun. Wirtanen
has been seen during several apparitions since its discovery in 1948, but
only recently have astronomers obtained detailed observations that have
allowed them to estimate the comet's size and behaviour, cf. ESO PR Photos

The most recent of these observations was performed in December 2001 with
the ESO VLT at the Paranal Observatory in Northern Chile, cf. PR Photos
06a-b/02, reproduced here. As a result of these observations ESA will be
able to refine plans for its Rosetta mission.

Good news for Rosetta

Rosetta will be launched next year and it will reach Comet Wirtanen in 2011.
By that time the comet will be nearly as far from the Sun as Jupiter,
charging headlong towards the inner Solar System at speeds of up to 135,000
km/h. To get there and to be able to match the comet's orbit, Rosetta will
need to be accelerated by several planetary swing-bys, after which the
spacecraft - following a series of difficult manoeuvres - will get close to
the comet, enter into orbit around it and release a lander from a height of
about 1 km.

The VLT observations were planned specifically to investigate the 'activity'
of Wirtanen at about the same solar distance as at the time of the landing
manoeuvres. Because of this timing requirement, they had to be carried out
at a certain moment - unfortunately, when the comet was low in the twilight
evening sky and descending rapidly towards the western horizon. However,
even though the exposures therefore had to be quite short, the VLT with its
superb light-gathering capability and opto-mechanical perfection was still
able to produce excellent images of this rather faint, moving object (about
6 million times fainter than what can be perceived with the unaided eye).

These observations have now confirmed that - at the same distance from the
Sun at which the landing will take place (about 450 million km from the Sun)
- the activity on Wirtanen is very low, cf. PR Photo 06a/02.

This is very good news for the mission, because it means that there will not
be so much dust near the nucleus as to make the landing dramatically

Landing on a 1-km snowball

Cometary nuclei are small frozen bodies made of ice and dust ('dirty
snowballs'). When they get close to the Sun the heat causes ices on the
surface to 'evaporate'. Gas and dust grains are ejected into the surrounding
space forming the comet's atmosphere (coma) and the tail.

In addition to dropping a lander on Wirtanen's nucleus for detailed in-situ
observations, Rosetta's task is to investigate the evolution of the comet on
its way to the Sun: in fact, Rosetta will keep orbiting around Wirtanen up
to the end of the mission in July 2013, at which time the comet is at its
closest approach to the Sun, at about 160 million km from it.

These and earlier VLT observations have also provided Rosetta mission
planners with an accurate measurement of their target's size: Wirtanen's
nucleus is only 1.2 km in diameter, a true cosmic bullet.

"Rosetta is certainly a very challenging space mission. No one has ever
tried to land on a comet before," says Gerhard Schwehm, Rosetta's Project
Scientist. "We need to learn as much as possible about our target. The new
VLT data will allow us to improve our models and make decisions once we get

"It is a pleasure to help our colleagues at ESA", says ESO astronomer and
comet specialist Hermann Boehnhardt. "We will continue to keep an eye on
this comet, in particular when Rosetta is approaching its target. We can
then provide the spacecraft controllers and the astronomers with very
useful, regular updates, e.g., about the 'cometary weather' at the time of

More about Rosetta

Rosetta's prime scientific goal is to unravel the origin of the Solar
System. The chemical composition of comets is known to reflect that of the
primordial nebula that gave birth to the Solar System - in the planets, that
primeval material has gone through complex processing, but not in the
comets. Therefore, Rosetta will allow scientists to look back 4.6 billion
years, to an epoch when the Solar System formed.

Previous studies by ESA's Giotto spacecraft and by ground-based
observatories have shown that comets contain complex organic molecules -
compounds that are rich in carbon, hydrogen, oxygen and nitrogen.
Intriguingly, these are the elements which make up nucleic acids and amino
acids, essential ingredients for life as we know it. Did life on Earth begin
with the help of comet seeding? Rosetta may help us to find the answer to
this fundamental question.

Rosetta carries 21 experiments in total. These are provided by scientific
consortia from institutes across Europe and the United States.

The Wirtanen observations by the VLT fall into a tradition of fruitful
collaboration between the European Space Agency (ESA) and the European
Southern Observatory (ESO). The two organizations, both members of the
EIROFORUM collaboration (ESO PR 12/01), are already combining their efforts
in several strategic areas, in order to facilitate the synergy between space
and ground facilities, where mutual sharing of technology and procedures can
result in substantial gains and savings.


[1]: This is a joint Press Release by ESO and ESA. The ESA release is
available at this website.


Hermann Boehnhardt
ESO Chile

Gian-Paolo Tozzi
Osservatorio Astronomico di Arcetri
Firenze, Italy
Tel: +39-055-2752-252

ESA - Communication Department
Media Relations Office
Paris, France
Tel: +33(0)1-53-69-7155
Fax: +33(0)1-53-69-7690

Rita Schulz
Rosetta deputy project scientist
Noordwijk, The Netherlands
Tel: +31 71 565 4821

Technical information about the photos

PR Photo 06a/02 is based on four exposures obtained with the FORS2
multi-mode instrument at the 8.2-m VLT YEPUN telescope in the evening
twilight on December 8, 2001 (local time). The comet was located in the
southern constellation of Sagittarius, rather low over the western horizon.
The first two exposures were made through a standard R-filter and lasted 300
sec each (airmass 2.4 - 2.5); they were followed by two V-exposures of 250
(airmass 2.7) and 200 (2.8) sec, respectively. The seeing at this low
altitude was 1.0 - 1.2 arcsec. The measured magnitudes of the nucleus were R
= 22.6 and V = 23.4 (both +- 0.1). North is up and east is to the left. The
field in Photo 06a/02 measures approx. 2.0 x 2.0 arcmin2; 1 pixel = 0.2

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


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