[meteorite-list] ESO's VLT Takes Images of Disintegrating Comet P73/Schwassmann-Wachmann 3

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Apr 26 14:10:26 2006
Message-ID: <200604261514.IAA20065_at_zagami.jpl.nasa.gov>

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO
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http://www.eso.org/outreach/press-rel/pr-2006/pr-15-06.html
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For immediate release: 25 April 2006

ESO Press Photo 15/06

The Comet With a Broken Heart

VLT Takes Images of Disintegrating Comet P73/Schwassmann-Wachmann 3

On the night of April 23 to 24, ESO's Very Large Telescope observed
fragment B of the comet Schwassmann-Wachmann 3 that had split a
few days earlier. To their great surprise, the ESO astronomers
discovered that the piece just ejected by fragment B was splitting
again! Five other mini-comets are also visible on the image. The
comet seems thus doomed to disintegrate but the question remains
in how much time.

Comet P73/Schwassmann-Wachmann 3 (SW 3) is a body with a very
tormented past. This comet revolves around the Sun in about 5.4
years, in a very elongated orbit that brings it from inwards of
the Earth's orbit to the neighbourhood of giant planet Jupiter.
In 1995, when it was coming 'close' to the Earth, it underwent a
dramatic and completely unexpected, thousand-fold brightening.
Observations in 1996, with ESO's New Technology Telescope and
3.6-m telescope, at La Silla, showed that this was due to the
fact that the comet had split into three distinct pieces. Later,
in December 1996, two more fragments were discovered. At the last
comeback, in 2001, of these five fragments only three were still
seen, the fragments C (the largest one), B and E. No new
fragmentations happened during this approach, apparently.

Things were different this time, when the comet moved again
towards its closest approach to the Sun -- and to the Earth. Early
in March, seven fragments were observed, the brightest (fragment
C) being of magnitude 12, i.e. 250 fainter than what the unaided
can see, while fragment B was 10 times fainter still. In the
course of March, 6 new fragments were seen.

Early in April, fragment B went into outburst, brightening by a
factor 10 and on 7 April, six new fragments were discovered,
confirming the high degree of fragmentation of the comet. On 12
April, fragment B was as bright as the main fragment C, with a
magnitude around 9 (16 times fainter than what a keen observer
can see with unaided eyes). Fragment B seems to have fragmented
again, bringing the total of fragments close to 40, some being
most probably very small, boulder-sized objects with irregular
and short-lived activity.

The new observations reveal that this new small fragment has split
again! The image clearly reveals that below the main B fragment,
there is a small fragment that is divided into two and a careful
analysis reveals five more tiny fragments almost aligned. Thus,
this image alone shows at least 7 fragments. The comet has produced
a whole set of mini-comets!

Will the process continue? Will more and more fragments form and
will the comet finally disintegrate? How many new fragments will
have appeared before the comet reaches its closest approach to the
Sun, around 7 June, and how bright will they be when the comet
will be the closest to the Earth, on 11 to 14 May?

Fragment C of the comet should be the closest to Earth on 11 May,
when it will be about 12 million km away, while fragment B will
come as 'close' as 10 million km from Earth on 14 May. Although
this is the closest a comet ever approached Earth in more than
twenty years -- even Comet Hyakutake's smallest distance was 15
million km -- this is still 26 times the distance between the Earth
and the Moon and therefore does not pose any threat to our planet.

If nothing else happens, at the time of closest approach, fragment
B will be just visible with unaided eye by experienced observers.
It should be an easy target however to observe with binoculars. If
we are lucky, however, fragment B presents another outburst,
becoming a magnificent sight in the night sky. On the other hand,
it could just as well fade away into oblivion. But then, the main
fragment C should still be visible, even possibly with the unaided
eye.

ESO telescopes will observe the comet in the greatest detail at
the end of May, when it is best observed from Chile and is brighter.
These observations will obtain invaluable information, especially
as the fragmentation process is revealing all the pristine material
buried below the crust of the comet. As such, these observations
will prove an ideal complement to the most comprehensive
observation campaign made with ESO telescopes of Comet Tempel 1
when it was being bombarded by the Deep Impact spacecraft, on
4 July 2005.

High resolution images and their captions are available at
     http://www.eso.org/outreach/press-rel/pr-2006/phot-15-06.html

Notes

About 30 comets have been observed to split in historical times
and this process is almost always accompanied by a significant
brightening. For instance, the nucleus of comet Shoemaker-Levy 9
broke up into at least 21 individual pieces when it passed very
close to Jupiter on July 8, 1992; this was the reason that it became
bright enough to be detected some eight months later. In the case of
SW-3, the opening of rifts and the subsequent splitting took place
far from any planet and must in some way have been caused by
increased solar heating. It is possible that major cracks and
rifts opened in the irregularly shaped icy nucleus already before
perihelion as the surface temperature began to increase. Completely
"fresh" cometary material was thereby exposed to the solar light
and the evaporation rate increased quickly, releasing more gas and
dust into space. In the course of this process, the rifts gradually
widened until the definitive breakage occurred somewhat later.

Comet Schwassmann-Wachmann 3 was discovered on May 2, 1930, on a
photographic plate obtained at the Hamburg Observatory (Germany)
by two astronomers at this institution, Arnold Schwassmann and
Arthur Arno Wachmann. The subsequent observations showed that the
comet moved in an elliptical orbit with a revolution period of
somewhat more than 5 years. Great efforts were expended to observe
the comet during the next returns, but it was not recovered until
nearly 50 years and eight revolutions later, when its faint image
was found of a plate obtained in August 1979 with a telescope at
the Perth Observatory in Western Australia. It was missed in 1984,
but was sighted again in 1989 and since then, it is observed at
each close approach. Thus this comet has only been observed during
six out of fifteen approaches since 1930. While this may be partly
due to a less advantageous location in the sky at some returns, it
is also a strong indication that the comet behaves unpredictably
and must have a quite variable brightness. Orbital calculations
have shown that it was inserted into the present, short-period
orbit by the strong gravitational pull of Jupiter during several,
relatively close encounters with this giant planet. For instance,
it passed Jupiter at a distance of about 30 million kilometres in
1882 and 1894, and again at 40 million kilometres in 1965. SW-3
belongs to the so-called "Jupiter family" of comets.

The observations were done with the FORS1 multi-mode instrument on
Kueyen, the second 8.2-m Unit Telescope of the Very Large Telescope
located at Cerro Paranal (Chile). The fragment was observed in four
bands (B, V, R, and I) for a total of 30 minutes by Emmanuel Jehin,
Olivier Hainaut, Michelle Doherty, and Christian Herrera, all from
ESO. The astronomers had the telescope track the comet, which
explains why the stars appear as trails of coloured dots, each
colour corresponding to the order in which the observations were
done in the various filters. At the time of the observations, the
comet was 26.6 million km away from the Earth, in the constellation
Corona Borealis. The seeing was 1.5" as the comet was observed when
it was rather low on the horizon as seen from Paranal. The final
processing of the image was done by Haennes Heyer and Olivia
Blanchemain (ESO).

More observations of the comet are foreseen from 20 till 30 May,
with ESO telescopes on the three sites of La Silla (NTT, 3.6m),
Paranal (VLT) and Chajnantor (APEX). These observations will study
aspects as diverse as the presence of organics in the dust, the
composition of this dust, the structure of the coma, and the
presence of deuterated water.

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Riitta Tirronen, +358 9 7748 8369
Denmark: Dr. Michael Linden-V?rnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, benacchio _at_ inaf.it
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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Received on Wed 26 Apr 2006 11:14:34 AM PDT