PLEASE NOTE:


*

CCNet DIGEST, 3 August 1998
---------------------------


(1) DON'T WORRY, BE HAPPY: COMETARY IMPACTS WITH EARTH UNLIKELY IN THE
    NEXT 500,000 YEARS
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(2) SPACEGUARD UK PROJECT
    Jonathan TATE <fr77@dial.pipex.com>

(3) WHAT CAUSED THE MASS EXTINCTION AT THE CENOMANIAN-TURONIAN
    BOUNDARY?
    A.C. Kerr, UNIVERSITY OF LEICESTER

(4) THE FRASNIAN-FAMENNIAN BRACHIOPOD EXTINCTION EVENT
    G. Racki, UNIVERSITY OF SLASKI

(5) THE FRASNIAN-FAMENNIAN MASS EXTINCTION
    P. Copper, LAURENTIAN UNIVERSITY

(6) FRASNIAN-FAMENNIAN BIOTIC CRISIS
    G. Racki, SILESIAN UNIVERSITY

(7) ESO PHOTOS OF COMET WILD-2 & COMET WIRTANEN AVAILABLE
    Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>

====================
(1) DON'T WORRY, BE HAPPY: COMETARY IMPACTS WITH EARTH UNLIKELY IN THE
    NEXT 500,000 YEARS

From Andrew Yee <ayee@nova.astro.utoronto.ca>

Ohio State University
 
Contact: Jay Frogel, (614) 292-5651; frogel.1@osu.edu
 
Andrew Gould, (614) 292-1892; gould@astronomy.ohio-state.edu
 
Written by Earle Holland, (614) 292-8384; holland.8@osu.edu
 
7/31/98
 
COMETARY IMPACT WITH EARTH UNLIKELY IN THE NEXT 500,000 YEARS
 
COLUMBUS, Ohio -- Contrary to Hollywood's latest predictions, it is
highly unlikely that a comet will rain death and destruction on the
earth during the next half-million years, according to a new study.
 
Two Ohio State University astronomers reported in Astrophysical Journal
Letters that a new review of the motions of thousands of nearby stars
failed to show any rogue stars capable of pulling comets out of their
orbits and into the earth's path.
 
Jay Frogel and Andrew Gould, professor and associate professor of
astronomy at Ohio State, were looking for evidence of the so-called
"death star" scenario where a passing star might alter the current
orbits of comets near our solar system and send them our way.
 
There is ample evidence both on earth and on other planets, they say,
that shows comets and asteroids have impacted with devastating results.
Two new movies -- "Deep Impact" and "Armageddon" -- depend on this
premise for their drama. Frogel's interest, however, was spurred by
geological evidence of such past impacts, he says, and not by the new
movies.
 
He and Gould turned to a relatively new resource to conduct their search --
the HIPPARCOS catalogue. In 1989, the European Space Agency launched the
HIPPARCOS satellite with its mission to accurately measure the location and
motion of more than 120,000 stars.
 
Astronomers believe a massive cloud of comets -- the Oort Cloud -- lies
as much as 100,000 AUs out from the sun, surrounding our solar system.
(An AU is the distance between the earth and the sun -- approximately
93 million miles.) If a star passed through that cloud, its
gravitational field might nudge a comet out of orbit and towards the
earth.
 
Frogel and Gould looked in the HIPPARCOS Catalogue specifically for
stars with near zero proper motion -- stars that were either coming
directly in our direction, or moving directly away. Any star that had
already passed would appear to be moving directly away.
 
"For all intents and purposes, you should just see a star that appeared
not to be moving at all," Gould said. The one potential candidate the
researchers did find turned out to be a star previously identified by
other scientists. They failed also to find evidence of stars that may
have already passed nearby.
 
Gould's analysis of the HIPPARCOS catalogue showed that it should be
sensitive enough to detect zero proper motion of any stars brighter
than 8th magnitude. Eighth magnitude stars appear about 25 times
fainter than those visible to the naked eye.
 
Gould said that these bright stars are important candidates for the
death star scenario. "They're bright either because they are close by
or because of their size," he said. The larger the star, the greater
it's gravitational effect might be on nearby comets.
 
"We showed that theoretically, about 96 percent of the possible
damaging events (the passing of such stars) should show up in the
HIPPARCOS catalogue," Gould said. They had defined a "damaging event"
as a star passing within 20,000 Aus of the sun.
 
Frogel and Gould are cautious with their predictions -- "We can't
guarantee that a comet won't hit the earth next year." Their analysis
of the catalogue, however, makes it "unlikely that a major (comet)
shower will occur in the next half-million years."
 
Gould said, "The chance that a big enough star to cause significant
damage would go through (our region) in the next 10 million years is
extremely small."
 
Frogel said he and Gould are confident about their analysis of the
HIPPARCOS catalogue. The next step would be to seek a "death star"
candidate among stars that were too faint to be included in HIPPARCOS.
 
Another satellite -- GAIA -- has been proposed by ESA which would
measure the motions of 50 million objects, including stars as faint as
15th magnitude. If approved, GAIA would be launched no sooner than the
year 2009.
 
Some support for this research came from the National Science
Foundation.

======================
(2) SPACEGUARD UK PROJECT

From Jonathan TATE <fr77@dial.pipex.com>

After two years of trying, and having achieved nothing practical, the
time has come for Spaceguard UK to think about doing something useful. 
I have begun to draft a set of options, and would hugely welcome
advice, help ... anything!  The main areas are the basic concepts (are
we barking up the wrong tree?), and costings.  I know that costs are
difficult to estimate, but the people who I am going to tap for cash
are going to ask how much the project is going to cost, and I will have
to have good numbers.  It is no use to say "well.....it could be as
much as .... depending on ....".

I am under no illusions as to the difficulty in raising the necessary
finance, but with the support, passive or active, of the membership, I
am sure that we can at least have a good crack at raising it.  No one
else is in the UK is trying, so we must at least have a go.  The
alternative is the abrogation of our responsibilities to the future, on
a colossal scale. That is, I think that you will agree, unacceptable.

Please be aware, this is FIRST draft of a still-flaky idea.  Only with
your assistance can I get a decent reality check.

Jay
_______________________________________________

DRAFT

SPACEGUARD UK NEO SEARCH PROJECT

INTRODUCTION

While Spaceguard UK has so far not been practically involved in
specific NEO search or follow-up projects, we have been campaigning for
the modification of the UK Schmidt Telescope (UKST) to be used in such
a role.  This project was viable as the UKST was due for
decommissioning in the near future. However, it has recently become
clear that the UK UKST has been "reprieved" and is going to be fitted
with a new Multi Object Spectrograph, called 6DF. It is highly likely
that this will preclude any possibility of using the UKST as part of
the global NEO detection network for at least the next three to five
years.  Consequently, alternative contributions that the UK can make
to the international effort must be considered.

A number of comprehensive and well-documented studies have been
undertaken, mainly in the USA, to determine the requirements for
detection and follow-up programmes.

PRIORITIES

The priorities for any Spaceguard UK project must be:

1.     Detection and cataloguing of all Near Earth Objects.
2.     Detection and classification of potentially threatening objects.
3.     Follow-up astrometric observations.
4.     Physical studies of asteroids and comets.

DEDUCTIONS

*     The only way to achieve priorities 1-3 is to conduct an
observational programme. Without the data resulting from observation,
priority 4 is largely irrelevant. Therefore, at this stage, an
observational programme should be the first priority for Spaceguard UK.

*    The requirements for such a programme are well documented, and
undisputed.

*     The necessary trained personnel to design, build and operate the
required instrumentation exist, but unless their expertise is used,
this resource will be lost.

*     The sole limiting factor is funding.

OPTIONS OPEN

The options open to Spaceguard UK are:

1.     Build, or participate in the building and operating of a new,
dedicated search and follow-up telescope.

2.     Find an existing, available telescope suitable for use in a
detection programme.

3.     Establish a dedicated team to study NEO's, and possibly to
conduct "precovery" searches, using the UKST Plate Library at ROE.

4.     Establish an amateur network for follow up observation.

Option 1 - Build A New, Dedicated Search And Follow-Up Telescope (c. 2-m).

General

Without doubt this is the "Rolls-Royce" option, and the most expensive.
However, a project such as this could be undertaken in collaboration
with other nations or organisations such as the Spaceguard Foundation
who already have plans for a search telescope in Namibia.

Development and building costs will be substantial, and in addition to
the capital costs, running costs would have to be factored into the
budget. It is almost certain that external funding would be required,
probably from private and commercial sponsorship.  However, the
advantages of a purpose built instrument, operating in the most
advantageous location (probably in the Southern Hemisphere) are
considerable.

Given the political and fiscal climate, this option may well be the
most likely to come to fruition, but will depend on adequate funding
being obtained from private and commercial sources.

Project Phases

*     Phase 1 - Groundwork       Year  0 - 0.5

Feasibility Study
Acquisition of "seed" funding
Initial team selection
Initial sponsorship trawl.

*     Phase 2 - Preparation       Y 0.5 - 1

Compilation of a business plan.
Co-ordination with collaborators (if any)
Initial design and detailed costing of the Spaceguard Telescope.
Costing of necessary infrastructure.
Site selection
Personnel Selection

*     Phase 3 - Funding and Support       Y 1 - 2

The search for sponsorship.
Lobbying of national and international organisations for funding and
support.  (Commercial interests, IAU, NASA, UN, DTI, BNSC, PPARC, Academia,
SGF)

*     Phase 4 - Development       Y 2 - 3.5

Hardware and software development
Establishment of link with MPC
Site preparation
Personnel training

*     Phase 5 - Construction       Y 3.5 - 4.5

Construction of necessary infrastructure
Telescope construction
Personnel Training (continues)

*     Phase 6 - Trials and Testing      Y 4.5 - 5.5

Initial operations, trials and testing
Hardware testing and integration
Software validation
System integration

*     Phase 7 - Operations       Y 5.5 ->

Operations
Upgrades

Costs

Capital costs (Telescope and infrastructure)   32,000,000
Capital costs (CCD array, processor)    2,000,000
Running and maintenance costs    450,000 pa

The total cost of a ten-year programme would therefore be 38,500,000

OPTION 2 - Find An Existing Telescope Suitable For Use In A Detection
Programme.

There are a number of possibilities here.  It was hoped that the
so-called "Congo" telescope at Herstmonceux might be suitable, once
refurbished. It is a 38" Schmidt, but its physical condition appears to
be extremely poor, especially the mirror which is now misshapen and
essentially useless. Also at Herstmonceux is a 24" Baker telescope. 
This is not the same as the US Air Force Baker-Nunn instruments, being
British built, but may be similar enough to use as an APT.  The APT
concept, using an original US Baker-Nunn telescope has been
successfully trialed by the University of New South Wales.

Given the likely timeline for Option 1, it may be feasible to consider
the use of the UKST again.  It is possible that the Multi-Object
Spectrograph project will be complete before Spaceguard is ready for
system integration. Should this be the case, the UKST will become the
most powerful search instrument in the world, and give the United
Kingdom a global lead.

It is impossible at this stage to estimate the costs involved in the
re-roling of any available instruments (other than the UKST), but they
are certain to be less than those for Option 1.

Project Phases

*     Phase 1 - Groundwork       Y 0 - 0.5

Initial team selection
Feasibility Study
Initial sponsorship trawl.

*     Phase 2 - Preparation       Y 0.5 - 1

Compilation of a business plan.
Co-ordination with the AAO
Initial design and costing of the Spaceguard CCD system.
Personnel Selection
Costing of necessary support resources (IT, Comms).

*     Phase 3 - Funding and Support       Y 1 - 2

Search for sponsorship.
Lobbying of national and international organisations for funding and
support.  (Commercial interests, IAU, NASA, UN, DTI, BNSC, PPARC,
Academia, SGF)

*     Phase 4 - Development       Y 2 - 3.5

Hardware and software development
Establishment of link with MPC
Personnel training

*     Phase 5 - Trials and Testing      Y 3.5 - 4

Initial operations, trials and testing
Hardware testing and integration
Software validation
System integration

*     Phase 6 - Operations       Y 4 ->

Operations
Upgrades

Costs

The costs described in the original UKST proposal, submitted to PPARC in
September 1997 were as follows:

Capital costs (CCD array, processor)    2,000,000
Running and maintenance costs    450,000 pa

The total cost of a ten-year programme would therefore be 6,500,000.

OPTION 3 - Establish A Dedicated Team To Study NEO's, And To Conduct
"Precovery" Searches, Using The UKST Plate Library At ROE.

There is an essential requirement to study of the physical and dynamic
properties of asteroids and comets, with particular emphasis on Near
Earth Objects. There is already substantial work being done in this
field in the UK, but the creation of a study team dedicated to the
investigation of NEO's, and possibly to utilise the UKST Plate Library
at ROE for "precovery" work would enhance worldwide knowledge about the
threat and possible countermeasures.

Elements of this option will almost certainly be an adjunct to Options
1 and 2, as any search and/or follow-up instrument will require
operational and analytical staff.

Project Phases

*     Phase 1 - Groundwork       Y 0 - 0.5

Initial team selection
Feasibility Study
Initial sponsorship trawl.

*     Phase 2 - Preparation       Y 0.5 - 1

Compilation of a business plan.
Operational Personnel Selection
Costing of necessary support resources (IT, Communications).

*     Phase 3 - Funding and Support       Y 1 - 2

Search for sponsorship.
Lobbying of national and international organisations for funding and
support (Commercial interests, IAU, NASA, UN, DTI, BNSC, PPARC,
Academia, SGF)

*     Phase 4 - Development       Y 2 - 2.5

Hardware and software development
Establishment of link with MPC
Personnel training

*     Phase 5 - Operations       Y 2.5 ->

Operations
Upgrades

Costs

Project costs will depend on the size of team chosen, and the
requirement for hardware and support.  It is unlikely that costs would
exceed:

Set-up costs     300,000
Running costs     400,000 pa

OPTION 4 - Establish An Amateur Network For Follow Up Observation

Amateurs around the world already do much of the follow-up
observational work. However, the task requires dedication, and a
certain amount of fairly expensive equipment. It is unlikely that it
would be possible to establish an effective network in the UK for an
extended period of time.

DRAFT

==================
(3) WHAT CAUSED THE MASS EXTINCTION AT THE CENOMANIAN-TURONIAN
    BOUNDARY?

A.C. Kerr: Oceanic plateau formation: a cause of mass extinction and
black shale deposition around the Cenomanian-Turonian boundary? JOURNAL
OF THE GEOLOGICAL SOCIETY, 1998, Vol.155, No.Pt4, pp.619-626

UNIVERSITY OF LEICESTER, DEPT GEOL, UNIV RD, LEICESTER LE1 7RH, LEICS,
ENGLAND

The Cenomanian-Turonian boundary (90.4 Ma) represents a major period of
worldwide environmental disturbance. The physical manifestations of
this are: elevated atmospheric and oceanic temperatures; a significant
sea-level transgression; and a period of widespread anoxia, leading to
the formation of oceanic black shales, and the extinction of 26% of all
genera. Elevated delta(13)C values and enrichment of trace elements in
Cenomanian-Turonian boundary sediments, combined with a reduction in
Sr-87/Sr-86, also imply a severe environmental perturbation. At this
time oceanic crustal production rates reached their highest level of
the last 100 million years. This was principally caused by extensive
melting of hot mantle plumes at the base of the oceanic lithosphere,
and the development of vast areas (up to 1 x 10(6) km(2)) of thickened
oceanic crust in the Pacific and Indian Oceans. The anomalous volcanism
associated with the formation of these oceanic plateaux may have been
responsible for the environmental disturbances c. 90 Ma. These
eruptions would also have resulted in the emission of large quantities
of CO2 into the atmosphere, leading to global warming. Additionally,
the emission of SO2, H2S, CO2 and halogens into the oceans would have
made seawater more acidic resulting in the dissolution of carbonate,
and further release of CO2. This run-away greenhouse effect was
probably put into reverse, by the decline of the anomalous Volcanic
activity, and by increased (CO2-driven) productivity in oceanic surface
waters, leading to increased organic carbon burial, black shale
deposition, anoxia and mass extinction in the ocean basins. Copyright
1998, Institute for Scientific Information Inc.

=======================
(4) THE FRASNIAN-FAMENNIAN BRACHIOPOD EXTINCTION EVENT

G. Racki: The Frasnian-Famennian brachiopod extinction events: A
preliminary review. ACTA PALAEONTOLOGICA POLONICA, 1998, Vol.43, No.2,
pp.395-411

UNIVERSITY OF SLASKI, KATEDRA PALEONTOL & STRATYG, UL BEDZINSKA 60,
PL-41200 SOSNOWIEC, POLAND

Preliminary review of taxonomy of the brachiopod order Atrypida and its
stratigraphic distribution in the late Frasnian Kellwasser Crisis of
several regions of Laurussia, western Siberia and South China point to
their moderate diversity and stepdown but irregular extinction pattern.
The distinctive character of the late Frasnian atrypid fauna is
emphasised by several relict genera, marked by recurrent and possibly
aberrant characters (mainly in ornamentation types), tendency to size
reduction and homeomorphy in some taxa. The transgressive/hypoxic Lower
Kellwasser Event and preceding eustatic changes during the Palmatolepis
rhenana Zone had only a regional destructive effect, and were linked
rather to an enhanced dispersal of the last generic set of atrypids.
The Variatrypinae, Spinatrypinae and Iowatrypa-group seem to belong to
the latest surviving atrypids. The final demise of the remaining
atrypids (and some other articulate brachiopods, e.g., gypidulids)
coincided with the transgressive/hypoxic Upper Kellwasser Event,
followed by catastrophic eustatic fall during the late Palmatolepis
linguiformis Zone (F-F Event). This was probably exacerbated by
accelerated submarine volcano-hydrothermal activity, and consequent
progressive regional eutrophication, and climatic destabilization. The
level-bottom rhynchonellid-inarticulate biofacies crosses the fatal F-F
boundary horizon without major changes. No reliable data exist for the
presence of atrypids in the Famennian survival and recovery biota, even
for the smooth lissatrypid Peratos. Sustained competition from
radiating and diversifying productid-cyrtospirifrid-athyrid faunas may
have provide an additional biotic factor in the collapse of the
Frasnian shelly benthos at the time of stress, as well as in a
post-extinction offshore repopulation from inner shelf habitats.
Copyright 1998, Institute for Scientific Information Inc.

======================
(5) THE FRASNIAN-FAMENNIAN MASS EXTINCTION

P. Copper: Evaluating the Frasnian-Famennian mass extinction: Comparing
brachiopod faunas. ACTA PALAEONTOLOGICA POLONICA, 1998, Vol.43, No.2,
pp.137-154

LAURENTIAN UNIVERSITY, DEPT EARTH SCI, SUDBURY, ON P3E 2C6, CANADA

The Frasnian-Famennian (F-F) mass extinctions saw the global loss of
all genera belonging to the tropically confined order Atrypida (and
Pentamerida): though Famennian forms have been reported in the
literature, none can be confirmed. Losses were more severe during the
Givetian (including the extinction of the suborder Davidsoniidina, and
the reduction of the suborder Lissatrypidina to a single genus), but
origination rates in the remaining suborder surviving into the Frasnian
kept the group alive, though much reduced in biodiversity from the late
Early and Middle Devonian. In the terminal phases of the late
Palmatolepis rhenana and P. linguiformis zones at the end of the
Frasnian, during which the last few Atrypidae declined, no new genera
originated, and thus the Atrypida were extirpated. There is no evidence
for an abrupt termination of all lineages at the F-F boundary, nor that
the Atrypida were abundant at this time, since all groups were in
decline and impoverished. Atrypida were well established in dysaerobic,
muddy substrate, reef lagoonal and off-reef deeper water settings in
the late Givetian and Frasnian, alongside a range of brachiopod orders
which sailed through the F-F boundary: tropical shelf anoxia or hypoxia
seems implausible as a cause for atrypid extinction.
Glacial-interglacial climate cycles recorded in South America
for the Late Devonian, and their synchronous global cooling effect in
low latitudes, as well as loss of the reef habitat and shelf area
reduction, remain as the most likely combined scenarios for the mass
extinction events. Copyright 1998, Institute for Scientific Information
Inc.

=========================
(6) FRASNIAN-FAMENNIAN BIOTIC CRISIS

G. Racki: Frasnian-Famennian biotic crisis: undervalued tectonic
control? PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY, 1998,
Vol.141, No.3-4, pp.177-198

SILESIAN UNIVERSITY, DEPT EARTH SCI, PL-41200 SOSNOWIEC, POLAND

The prime cause of the Late Devonian Kellwasser crisis, culminating in
a mass extinction event near the Frasnian-Famennian (F-F) boundary,
remains conjectural. Nevertheless, rapid sea-level fluctuations of
uncertain origin during tectono-eustatic highstand, paired with
repeated oceanic anoxia and climatic changes, are usually thought at
present to be one of the main immediate triggers. The Cathles-Hallam
model of stress-induced changes in plate density, accompanying rapid
rift formation, furnishes an alternative for understanding the
enigmatic sudden eustatic variations in the non-glacial time. Late
Devonian tectonic extension, causing rifting and volcanicity, appears
to be strongly marked in several regions of Eurasia, particularly in
Kazakhstan and eastern Laurussia. All larger Devonian continents were
more or less tectonically affected. A subtle record of this tectonic
rearrangement is implied even for distant and apparently quiet
carbonate platforms in local extensional block faulting and tilting,
hydrothermal mineralisation, geochemical anomalies, and localized
blooms of siliceous biota. Interpreting the late Frasnian
regressive-transgressive pattern in terms of the Cathles-Hallam
tectono-eustatic model, two major rifting events are hypothesized: one
at about the beginning of the rhennna Zone, and a second principal
pulse in the late linguiformis Zone, that encompassed the F-F
transition. Developing the Veimarn-Milanovsky scenario of the global
extensional pulse, it is assumed that the key endogenous factors were
related to episodic (super)plume activity. The tectonically triggered
changes climaxed in thermal and nutrient pulses, and induced the
stepdown ecosystem destabilization observed in the F-F bio-crisis.
Minor cometary strike(s) might have eventually participated in this
prolonged multicausal environmental stress, mainly due to additional
thermal shocks, but perhaps effective on a regional scale only. (C)
1998 Elsevier Science B.V. All rights reserved.

======================
(7) ESO PHOTOS OF COMET WILD-2 & COMET WIRTANEN AVAILABLE

From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
 
ESO Press Photos 28a-b/98
31 July 1998                                                        
For immediate release
http://www.eso.org/outreach/press-rel/pr-1998/phot-28-98.html

------------------------------------------------------------------------
 
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
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 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.
 
------------------------------------------------------------------------

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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