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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