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CCNet DIGEST, 6 May 1999
------------------------

(1) ASTEROIDS CLOSE TO THE EARTH?
    NATURE News Service

(2) POSSIBLE LONG-LIVED ASTEROID BELTS IN THE SOLAR SYSTEM
    NATURE, 6 May 1999

(3) DO UNSEEN ASTEROID BELTS STILL LURK?
    MSNBC Space News

(4) A LINEAR MODEL FOR THE YARKOVSKY EFFECT
    D. Vokrouhlicky, CHARLES UNIVERSITY

(5) SPACEGUARD TALK DOWN UNDER
    Michael Paine <mpaine@tpgi.com.au>

(6) COLLISIONS BETWEEN GALAXIES MORE COMMON THAN THOUGHT
    UniSci, 5 May 1999

(7) TURN LEFT AT CALLISTO
    NASA Science News <expressnews@sslab.msfc.nasa.gov>

(8) US SPACE ROCKETS GROUNDED
    BBC Online Network, 6 May 1999


=================
(1) ASTEROIDS CLOSE TO THE EARTH?

From NATURE News Service
[http://helix.nature.com/nsu/990506/990506-2.html]

By HENRY GEE

When astronauts set off for Mars, they might have to navigate through a
hitherto undiscovered belt of asteroids. A computer study by two
UK-based researchers suggests that there could be a narrow ring of
around 1,000 asteroids, left over from the birth of the Solar System
4.5 billion years ago, in stable, circular orbits around the Sun, just
beyond the Earth's own orbit. These remnant asteroids could since have
been joined by bodies ejected from the 'main belt' of asteroids between
Mars and Jupiter.

The researchers, N. Wyn Evans and Serge Tabachnik of Oxford University,
present their study in the 6 May issue of the science magazine Nature.
They were following up a slew of recent discoveries of asteroids or
'minor planets' throughout the Solar System.

Ceres, the first known member of the main belt of asteroids between
Mars and Jupiter, was discovered in 1801: thousands have been added to
the tally since then, though many asteroids range widely throughout the
Solar System, from beyond Saturn to well within the orbit of the Earth.
Some of these latter 'near-Earth objects' or NEOs may be cause for
concern, as they would cause enormous devastation if they hit the
Earth.

However, astronomers have long realized that many asteroids outside the
main belt could represent distinct populations of bodies -- in other
words, there could be more than one distinct asteroid belt. Over the
past decade, for example, an increasing number of small, icy bodies has
been found orbiting in the 'Kuiper belt', a broad swath in the frigid
wastes beyond the orbit of Neptune. More recently, computer simulations
have predicted that asteroids could be oribiting between Neptune and
Uranus.

Computer simulations are heavy-duty games of planetary billiards, in
which astronomers seek to find regions of the Solar System in which
small bodies could orbit stably for hundreds of millions of years,
without being deflected by the gravitational fields of the larger
planets, in particular the giant planet Jupiter. The Kuiper belt beyond
Neptune is one such zone, as is the zone between Uranus and Neptune
and, of course, the main belt itself between Mars and Jupiter. Evans
and Tabachnik wondered if there might be zones of stability closer to
home, in the inner Solar System.

Using a series of 20 personal computers working full-time for more than
four months, the researchers showed that there could be asteroids
between all the inner planets in orbits stable for periods of around
100 million years. But this represents only two per cent of the entire
history of the Solar System. Running their simulation to represent the
entire history of the Solar System would take almost 16 years of
computer time, so the researchers had to extrapolate as best they
could. Once this was done, they identified two zones of extraordinary
stability. One was just outside the Earth's orbit. No known asteroid
has been definitely shown to occupy a circular, non-Earth-crossing
orbit in this region, although current catalogues hold three
candidates, all discovered since 1996.

The other zone of stability lies in the super-hot zone between Mercury
and the Sun, where asteroids -- presumably fried to cinders -- could
have lain undetected since the birth of the Solar System. Detecting
these so-called 'Vulcanoids' would be very hard, though they might be
revealed through their copious emission of infra-red radiation -- heat.
Searches for Vulcanoids have so far found nothing, but they have
hitherto been rather limited in scope.

Macmillan Magazines Ltd 1999 - NATURE NEWS SERVICE

==================
(2) POSSIBLE LONG-LIVED ASTEROID BELTS IN THE SOLAR SYSTEM

From NATURE, 6 May 1999
[http://www.nature.com/server-java/Propub/nature/399041A0.abs_frameset]

N. WYN EVANS AND SERGE TABACHNIK
 
Recent years have seen the discovery of several objects in stable
orbits in the outer Solar System; these bodies include objects in the
Kuiper belt (also known as the Kuiper-Edgeworth belt) as well as the
Centaurs. Moreover, another region of orbital stability has been
identified between the orbits of Uranus and Neptune. Here we report
evidence from numerical simulations of zones of orbital stability in
the inner Solar System. We find that there are two possible long-lived
belts of asteroids. The first region lies between the Sun and Mercury,
in the range 0.09-0.21 astronomical units, where remnant planetesimals
may survive for the age of the Solar System provided that their radii
are greater than ~0.1 kilometres. The second region of stability is
between Earth and Mars (range 1.08-1.28 astronomical units), where a
population of bodies that are on circular orbits may survive. A search
through the catalogues of near-Earth objects reveals an excess of
asteroids with low eccentricities and inclinations occupying this
latter region: several examples are the recently discovered objects
1996 XB27, 1998 HG49 and 1998 KG3.

Copyright 1999, NATURE

===============
(3) DO UNSEEN ASTEROID BELTS STILL LURK?

From MSNBC Space News
http://www.msnbc.com/news/265993.asp

Two extra rings are theoretically possible, researchers say

By Alan Boyle, MSNBC

May 5— Researchers say our solar system could accommodate two 
as-yet-undetected rings of space rocks: one between Earth and Mars, the
other between Mercury and the sun. If such asteroid belts really exist
and harbor large objects, they could represent a new worry — but
asteroid-watchers say they’ve seen no sign of the belts.

FULL STORY at http://www.msnbc.com/news/265993.asp

===================
(4) A LINEAR MODEL FOR THE YARKOVSKY EFFECT
 
D. Vokrouhlicky: A complete linear model for the Yarkovsky thermal
force on spherical asteroid fragments. ASTRONOMY AND ASTROPHYSICS,
1999, Vol.344, No.1, pp.362-366

CHARLES UNIVERSITY, ASTRON INST,CZ-18000 PRAGUE 8,CZECH REPUBLIC

A linear theory for heat conduction in a spherical, solid and rotating
body illuminated by solar radiation is developed. The recoil force due
to the thermally re-emitted radiation by the surface of the body is
computed, including all the terms depending both on the body's rotation
frequency and the mean motion of its revolution about the Sun. The
present solution thus overcomes a drawback of the previous approaches,
which have been tailored separately either to the diurnal or to the
seasonal variant of the so-called Yarkovsky effect, corresponding to
different Limiting cases of the current theory. We pay a special
attention to compute the secular effects on the semimajor axis of the
body's orbit about the Sun. The results from the general model coincide
with those of the previous approaches to a high level of accuracy, as
the relative size of the additional ''mixed'' terms is smaller than
10(-3) for plausible parameter choices. This confirms that the use of
the simplified formulae is warranted in the relevant Solar System
applications. Copyright 1999, Institute for Scientific Information Inc.

===================
(5) SPACEGUARD TALK DOWN UNDER

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny,
 
I have just returned from Canberra where I helped AAO astronomer Ken
Russell deliver a talk on the asteroid threat as part of the Australian
Science Festival. It was very well attended and has helped to generate
more interest in the nation's capital. I also stood in for Ken during a
TV news interview which was broadcast nationally (probably got a whole
ten seconds of air time!).
 
Following this attention we are about the lobby the Australian
politicians once again so any contribution by CCNet subscribers would
be appreciated (approaches from the governments/departments of other
countries make a good impression!).
 
We are thinking of a new motto for the Australian asteroid search
"Cover your ass" with reference to:

* the lack of major NEO search program "down under",  
* a preoccupation of politicians
* the (unfortunate) acronym of Australian Spaceguard Survey (which 
  is an unofficial title) :)
 
By an extraordinary coincidence, Ken's talk was on the same day as
Duncan Steel's talk in Cardiff UK (CCNET Digest 5 May 1999). Ken and
Duncan worked on NEO detection in Australia until the program was cut in
1996.
 
Michael Paine
The Planetary Society Australian Volunteers

=====================
(6) COLLISIONS BETWEEN GALAXIES MORE COMMON THAN THOUGHT

From UniSci, 5 May 1999
http://unisci.com/stories/19992/0505991.htm

Astronomers compiling a catalog of spiral galaxies have discovered that
collisions between such galaxies, as well as near-collisions, are more
common than had been thought.

FULL STORY at http://unisci.com/stories/19992/0505991.htm

================
(7) TURN LEFT AT CALLISTO

From NASA Science News <expressnews@sslab.msfc.nasa.gov>

NASA Space Science News for May 5, 1999
 
Turn Left at Callisto: This morning NASA's Galileo spacecraft zoomed 
past Jupiter's moon Callisto. The maneuver was designed to bring
Galileo closer to Jupiter in preparation for a daring encounter with a
volcano on Io.
 
FULL STORY at http://science.nasa.gov/newhome/headlines/ast05may99_1.htm

=======================
(8) US SPACE ROCKETS GROUNDED

From the BBC Online Network, 6 May 1999
http://news.bbc.co.uk/hi/english/sci/tech/newsid_336000/336592.stm

All United States rockets capable of launching large satellites are
grounded as an investigation begins into a series of launch failures.

The latest launch to misfire was a Delta III carrying a communications
satellite. It was an unprecedented sixth failure in nine months.

FULL STORY at
http://news.bbc.co.uk/hi/english/sci/tech/newsid_336000/336592.stm

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