PLEASE NOTE:
*
CCNet DIGEST, 25 May 1999
--------------------------------------------------------
QUOTE OF THE DAY
Williams bristles at the idea that potential
asteroid
threats should be dealt with out of view of the
public. He pointed
out that the latest round of observations were made
by Frank
Zoltowski, an amateur astronomer in Australia - most
likely in
response to the questions raised publicly over 1999
AN10.
"These objects are brought to the attention of
the public because it
shows that work is being done, that the people
responsible for
keeping track of these things are keeping track of
them, and it’s
being done in an open fashion," he said.
(Gareth Williams, MSNBC, 24 May 1999)
(1) NASA INCREASES IMPACT PROBABILITY FOR ASTEROID 1999 AN10 YET
AGAIN
Benny J Peiser <b.j.peiser@livjm.ac.uk>
(2) EARTH'S CLOSE SHAVE WITH AN ASTEROID
MSNBC Online
(3) FIREWORKS IN THE SKY?
Andy Nimmo <andy.nimmo@net.ntl.com
(4) GREAT BALLS OF ... DUST
Duncan Steel <dis@star.arm.ac.uk>
========================================
(1) NASA INCREASES IMPACT PROBABILITY FOR ASTEROID 1999 AN10 YET
AGAIN
From Benny J Peiser <b.j.peiser@livjm.ac.uk>
The story behind asteroid 1999 AN10 is getting more fascinating
by the
day. You may recall that the results of the initial impact
probability
calculations were so tiny that the idea of making them public was
considered unnecessary. In addition, it was believed by some that
no
further observations of AN10 could be made for months to come.
However, the sensitive impact-threat data was posted on the
internet
nevertheless. The publicity generated on the CCNet about this PHA
and
the akward way the information was handled, attracted the
interest of
NEO observers and consequently lead to new and relevant
astrometric
data obtained by Frank Zoltowski, an American amateur astronomer
in
Australia.
On the basis of this new data - which we, I would argue, might
not
know without the AN10 debate on CCNet - the impact probability
has
increased ever since. Starting from an initial impact risk of one
in a
billion, NASA announced last week that this value had to be
reduced to
an impact probability of one in 10 million for a hypothetical
impact in
2039. But this is not the end of the story. In a low-key
announcement
made to MSNBC online yesterday, Don Yoemans, the director of
NASA's NEO
Programme Office, has increased the impact risk probability yet
again
to 1:500,000 - this time for a hypothetical impact in the year
2044.
It is important to stress that even this latest revision does not
make
AN10 an actual threat about which we would have to worry about at
this
stage. But it is now clear for the first time that there is a
potential
impact scenario which can no longer be ignored. While nobody
needs to
lose sleep about this latest revelation, the AN10 affair
demonstrates
once and for all just how important hard observational data is -
rather
than impact probability statistics - when it come to dealing with
the
NEO problems we are facing. What is perhaps even more revealing
is the
fact that there are still inherent biases and uncertainties in
the used
probability methods which make calculations based on them rather
vague.
The latest developments of the AN10 story would suggest that
drawing
attention to the controversial web paper on the CCNet and the
resulting
debates have helped to provide additional observations and
astrometric
data which make asteroid 1999 AN10 an even more interesting
object. I
am sure that most subscribers are just as curious as I am to find
out
what the additional observations of AN10 in the next few months
will
reveal about this peculiar asteroid.
Benny J Peiser
===================
(2) EARTH'S CLOSE SHAVE WITH AN ASTEROID
From MSNBC Online
http://www.msnbc.com/news/272798.asp
This time, debate over collision threat in 2039 seems calmer
By Alan Boyle, MSNBC
May 24 — Decades from now, a stroke of astronomically
bad luck could
set an asteroid on a collision course with Earth. The strange
case of
Asteroid 1999 AN10 is an echo of last year’s biggest space
scare — but
this time, the debate over the potential risk is much more muted.
ASTEROID-WATCHERS HAVE known about 1999 AN10 for months: The
space rock
was spotted in January by the Lincoln Near Earth Asteroid
Research
Project, also known as LINEAR. Within weeks of its discovery,
astronomers began debating just how close 1999 AN10 might come to
Earth
— and a team led by Italian astronomer Andrea Milani saw a
slight
chance of a collision in 2039.
It all sounds much like the public controversy that erupted in
March
1998 over another asteroid, known as 1997 XF11. Back then, even
the
suggestion that an asteroid might hit Earth in 2028 generated a
media
alarm — until additional analysis of the object’s path
showed that it
would pass safely by.
But this time, there’s been little
"sky-is-falling" hysteria, even
though the latest observations have led NASA to raise the chances
of an
impact. As things stand now, the risk is still incredibly slim
— one
chance in 10 million for 2039, and 1 in 500,000 for an encounter
in
2044, according to astronomer Donald Yeomans, who heads NASA’s
asteroid-observing effort at the Jet Propulsion Laboratory.
Those odds are longer than the chance of an earth-shattering
object
coming out of the blue at any time, NASA scientists say. In any
given
year, they figure there’s a one-in-100,000 to
one-in-a-million chance
that an undetected object bigger than a kilometer across will hit
Earth.
"We’re still down below the background level" with
1999 AN10, Yeomans
said. "There’s an unofficial guideline that we don’t
really have to
worry about objects whose impact probability is below that, and
we
start worrying when their impact probability is above that."
Yeomans noted that the 2044 impact scenario was starting to come
within
that threshold of perceived threat.
THE PROBABILITY OF PERIL
Asteroid 1999 AN10 is thought to be on the order of a kilometer
across
— which would be big enough to cause a thermonuclear-scale
explosion
and a global catastrophe if it hit Earth. Scientists surmise that
similar collisions have sparked mass extinctions in the past,
including
the demise of the dinosaurs 65 million years ago.
Current projections have the asteroid passing no closer than
19,000
miles from Earth’s surface on Aug. 7, 2027. Under the
closest-approach
scenario, the asteroid would be visible from Earth as an object
of the
third or fourth magnitude — about as bright as one of the
stars in the
Pleiades cluster. But it could just as well pass beyond the orbit
of
the moon.
The impact scenarios for 2039 and 2044 rely on a complex
combination of
near-misses in 2027 as well as in 2034: The asteroid would have
to be
diverted just enough by Earth’s gravitational pull to put it
in line
for a collision during a later orbit.
The circumstance has been compared to firing an arrow or a bullet
through a front-door keyhole and a back-door keyhole to hit
someone in
the back yard.
"What’s happening with this AN10 is that it’s
making multiple close
approaches. ... You quickly enter the realm of chaos. You can’t
use
traditional approaches, you have to use Monte Carlo (probability)
techniques," Yeomans said.
Some asteroid-watchers say 1999 AN10 could come close to Earth’s
orbital path repeatedly over the next few centuries — which
further
complicates the picture.
NO WORRIES ... FOR NOW
So is 1999 AN10 anything to get upset over? Most astronomers say
they’re waiting for more observations during the asteroid’s
current
pass as well as a 2004 encounter.
"I’m not worried in the least about this object, but I’m
not going to
ignore it," said Gareth Williams, an astronomer at the Minor
Planet
Center in Cambridge, Mass. "Of course, if the 2039 (impact
scenario) is
still a possibility in 2004, I might change my mind about this.
But at
least for the time being there’s no reason to get
uptight."
The early discussions about 1999 AN10 were remarkably low-key
— so
low-key, in fact, that some observers feared the 1997 XF11
controversy
had scared astronomers into silence.
Yeomans, however, saw the relative quietude as a welcome change
after
the hysteria over 1997 XF11.
"The problem there was that the press release preceded the
analysis
(sic!), whereas in this case the analysis was done as it should
have
been done," he said.
He also took it as a positive sign that teams in Italy and at the
Jet
Propulsion Laboratory were coming up with "reasonably
consistent
results" independently.
"I don’t look at this object as a threat, but I do look
at it as
something that provoked the astronomical community to come
together
fairly quickly," he said. "It exercised the system
rather well, and it
worked rather well."
Astronomers are due to meet in Turin, Italy, next week to discuss
ways
to make the system for monitoring near-Earth objects more
efficient —
but the strange case of 1999 AN10 is likely to be on the agenda
as
well.
Williams bristles at the idea that potential asteroid threats
should be
dealt with out of view of the public. He pointed out that the
latest
round of observations were made by Frank Zoltowski, an amateur
astronomer in Australia — most likely in response to the
questions
raised publicly over 1999 AN10.
"These objects are brought to the attention of the public
because it
shows that work is being done, that the people responsible for
keeping
track of these things are keeping track of them, and it’s
being done in
an open fashion," he said.
Copyright 1999,
MSNBC
===============
(3) FIREWORKS IN THE SKY?
From Andy Nimmo <andy.nimmo@net.ntl.com
Dear Dr Peiser,
I noted in the BBC OnLine excerpt in the CCNet Special of 20th
May, it
says:-
"Astronomers at the Minor Planet Center at the US
Smithsonian
Astrophysical Observatory used Zoltowski's work to work out an
estimated approach distance for AN10 of 56,500 kilometres from
Earth.
The fly-by will occur on August 7, 2027. "
As the thickness of Earth's magnetosphere, at its thinest, is
never
less than around 64,000 kilometres from Earth, I presume this
means
there is a very good chance that on 7th August 2027, AN10 will
come
well within Earth's magnetosphere.
When your CCNet Digest came in, I happened to be reading
Professor Mike
Baillie's interesting book, "Exodus to Arthur", in
which, on page 178,
he enquired of Gerry McCormac "a colleague who had trained
and worked as
an atmospheric physicist" as to what would happen if a comet
came very
close to the earth? He replied, "If it came within the
earth's
magnetosphere it would probably be spectacular ...the sky would
go
purple or green, particles from the comet would spiral down the
lines of
force and it is likely that you would have amazing auroral
displays and
coloured streamers..."
Well, AN10 is an asteroid rather than a comet, but perhaps it may
have
small stones and/or dust in its wake. What chance do the CCNet
experts
think there may be that we'll have a spectacular display that
night?
Andy Nimmo
=============
(4) GREAT BALLS OF ... DUST
From Duncan Steel <dis@star.arm.ac.uk>
Dear Benny,
A note has been passed to me from Andy Nimmo in which he very
reasonably
asks about what sorts of things might be witnessed when (if) 1999
AN10
makes a very close fly-by of the Earth in 2027. Basically, he is
wondering whether we might see great balls of fire (copyright,
Jerry Lee
Lewis). I would suggest here that great balls of dust are
more likely.
My answer is based on the assumption that 1999 AN10 is indeed
asteroidal
(i.e., no volatile component producing outgassing and hence
auroral
effects etc. as Nimmo mentions).
First, a simple prediction based upon its observed brightness
indicates
that it would be between magnitudes +3 and +4 as it passes from
day- to
night-side during the close approach. That is, brighter than all
but a
handful of stars.
Now, let me assume that it has a dusty regolith, and that it
passes close
enough to penetrate the magnetosphere. I believe that under such
circumstances the individual dust grains (I am talking about
sub-micron to
100 micron particles here) would charge to some thousands of
volts.
Electrostatic repulsion would then make them fly off the
asteroid. That
is, a dust coma co-moving with the asteroid might be formed. This
would
perhaps scatter much more light than the solid body itself, and
extend
over a large volume of space, gradually dispersing.
Let me give a background reference which some others might find
interesting. The dust detection instrument on board the
HEOS-2 satellite
operated by the Heidelberg group during the 1970s found evidence
for
bursts of impacts by dust grains only whilst the satellite was
close to
the Earth, as opposed to apogee out near the lunar
distance. Their
interpretation (see the item by Hugo Fechtig in Comets, ed. L.
Wilkening,
Univ. of Arizona Press, 1982) was that dust balls/agglomerations
of mass
of order 1 tonne were remaining intact in space until such time
as they
approached the Earth/magnetosphere, and consequent charging led
to
electrostatic bursting of the mass, producing a cloud of
co-moving dust
particles. (I might mention in passing that at one stage I gave
some
thought to whether this phenomenon, if it is real, might explain
whatever
it is that Lou Frank claims to detect, the so-called dark
atmospheric
holes, avoiding his preposterous explanation of substantial
minicomets
depositing water as they break up.)
What I am suggesting here is the possibility that, if 1999 AN10
has a
dusty regolith (surface layer), then the small dust grains might
be
levitated through charging, producing a bright dust coma (*not* a
gas
coma, the dominant factor in the brightness of a comet). If this
does
happen, then we might see a celestial ghost in 2027.
Duncan Steel
dis@star.arm.ac.uk
----------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
----------------------------------------
The CCNet is a scholarly electronic network. To
subscribe/unsubscribe,
please contact the moderator Benny J Peiser <b.j.peiser@livjm.ac.uk>.
Information circulated on this network is for scholarly and
educational use only. The attached information may not be copied
or
reproduced for any other purposes without prior permission of the
copyright holders. The fully indexed archive of the CCNet, from
February 1997 on, can be found at http://abob.libs.uga.edu/bobk/cccmenu.html