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

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