PLEASE NOTE:


*

CCNet ESSAY: KEEP WATCHING THE SKIES: COMET-CHASERS & PLANETARY PROTECTION
--------------------------------------------------------------------------

"The chances that LUCIFER'S HAMMER would hit the Earth head-on were
one in a million, then one in a thousand, then one in a hundred. And
then..."

by Duncan Lunan [ astra@dlunan.freeserve.co.uk ] and Gordon Ross
 

[In February 2002, the Cambridge Conference Network carried articles on
using solar sails to  deflect incoming NEO's, and on storing nuclear weapons
in the Lagrange points to protect Earth.   The two concepts are combined in
the article below, which proposes a small fleet of parabolic solar sails,
'Comet-chasers' to be stationed at the Lagrange points.

The style is informal because the original was commissioned by David
Langford for the short-lived British magazine Extro, in 1982, during the
first scare about Comet Swift-Tuttle, but did not appear because Extro
ceased publication. Gordon Dick (now Gordon Ross) designed the Comet-Chaser
for a version submitted to the Hughes Aircraft/Griffith Observer essay
contest, in 1986. The essay wasn't placed, but Space Policy published two
letters from us on the subject. After Swift-Tuttle returned in 1992 and the
threat again became apparent, the revised article was published in Analog in
October 1994, with illustrations by Sydney Jordan. A longer one was
published by ASTRA, the Association in Scotland to Research into
Astronautics, in our journal Asgard in May 1995. A shorter piece appeared in
the Glasgow Herald in March 13th, 1998, as part of the extensive coverage
which another scare had gained for Jay Tate's lectures to ASTRA in Airdrie
and Glasgow. Since then the Glas-weg-ian has run a piece, illustrated with a
photo of a Comet-chaser model by Gordon Ross. We are about to print the
version below in Asgard as part of 'The Politics of Survival', an ASTRA
discussion pro-ject on threats to Earth of all kinds and how to avert them.
DL]


Comet Swift-Tuttle  (1862 III) returned to the northern skies in late 1992.
With the possibility that it might hit the Earth later, it made waves in
amateur astronomy, professional astronomy, the media, the political sphere,
and the military-industrial one;  and it was interesting that the various
groups seemed hardly to be talking to one another, much of the time. The
whole debate about the need to protect the Earth was reopened.

Lewis Swift discovered 1862 III, the Great Comet of that year, on July
15th.1) At first it was so like Schmidt's 1862 II that Swift didn't realise
it was different, until Tuttle independently found it and announced it three
days later. The astronomical world split the honours, hence 'Comet
Swift-Tuttle'. In August 1862 the comet put on a spectacular display in
northern hemisphere skies, travelling past Polaris from Camelopardis and
developing a tail 25 degrees long. In the telescope it was seen to be
throw-ing out luminous jets which looped around the more normal tail in a
most unusual way. And when the orbit of the comet was calculated, it
appeared to coincide with the Perseid meteor shower through which the Earth
passes every August; the biggest display takes place between August 12th and
14th. The Swiss astronomer Plantamour lectured on the link with the Perseid
meteors in the winter of 1871-72, mentioning that the Earth would next
encounter the meteors on August 12th, 1872. The newspapers reported that the
Earth would collide with the comet on that date, causing considerable public
alarm.2
    
19th century astronomers thought such fears were ground-less. Comets had
passed close to Mercury, and between the moons of Jupiter, without producing
any noticeable perturb-ations; so their masses had to be low. And Earth had
passed through the tail of the great comet of 1861 without any notice-able
effect, so the tails had to be tenuous gases;  and when the head of one
passed in front of the star Arcturus without dimming it significantly, that
seemed to clinch the matter - comets were entirely gaseous. In the 20th
century it came to be accepted that there must be something inside the great
heads of the comets, but whatever it was, no doubt it was too small and
flimsy to be dangerous.

Science-fiction writers, of course, ignore the astronomers when it suits
them. (E.g., for much of the 20th century astronomers believed this was the
only planetary system in the Galaxy.) In "Hector Servadac" and its sequel,
Jules Verne described a comet made of solid gold telluride, so that it could
knock lumps off the Earth for the purposes of his story. H.G. Wells stuck to
prevailing theory for "In the Days of the Comet", in which there was no
actual impact and the gases mix peacefully with the Earth's atmosphere. In
Arthur C. Clarke's short story 'Into the Comet', the core was a loose
cluster of icebergs, giving off jets of methane and ammonia.

Clarke claimed to be quoting F.L. Whipple, but Whipple himself envisaged the
'dirty ice' as a single mass typically 1-10 km in diameter.3 I wrote a
story, based on my own amateur observations of Comet Bennett in 1970, in
which the nucleus was one loosely compacted mass of ice and rock, and Isaac
Asimov said that "pictures a Whipple-like comet with con-siderable accurate
detail".4 "Lucifer's Hammer", the comet which hits the Earth in the
Niven-Pournelle novel, has a nucleus initially like the one in my story.
Trying to visualise the impact, the characters use the analogy of 'hot fudge
sundae' - with the ice cream representing the 'foamy ice', overall density
consid-erably less than water, and embedded crushed nuts representing the
rocks. But there's so much vaporisation as the comet rounds the Sun that
what approaches Earth is a boulder field embedded in gas, much like Clarke's
description. We've all used the model which best fits the needs of our
plots.

At the end of March 1982, the Daily Telegraph and the Daily Star - at
opposite ends of the UK press spectrum - announced almost together that
there was a chance Comet Swift-Tuttle would hit the Earth in August that
year. According to the Telegraph, there would be no dan-ger unless the comet
went through perihelion (its closest point to the Sun) on August 12th, and
the odds against that were given as two mill-ion to one. But as it says on
the back of the novel, "The chances that LUCIFER'S HAMMER would hit the
Earth head-on were one in a million, then one in a thousand, then one in a
hundred. And then..."

The Telegraph gave as its source Dr. Brian Marsden, of the
Harvard-Smithsonian Centre for Astrophysics. Marsden and Yeomans, of the Jet
Propulsion Laboratory, had independently recalculated the orbit and
predicted perihelion passage in  June or September 1981.5  The problem was
that the comet didn't seem to have any respectable antecedents. Giovanni
Schiaparelli, better known for the first account of canali on Mars,
calculated that the period of the orbit was 120 years and coincided with
that of the meteors. Other 19th century estimates were 121.5 years6 and 142
years.7 But on any of these, then even allowing for planetary perturbations
the historical record should contain bright August comets which were
previous visits by Swift-Tuttle.   Marsden didn't find such sightings: the
best available correlation was with Kegler's Comet of 1737, which would give
no threat to Earth in 1982. However, it would mean that the comet was being
acted on by powerful non-gravitational forces, and it could return to
perihelion in November 1992, with a serious prospect of a collision one or
two orbits later.8

Nevertheless, the amateur observers were now aroused. Meteor studies are one
of the many areas of astronomy still dependent on amateurs for detailed,
labour-intensive work. From these are calculated the Zenith Hourly Rates for
each year's shower, mapping its structure as the Earth passes through it.
The Perseids' rates fell to five or ten per hour in the 1920's, then rose
slowly as the century went on. In the 1970's rates rose to 80 or more (the
1977 shower was well observed)  and in 1980 there was a 50% increase. In
1981 and '82 the rates appeared lower, but the Moon interfered with both. In
1983 there was no sharp rise, and if the 120-year period was roughly
correct, the comet had probably passed on the far side of the Sun
unobserved.

Marsden had made his 1992 prediction diffidently in 1973, and repeated it
still more diffid-ently in July 1991. Just twelve days later, an unexpected
peak in the Perseid meteors sug-gested something was about to happen; it was
repeated in 1992;  and on 26th September the comet was recovered by
Tsuruhiko Kiuchi of Japan. Getting to grips with the inconsistencies in
previous sightings of what could be the same comet, Marsden found himself
facing the possibility of an impact in 2126. This comment appeared in the
January 1993 Sky & Tele-scope, accompanied by a Don Davis cover showing the
nucleus grazing the Earth's atmosphere at the terminator, with a sea of fire
below as the dust grains on parallel tracks meet their end. Will the main
body strike, or miss?

By late October '92, the press was saying that the chances of an impact in
2126 were only one in 400 - no problem, though some of these same
journalists had considered one in two million to be dangerous ten years
before. Dr. Ken Russell, at the Anglo-Australian Telescope, was quoted as
retorting, "A chance of one in 400 is not small when you are talking about
the ex-tinct-ion of the human race",9 which seemed a bit much for an object
now estimated to be around 5 km. across.   If com-posed mostly of ice, it
would have perhaps one-fortieth of the mass of the object which wiped out
the dinosaurs - now very strongly linked with the Chix-ulub impact crater
off the coast of Yucutan.

Both press reports of 1982 cited the Tunguska event of July 1908 as an
example of what could happen. That explosion was in the multimegaton range,
and threw enough dust into the upper atmosphere to produce spectacular
sunsets for months. Yet no part of the object reached the ground:  there
were no impact craters, and trees remained standing at ground zero, though
stripped of bark and branches, as they did at Hiroshima. The standard
scientific view is that the object was a small comet which approached Earth
unseen on the sunward side: a loosely struct-ured object would fragment when
the mass of atmosphere between it and the ground was equal to its own mass,
and then supp-os-edly all the ice evaporated and its kinetic energy was
dumped into the atmosphere as heat, simulating an explosion.

But the Tunguska object was about 3 km. up and travelling at several km. per
second when it exploded. Could so much ice vaporise in a fraction of a
second? The big thing about ice as a material is its recalcit-rance. In the
1960's, the U.S. Coastguards set out to destroy a small berg to see if they
could keep hazards out of the shipping lanes: high explosives only blew
chips off, and the incendiaries just glazed the surface without even
altering the overall shape. The chastened Coastguards admitted that to
destroy bergs in a hurry, even nuclear weapons might be ineffective.

Now, remember those jets of gas from Comet Swift-Tuttle? Sir John Herschel
reported only one,10 but Chacornac saw 13 jets over 17 days.11 In 1978
Whipple announced a new analysis of comet rotations showing that in many
cases only small areas of the nuclei were active. Computer modelling allowed
the rotation rates to be determined - 33 hours in the case of Swift-Tuttle12
- and Chacornac proved to be nearer the mark. The best fit with the
observ-at-ions, modelled by Zdenek Sekanina at the Jet Propulsion
Laboratory, indicated seven active areas on the nucleus, none of them
large.13 The rotation periods for comets ranged from four hours to five
days, and implied that to hold together, the nuclei couldn't be loosely
compacted but had to be frozen solid - the nucleus of Comet Giacobini-Zinner
seems to be disc-shaped, with an equatorial radius eight times the polar
one. If loosely struct-ured, it would almost certainly come apart under
solar heating, or atmosphere entry.

Until those astonishing tiles were invented for the Space Shuttle the only
way to protect an incoming vehicle was by ablation - the surface layer of
the heat shield vaporising and carrying away the heat it's absorbed while
the shockwave pro-tects the material behind. Some materials achieve it
nat-urally (Chinese re-entry capsules have shields of peanut fibre, and it
turns out that Soviet ones were wooden all along). The tektite class of
meteorites have been ablated into aerodynamic shapes; larger stones are
often covered with ice after they fall, even if red-hot at first, proving
that the interiors are still at very low temperatures.  The Coastguards were
right:  ice would ablate in fireball conditions. Arthur Kantrowitz and
others have envisaged ice-filled rockets, ener-gised by laser or electron
beams from the ground; in March 1974, the idea feat-ured on an Analog cover
for a story by Jerry Pournelle. The same physics apply to an ice mass coming
down through the atmosphere - even if it did break up when the sonic boom
became trapped between it and the surface, if it remained intact down to
three kilo-metres it would fragment into pieces much too large to vaporise
before they struck the ground below.

In 1976, an inter-national programme to photograph bright fireball meteors
and locate where they fell, revealed that most fireball objects are much
bigger than had been thought, yet very fragile, disinte-grating high above
the ground.14 The largest one photographed was estimated to mass 200 tons,
yet be so fragile that it would have crumbled under Earth-surface gravity.
In his book "Messages from the Stars" Ian Ridpath suggested very plausibly
that the Tunguska object was similar in composition - possibly a comet
nucleus from which all the ice had been driven off.15

This could be the nature of many of the 'Earth-grazing' asteroids. 38 were
known as of 1978, and more than 90 by now - all of them liable to hit the
Earth, Moon or Venus within the next 100 million years. If many of them are
fragile objects, less threatening than their 1-10 km. diameters would imply,
then the situation might not be too bad. On the other hand, if it is impacts
which cause reversals in the Earth's magnetic field, as has been suggested
in some quarters, then the average interval between events big enough to do
serious damage is 170,000 years, and we may count ourselves lucky as a
species that there hasn't been one for 700,000 years. And on the other hand
again - or is it the same hand? - it's been argued that as ice sublimes off,
the comets may form a protective crust of dust which can close over
altogether, shielding a substantial mass of ice inside.13,16 Halley's Comet,
the only nucleus to have been photographed, seems to support that idea, and
Earth-grazing asteroids masquer-ading as fluffy dust balls may actually be a
great deal more dangerous.

Asteroid 1989-FC caused some stir when it passed the Earth at 450,000 miles
in March 1989. In March 1990 the National Space Society Golden Gate
Chapter's Space-faring Gazette reported that its orbit gave recurring
encounters every 13 months and it would collide with either the Earth and
Moon within 30 to 40 years.17 Its diameter was given as between 500 and 1000
feet, depending on its compos-ition and reflectivity. A dark, large but
loosely structured object would probably explode above ground without
forming a crater, like the Tunguska one. 1989-FC had no cometary head and
presumably isn't icy, but see above. But if the object is of rock or metal,
with ten times the mass of a similarly sized iceberg, then the crater size
could be 10 miles or more, equivalent to the impact of an average-sized
comet nucleus.

There was no international response to the threat de-scribed in the Gazette
report. Through the editors, and through my colleague Danny Varney in
Australia, I exchanged letters with a number of experts including Dr. Duncan
Steel, none of whom regarded the prediction as valid. Nevertheless, the
American Institute of Aeronautics & Astronautics held a conference on
countering impact threats, only two months after the Gazette article
appeared.

The favoured answer was that the incoming object can be diverted by
exploding nuclear war-heads beside it: the material vaporised acts like a
crude rocket to thrust the comet or asteroid into a new trajectory.18  But
even passing objects are detected normally by the trails they made on
photographic plates, not developed until after the event is over. Something
coming towards the Earth would be still harder to detect, and if it came out
of the Sun like the Tunguska object it could arrive unannounced. An asteroid
coming straight at us wouldn't make a trail on a plate, even if it was
developed in time - and even from the distance of the Moon, at average
Earth-grazer velocities there would be less than four hours to impact. It
won't show on Ballistic Missile Early Warning radar even then, because the
computers are programmed to ignore objects at such distances  (ever since
the time in the 1960's when they picked up the rising Moon and announced
World War Three);  and if it passes through the NORAD radar fence, recording
satellites passing over the spine of North America, that would be only by
chance.

The chances that large numbers of missiles could be reprogrammed and
launched with the required accuracy seem very low. There would after all be
no capability for mid-course corr-ections. The other problem is that if the
object were shattered the consequences of multiple impacts could be worse
than the original prospect. To protect Earth adequately, longer-term
provision is necessary.

In June 1980, NASA's Advisory Council suggested a "Space-watch" to compile a
catalogue of all objects passing close to the Earth.19  No government action
followed, but watches for Earth-grazing objects were set up, first at Mt.
Palomar under Elinor Helin, then by Gene and Carolyn Shoemaker of the U.S.
Geological Survey, and later at the Anglo-Australian Tele-scope under Duncan
Steel and Robert McNaught. In January 1992, NASA recom-mended to Congress
setting up a 'Spaceguard Survey' (name lifted from Arthur C. Clarke's
"Rendezvous with Rama"), of six telescopes, to identify 90% of Earth-grazing
hazards within a decade.20 Towards the end of the year, NASA doubled its
study funding for the project, which still left only a dozen people in the
world working on the problem.21

But with a comprehensive catalogue, dangerous objects could be intercepted
at aphelion, their furthest point from the Sun, and deflected using neutron
bombs. This scenario is associated with Dr. Edward Teller, "the father of
the H-bomb" and with the 'Star Wars' SDI researchers at the Lawrence
Livermore National Laboratory, whom Dr. Teller has long supported. In a time
of decreasing tensions between the great powers, and with funding for the
Strategic Defence Initiative under threat, critics such as Louis Friedman of
the Planetary Society sug-gested that concern over impacts was just a cover
for getting nuclear weapons into space.22

A still more drastic nuclear proposal, not to deflect dangers but to detect
them, was put forward by Arthur C. Clarke in early 1993. 'Project Excalibur'
would explode a 1000-megaton device, on the far side of the Sun for Earth's
sake, and flood the Solar System briefly with microwaves, allowing the
identification by radar of everything more than three feet across within the
orbit of Jupiter - except objects on the Earth-Sun line at the time.23 If
you're going to break the Test Ban Treaty at all, for the sake of Earth's
survival, perhaps there's no point in half measures.

But as regards Teller's scenario, we don't even know the crustal composition
of a typical comet nucleus. Comet Halley's is extremely dark, supporting the
idea of dust build-up; but Sir Fred Hoyle and Prof. Chandra Wickramasinghe
took it to be evidence of complex chemistry, building up a shell of complex,
tar-like compounds. To confuse the issue still more, recent work suggests
that debris re-impacting Comet Halley should prevent the build-up of any
protective crust,24 and the 'explosion' on the comet in December 199025 may
have been such an event.

Then there's the rotation of the nucleus. In the case of Comet Encke, gas
emissions are shortening the orbital period, but the pole of rotation has
shifted through more than 100o over 191 years, and the effect of the
emissions has been greatly reduced.26 The effect of gases released by a
nearby nuclear explosion will be very much related to the spin:  it's not
hard to imagine that on a fast-rotating nucleus the net thrust of a new jet
might be zero, or much reduced, as seems to be the case with Comet
Swift-Tuttle  (see below).

But a technology 'annexed' by SDI may pro-vide the answer: a quick,
relatively inexpensive, non-violent answer which breaks no international
treaties and creates no political issues. The 'Comet-chaser', designed by
Gordon Ross of Glasgow School of Art, could be effective much sooner than my
own alternative to the nuclear weapon scenarios - indust-rialising the inner
Solar System, so that any detected hazard could be reached by a task force
and simply disman-tled.27,28

Gordon is an aerodynamicist and former sail-maker, winner of the Duke of
Edinburgh's Award for the double-surface sail design which is now in
competition use worldwide, design-er of the new 'Powersails' taking the
international market by storm, and head of ASTRA's Waverider project.   The
'Comet-chaser' he envisages marries the 'Solaris', his own design of
parabolic solar-sail, with adaptive optics such as the flexible para-bolic
mirror created at Strathclyde University, by a team led by Dr. Peter
Waddell. The University sold the patents to a U.S. aerospace company for
'Star Wars' applic-ations, and little has been heard of that type of
flexible mirror since, but similar systems may resolve the nuclear weapons/
comets dilemma.

The Strathclyde mirror requires a pressure differential, which in space
would be main-tained by a layer of gas contained by a transparent, flex-ible
membrane.29 Other adaptive optic systems, developed meantime, include
mirrors made of multiple seg-ments, individually mounted and computer
controlled,30 and very thin mirrors, mechanically adjusted from the rear.31
Any of these could be used in the 'Comet-chaser', probably more effectively
- especially for focus-sing the beam off-axis.

Three to five of these could be stationed at the five Lagrange points of the
Earth-Moon system, equidistant from the Earth and Moon, maintained in place
by sunlight pressure alone.32 Even in an emergency at least one should be
able to detect and intercept an in-com-ing comet by the time it reaches the
orbit of the Moon, and burn a hole in its crust to release a gas jet which
would deflect the comet past the Earth. At the distance of the Moon, the
deflection required would be only one degree, or four thousand miles in
space. For asteroids, more time would be required for deflection, but an
interplanetary version of the sail would be able to cope with that given
enough notice. And the more comprehensive the catalogue of Earth-grazers,
the more likely that the Comet-chasers can get to a threatening object in
time to turn it aside.

The optical system of the Comet-chaser combines four concentric parabolic
mirrors in what might be called a 'double Cassegrain' configuration. Once
rendezvous has been achieved, the vehicle concen-trates all the energy
gathered by the sail on the secondary mirror, from there to a tertiary one
behind the sail, and from there via a quaternary, adjustable mirror to the
optimum spot on the crust of the comet. While the beam goes down-Sun, net
thrust on the vehicle will be zero while it is in 'burn mode'. At first we
invisaged a fifth, free-flying mirror to keep the beam focussed on the
target spot as the nucleus ro-tated; if there was time, the first objective
might be to slow the rotation to make it Sun-synchronous, so that the
orbit-changing jet would be fully effective. The free-flyer would probably
be needed even then, unless the hot spot was right at the sub-solar point on
the stabilised nucleus.

But in 2001 a paper by Drs. David Asher and Nigel Holloway, given at the
Charterhouse conference and repeated to ASTRA, opened up a new angle to the
debate. Advocates of defending Earth have been agonising over the
'deflection dilemma' - what if developing such technology allows asteroids
to be used as weapons? Their study showed that to achieve that would require
much more effort, precision and secrecy than a simple deflection operation.
And that confirmed something Gordon and I had realised meantime:  since any
deflection will do, out of collision course with Earth, the obvious place to
focus the Comet-Chaser's beam is at one of the asteroid or comet's poles.
Even then off-axis focussing will probably be required, and as the comet's
orbit begins to change, the optical system will have to be defocussed
intermittently, to allow the sail to follow, unless there's enough energy
available for a beam-splitter system to balance up and readjust the forces.

And if on further research the burn technique isn't going to work, if
nuclear warheads have to be used, then at least the Comet-chaser can use
passive propulsion to place them in exactly the right place.

The apparent threat of 1989-FC brought a brief surge of interest in Gordon's
concept, and the Glasgow Herald (as it then was)  provisionally commissioned
an article, to emphasise that workers at Glas-gow School of Art and the
University of Strathclyde could have countered a great danger to the world;
but they dropped it when the specific threat could not be con-firmed.
Another Scottish newspaper provisionally took up the story - and then,
paradoxically, Comet Swift-Tuttle came back and spoiled it.

On November 3rd '92, Donald Yeomans ruled out non-gravit-at-ional effects on
the comet, accounted for the sightings in 1737, 1862 and 1992, and ruled out
a collision in 2126.21 Now that the orbit had been charted with such
accuracy, it seemed that the net effect of the jets was nil. (Which isn't to
say that it will stay that way, as witness the behaviour of Comet Encke and
the 1990 explosion on Comet Halley.) Marsden recommended intercepting Comet
Swift-Tuttle between the orbits of Saturn and Uranus in 21228 - too far from
the Sun for Comet-chasers to be effective.   But because of the comet's high
orbital inclination  (113o), he assumed rendezvous at the ascending node
where the comet next crosses the Earth's orbital plane. Sending conventional
space-craft out of the Ecliptic requires heavy fuel expenditure or
gravit-ational slingshot.   Low-thrust, continuous propulsion systems don't
have that limitation, so the Comet-chasers could catch threatening comets on
return to the Sun. Asteroid orbits are less far-ranging, so they can close
with them and divert them at leisure.

On December 12th, as the comet reached perihelion, Robert McNaught said that
there was no longer a danger either for 2126 or 2261.9 Even before the BBC's
Tomorrow's World ann-oun-ced that there was no danger after all, the second
newspaper had likewise lost interest, since their exclusive was now 'only'
the best way to counter incoming hazards. By March 24th, 1993, two
high-ranking officials of the SDI Office and the Air Force Space Command had
refused to appear before the House Science, Space and Technology
subcommittee, because the Pentagon "didn't want to see headlines that the
Air Force was chasing space rocks. The subject looked like it had the
potential for a high giggle factor when they are involved in so many larger
issues."33
   
Andy Nimmo, of ASTRA and the Space Settlers Society, then proposed forming
'the Light-Sail Consortium', to develop a simplified model of the Solaris
for a millennium project to plant pound coins on various Near Earth Objects
- so proving the technology while setting up a competition to retrieve them
later. Initially the idea gained the support of the British qual-ity papers
such as The Daily Telegraph, which put it on the front page;  the tabloids
were more sceptical, but the BBC was in favour, until David Mellor denounced
it as "in-sane" on News-night. Thereafter the Millennium Com-mission severed
communications with us and the Light-Sail Consortium remians on the back
burner, for now.

As John G. Kramer pointed out in Analog, how impact risks are assessed is a
matter of definition. The chances of dying in an asteroid impact are slight,
on a day to day basis, but so many deaths would result that the risk over 50
years is greater than from aeroplane accidents, tornadoes or
electro-cution.34 But in a sense, statistical arguments are little use:
there's one day on which it's actually going to happen and the number of
days on which it doesn't happen is academic.   For the few of us, still, who
see things in that light, the lesson of the last ten years is that it won't
be easy to arrange protection for the Earth without a quantifiable menace to
give it the necessary urgency. All we can do is keep watching, in hopes to
find something which is going to hit us, but not too soon to do anything
about it.

References

1.  J. Rambosson, "Rambosson's Astronomy", Chatto & Windus, 1897.
2.  R.A. Proctor, "Myths & Marvels of Astronomy", Longmans, Green & Co., 1896.
3.  Fred L. Whipple, "Earth, Moon and Planets", Third Edition, Pelican
Books, 1971.
4.  Isaac Asimov, 'Comets', introducing Duncan Lunan, 'The Comet, the Cairn
and the Capsule', in Asimov, Greenberg & Waugh, eds., "The Science Fictional Solar System",
Harper & Row, 1979.
5.  John Bortle, 'Comet Digest', Sky & Telescope, 62, 1, 29,   (July 1981).
6.  Camille Flammarion, "Les Étoiles", Paris, 1882.
7.  R.A. Proctor, "The Orbs Around Us", Longmans, Green & Co., 1899.
8.  Brian Marsden, 'Comet Swift-Tuttle:  Does It Threaten Earth?', Sky &
Telescope, 85, 1, 16-19 (January 1993).
9.  Julian Cribb, 'Scientists Allay Fears of Comet Collision', The Weekend
Australian, Dec. 12-13 1992.
10.  Sir John Herschel, "Outlines of Astronomy", (11th ed.), Longmans, Green & Co., 1871. 
11.  William Miller, "The Heavenly Bodies", Hodder & Stoughton, 1883.
12.  'Amateur Astronomers', Sky & Telescope, 56, 4, 312  (Oct. 1978).
13.  Carl Sagan & Ann Druyan, "Comet", Michael Joseph, 1985.
14.  Keith Hindley, 'Fireball Networks - a Mixed Blessing', New Scientist,
72, 1032, 695-698   (23/30 December, 1976).
15.  Ian Ridpath, "Messages from the Stars", Fontana/Collins, 1978.
16.  John K. Davies, 'Is 3200 Phaeton a Dead Comet?', Sky & Telescope, 70,
4, 317-318  (Oct. 1985).
17.  'Frontier', Spacefaring Gazette, 6, 3, 4  (March 1990).
18.  Adrian Berry, 'Film Plot Could Save World from Disaster', The Daily
Telegraph, 8th May 1990.
19.  Clark Chapman, David Morrison, 'Viewpoint:  the Next Doomsday Impact',
Astronomy, 17, 11, 8 (November 1989).
20.  Alan Fitzsimmons, 'Target Earth', Astronomy Now, 7, 2, 38-40  (Feb.1993).
21.  Liz Tucci, 'Funds to Track Comets Get Big Boost, but Killer Comet
Threat Dismissed', Space News, 3, 42, 8  (November 9-15, 1992).
22.  Fran Smith, 'A Collision over Collisions:  a Tale of Astronomy and
Politics', San José Mercury News, 22 March 1992;  reprinted Mercury, May-June 1992, 97-102
& 110.
23.  Adrian Berry, 'Writer's Explosive Theory', The Daily Telegraph, 31.3.93.
24.  Chris Kitchin, 'Spitting into the Wind', Astronomy Now, 7, 4, 11 (April 1993).
25.  'News Update:  Halley's Outburst', Astronomy Now, 5, 8, 9  (August 1991).
26.  Fred L. Whipple, 'The Spin of Comets', Scientific American, March 1980;
reprinted in John C. Brandt, ed., "Comets", W.H. Freeman & Co., 1981.
27.  Duncan Lunan, "New Worlds for Old", David & Charles, 1979  (US edition
William Morrow, Inc.)
28.  Duncan Lunan, "Man and the Planets", Ashgrove Press, 1983  (US import
Salem House).
29.  Hiroshi Shimuzu, 'Ultralightweight Reflector for Lidar Applications',
Applied Optics, 25, 9, 1467-1469 & 1475  (May 1986).
30.  Steven Miller, 'First Light at the Keck', Astronomy Now, 5, 4, 21-23
(April 1991);  'Last Pieces of Mountain Mosaic', Astronomy Now, 6, 1, 10-11  (Jan. 1992).
31.  'REOSC:  a Sharp Eye on Space Optics', News from Prospace, Prospace,
Paris, 33, 32-41  (Dec. 1991).
32.  H. Hornby, W.H. Allen, 'Mission to the Libration Centres', Astronautics
& Aeronautics,  July 1966, 78-82. 
33.  Leonard David, 'Defense Experts Duck Asteroid Threat Hearing', Space
News, 4, 13, 6  (March 29 - April 1, 1993).
34.  John G. Kramer, 'Killer Asteroids and You', Analog Science
Fiction/Science Fact, CXII, 1 & 2, 208-213  (Jan. 1992).

Copyright 2002, Duncan Lunan and Gordon Ross



CCCMENU CCC for 2002

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.