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.
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Copyright 2002, Duncan Lunan and Gordon Ross