CCNet 112/2001 - 31 October 2001
"To me it is becoming clear that the impact of a
few-hundred-meter-sized asteroid, or even a 50-meter asteroid, would
have societal repercussions far beyond what I had previously
imagined. And it might behoove politicians to consider the potentail
repercussions if we had chosen to do nothing about such impacts when
there were cost-effective things that we
could have done.
--Clark Chapman, 30 October 2001
"The NUMBER ONE science driver of the LSST is the search for NEOs
down to the 300-m scale. The NEO community should understand that
LSST has a lot of momentum, and that it will probably be built whether
they support it or not. What is at stake is maintaining the LSST as
PRIMARILY a solar system survey telescope. If the NEO community does not
embrace LSST, rest assured that alternate science drivers will float up
from the astrophysics community. Many of these are fascinating, of
course (supernova surveys, weak gravitational lensing) but the solar system
applications will surely be pushed into the background, just as has
happened with almost every other large telescope at the national and
--David Jewitt, 30 October 2001
(1) MYSTERY SPACE BLAST 'SOLVED'
BBC Online News, 20 October 2001
(2) SOHO AND DISCOVERY OF COMETS
Bernhard Fleck <email@example.com>
(3) SOHO AND DISCOVERY OF COMETS
Michael Oates <firstname.lastname@example.org>
(4) MORE ON RADIATIVE FORCES
John Michael Williams <jwill@AstraGate.net>
(5) SMALL ASTEROIDS, THE IMPACT HAZARD & INTELLIGENCE FAILURES
Benny J Peiser <email@example.com>
(6) MORE ON SURVEY COST-EFFECTIVENESS
David Morrison <firstname.lastname@example.org>
(1) MYSTERY SPACE BLAST 'SOLVED'
>From the BBC Online News, 20 October 2001
By BBC News Online science editor Dr David Whitehouse
Astronomers may have solved the puzzle of what it was that brought so much
devastation to a remote region of Siberia almost a century ago.
In the early morning of 30 June, 1908, witnesses told of a gigantic
explosion and blinding flash. Thousands of square kilometres of trees were
burned and flattened.
Scientists have always suspected that an incoming comet or asteroid lay
behind the event - but no impact crater was ever discovered and no
expedition to the area has ever found any large fragments of an
Now, a team of Italian researchers believe they may have the definitive
answer. After combining never-before translated eyewitness accounts with
seismic data and a new survey of the impact zone, the scientists say the
evidence points strongly to the object being a low-density asteroid.
They even think they know from where in the sky the object came.
"We now have a good picture of what happened," Dr Luigi Foschini, one of the
expedition's leaders, told BBC News Online.
The direction of the flattened trees is a vital clue
The explosion, equivalent to 10-15 million tonnes of TNT, occurred over the
Siberian forest, near a place known as Tunguska.
Only a few hunters and trappers lived in the sparsely populated region, so
it is likely that nobody was killed. Had the impact occurred over a European
capital, hundreds of thousands would have perished.
A flash fire burned thousands of trees near the impact site. An atmospheric
shock wave circled the Earth twice. And, for two days afterwards, there was
so much fine dust in the atmosphere that newspapers could be read at night
by scattered light in the streets of London, 10,000 km (6,213 miles) away.
But nobody was dispatched to see what had happened as the Czars had little
interest in what befell the backward Tungus people in remote central
The first expedition to reach the site arrived in 1930, led by Soviet
geologist L A Kulik, who was amazed at the scale of the devastation and the
absence of any impact crater. Whatever the object was that came from space,
it had blown up in the atmosphere and completely disintegrated.
Nearly a century later, scientists are still debating what happened at that
remote spot. Was it a comet or an asteroid? Some have even speculated that
it was a mini-black hole, though there is no evidence of it emerging from
the other side of the Earth, as it would have done.
What is more, none of the samples of soil, wood or water recovered from the
impact zone have been able to cast any light on what the Tunguska object
Researchers from several Italian universities have visited Tunguska many
times in the past few years. Now, in a pulling together of their data and
information from several hitherto unused sources, the scientists offer an
explanation about what happened in 1908.
They analysed seismic records from several Siberian monitoring stations,
which combined with data on the directions of flattened trees gives
information about the objects trajectory. So far, over 60,000 fallen trees
have been surveyed to determine the site of the blast wave.
Over 60,000 fallen trees have been surveyed to determine the site of the
"We performed a detailed analysis of all the available scientific
literature, including unpublished eye-witness accounts that have never been
translated from the Russian," said Dr Foschini. "This allowed us to
calculate the orbit of the cosmic body that crashed."
The object appears to have approached Tunguska from the southeast at about
11 km per second (7 miles a second). Using this data, the researchers were
able to plot a series of possible orbits for the object.
Of the 886 valid orbits that they calculated, over 80% of them were asteroid
orbits with only a minority being orbits that are associated with comets.
But if it was an asteroid why did it break up completely?
"Possibly because the object was like asteroid Mathilde, which was
photographed by the passing Near-Shoemaker spaceprobe in 1997. Mathilde is a
rubble pile with a density very close to that of water. This would mean it
could explode and fragment in the atmosphere with only the shock wave
reaching the ground."
The research will be published in a forthcoming edition of the journal
Astronomy and Astrophysics.
Copyright 2001, BBC
* LETTERS TO THE MODERATOR *
(2) SOHO AND DISCOVERY OF COMETS
>From Bernhard Fleck <email@example.com>
In response to Michael Oates comments regarding the discovery of comets on
CCNet 30 October 2001, we would like to clarify our views.
We certainly don't want to leave the impression that we are ashamed that the
majority of comet discoveries using SOHO observations are made by amateur
Quite the contrary, this is a fact which we take great pride in, and often
tout both in internal and public reports as well as to the media. In the
NASA Senior Review Report submitted this year, asking for continued funding
of SOHO, it says:
"In addition to professional access, amateurs routinely download LASCO FITS
files and GIF images to search for new comets. As a result, 300 of the 1210
comets for which orbital elements have been determined (since 1761) were
discovered by SOHO, and more than half of those by amateurs
accessing LASCO data via the Web."
We may not always remember to mention the discoverer of a comet, or his or
her status as "amateur" astronomer, but we do so quite often, when the
discoverer is known at the time of writing and when it is pertinent to the
"story" at hand. Michael Oates is in fact mentioned in two of our Hot Shot
stories, one of which credits him with the discovery of 31 comets (it's an
old story). See also news stories on e.g. Space.com:
In the material sent to the media for the most recent comet story, the fact
that the majority of comets discovered by SOHO have been discovered by
amateur astronomers was mentioned. However, as far as we know, only two
Norwegian news stories included this (see
We do not see any harm in using phrases like "the satellite has spotted more
than 365 comets" or "...most prolific comet finder in the history of
astronomy". As journalistic tools for expressing the fact that images taken
by the SOHO spacecraft have been used to discover comets, they are
ubiquitous and do not imply that there is not a person involved, any more
than do phrases like "Hubble discovers Missing Pieces of Comet Linear".
We have, however, added a sentence to the last paragraph of our current Hot
Shot page, referring to the fact that the majority of comet discoveries
using SOHO images are in fact due to amateur astronomers. This is a point we
would like the world to know.
SOHO Project Scientist
Dr. Bernhard Fleck | Tel: +1-301-286-4098
SOHO Project Scientist | Fax: +1-301-286-0264
ESA Space Science Department | firstname.lastname@example.org
NASA/GSFC Mailcode 682.3 | Courier: Bldg. 26, Room G-1
Greenbelt, MD 20771, USA |
(3) SOHO AND DISCOVERY OF COMETS
>From Michael Oates <email@example.com>
I would like to thank Bernhard Fleck for explaining the SOHO teams views. I
am very pleased to hear that the work of the amateur is valued and also that
the original press release was not published in full, omitting the role of
the amateur. The CNN article is not the first article to be published
without giving credit to the discoverer or to amateurs in general when the
main focus of the article was a discovery by an amateur, and I just thought
something needed to be said.
Apart from recognition from fellow amateurs, the main visible manner by
which the amateur (who discovers comets in LASCO data), has recognition from
the 'professionals' comes from such news articles, mainly on the internet.
So when such an article is published and the work of the amateur is not even
mentioned, it leaves a rather bitter feeling, hence my original email.
At this point I would like to thank the SOHO team, ESA and NASA for taking
the step of making the images available in real-time. Anyone with internet
access can view the images, possibly even before any professional has had a
chance to do so. The same also applies to the archive images from which most
of my discoveries have been made. With out such openness many comets may
have gone un-noticed and unreported.
The helpfulness of the SOHO team is also most appreciated, and without
which, I would not have made the progress that I have. When help was needed
getting data or finding out how to process the FITS images, help was there.
Doug Biesecker, the person who deals with all the comet reports is also very
helpful and appreciative of the amateurs work.
I hope something positive comes of this communication, if only to let the
SOHO team know of the 'impression' that is sometimes projected of the
(4) MORE ON RADIATIVE FORCES
>From John Michael Williams <jwill@AstraGate.net>
It is VERY easy for any group of specialists to drift into error because of
disinterest in outside criticism.
Correct or not, when a specialty reaches a conclusion which can be stated in
simple, unspecialized terms, it becomes an unending battle of explanation to justify
such conclusions to persons not adhering to that specialty. This is a
responsibility which has to be accepted by such specialists.
Or, such statements must be avoided, possibly balancing error with
My cure for this would be NEVER to allow peer review unless it include at
least one reviewer drawn at random from a pool of peer reviewers of ALL
I agree my previous message was somewhat raw: I was primarily interested in
finding a way for radiation to exert a nonradial force, one which might
accumulate to an orbital change.
I agree that the NET effect of Solar radiation will be to decrease the
effective value of the gravitational constant G, depending on
surface-to-volume (surface to mass) ratio of the orbitting body. However, I
think there also will be a component caused by heat flow which will
contribute in the direction of an increase in G.
Therefore, yes, I agree that for a teardrop-shaped body, the head would be
at higher temperature than the tail, but for two reasons, not one: (1) because it
receives more sunlight, and reradiates more, which is Duncan's first point
above; and, (2) because the constant loss of heat from the tail always would
exceed that from the head.
The radiative forces are because of momentum of radiated photons, and for
photons, momentum is proportional to energy:
E = h*f and p = h*f/c.
If the heat energy radiated from the tail totals to more than that from the
head, a net force will exist in the direction of tail-to-head. And, the tail
will be cooler BECAUSE it is exerting a greater force.
Briefly, I tried to suggest that there will be three distinct forces
determining orbital corrections caused by Solar radiation (ignoring Solar
wind): The Poynting-Robertson force, the Yarkovsky force (for rapidly
spinning bodies), and the thermal force I described in my recent CCNet
Now, let me answer the main thrust of the second of Duncan Steel's issues,
which, which addresses rotation of small bodies:
Solar radiation MUST dampen such rotation. Here is a proof, expanding my
previous explanation but stopping short of equations:
Assume a body significantly larger than a wavelength of the solar radiation
in question. Ignore solar wind.
Let the body be of ANY shape and be rotating on an axis with a component off
the direction of the radius between the center of gravity of the Sun and
that of the body.
Now, in this case, it will be possible to project the axis of rotation in the direction of
the Sun so that sunlight will be irradiating, in the temporal average, half
of the projected area of the body receding from the Sun and half approaching.
Now consider the momentum transferred by a Solar photon to the approaching
vs receding halves:
The photon will be blue-shifted in its interaction with the approaching half
vs the other. Momentum of a photon is proportional to frequency f, p =
So, the momentum transferred to the approaching half will be greater than
that transferred by a photon of equal wavelength to the receding half. This
holds for any wavelength (with the IR cutoff above); so, summing over all
wavelengths, the momentum opposing rotation always will exceed that
If so, then I think one must conclude that the rotation of small bodies is
damped by Solar radiation. Note that this argument does not hold for
symmetrical radiation, such as that of the CMB.
So, the Yarkovsky effect must apply only to relatively large bodies or those
relatively recently set into rotational motion. Or both.
I have suggested that there still will be a thermal effect, even after all
rotation, other than that of the orbital motion, has ceased.
John Michael Williams
(5) BETTER SAVE THAN SORRY: SMALL ASTEROIDS, THE IMPACT HAZARD &
>From Benny J Peiser <firstname.lastname@example.org>
Have the European Union and the UK Task Force on NEOs got it wrong when they
recommended that the search for potentially hazardous asteroids should be
extended to those smaller than 1km in size? If we follow the logical
conclusions of Al Harris' cost-benefit estimate (CCNet 29 October 2001), it
would apper that it is not prudent to spend any significant amount of money
on the search for smaller asteroids since the relatively minute risk they
pose does not justify the disproportionate cost necessary for their
detection. While Harris' argumentation is not new (the whole NASA strategy
of restricting search efforts to asteroids <1km is based on this
philosophy), it poses some pertinent questions.
Foremost, governments and funding-agencies will want to know why any
additional money should be spent on NEO searches once the NASA goal of
finding 90% of asteroids <1km is accomplished. Why should the UK, Europe or
the US fund any new NEO search or follow-up telescopes if the current search
programmes are running cost-effective for both large and smaller asteroids?
Given these conclusions, it is not surprising that David Jewitt (see below)
fears that Al Harris' line of reasoning undermines the bid to build the LSST
as a key NEO search instrument.
Duncan Steel is equally critical of Al's analysis, arguing that, in light of
the terrorist attacks on September 11, his cost/benefit treatment is flawed.
Duncan believes that "the economic consequences of NEO impacts are dominated
not by nominal values for the human lives to be lost in the next impact
(whether this is a Tunguska-type event - i.e. small - or a 1 or 2 km arrival
causing a global catastrophe), but the response of people at large by the
next damaging impact... Suddenly it would be apparent that rocks from space
delivering explosive yields in excess of most operable nuclear warheads
could arrive at any time, randomly scattered across the Earth's surface and
their government has done nothing to avoid this, nor is able to do so in the
near term. In short, we're naked against this threat."
The most remarkable conversion in response to the recent terrorist outrages
comes from Clark Chapman. Clark, who in the last two decades has repeatedly
gone on record for underrating the risk of smaller asteroids, now seems
particularly troubled by the social, economic and political consequences
even small-scale impacts may have on societies:
"To me it is becoming clear that the impact of a
few-hundred-meter-sized asteroid, or even a 50-meter asteroid, would
have societal repercussions far beyond what I had previously imagined.
And it might behoove politicians to consider the potential repercussions if
we had chosen to do nothing about such impacts when there were
cost-effective things that we could have done."
But what exactly are the "cost-effective things" we could do in order to
prevent a small or medium-sized impactor to collide with Earth in the
foreseeable future - if not to specifically search for them? Both Duncan's
and Clark's worries show vividly that Al Harris' has ignored or at least
miscalculated significant societal knock-on effects of small and medium
sized impact disasters. In view of the September 11 atrocities, the real
problem space agencies may face in the event of such an impact should be
compared to the culpability of the security and intelligence agencies for
failing to protect the US population from the September 11 attacks.
While many commentators have related the hazard from small and medium
asteroid impacts to the recent man-made disaster in the US, it is
interesting to note that nobody has pointed out another possible linkage:
the failure of U.S. intelligence agencies to provide adequate warning of the
September 11 terrorist attacks, let alone their failure to prevent them.
Given the huge amount of money spend on intelligence gathering and
anti-terrorism measures in the US, and in light of the fact there were clear
indications that attacks were going to happen, September 11 epitomizes an
embarrassing failure of the US security agencies.
As a result, many heads will role both in the CIA and the FBI for the
blunders and mistakes that contributed to this catastrophic failure. As long
as we downplay the societal risks of small and medium-sized impacts, and as
long as some of us continue to hold-up the development of planetary defence
capabilities, space bureaucrats may well have to face similar criticism and
condemnation should a security and intelligence failure ever result in
inadequate warnings or the inability to prevent of an impact catastrophe.
Let's be better safe than sorry.
Benny J Peiser
(6) MORE ON SURVEY COST-EFFECTIVENESS
>From David Morrison <email@example.com>
NEO News (10/29/01) More on survey cost-effectiveness
Dear Friends and Students of NEOs:
In NEO News for October 26, Alan Harris (JPL) argued that below the
NEA global catastrophe threshold (of 1-2 km), down to the atmospheric
cut-off at about 50 m, there is no strong gradient in risk from
impacts. However, the cost of discovering NEAs goes up sharply as we
move our goal toward smaller sizes. Thus the risk stays roughly
constant, but the cost-effectiveness of the survey becomes much worse
as we try to achieve completeness at ever smaller sizes. Several
responses to these comments follow. Also included is some background
material on these subjects from a draft chapter (intended for the
book Asteroids III) on dealing with the impact hazard.
My apologies that this is a longer and more technical edition of NEO
News than most, but the issues seem relevant, especially following
the events of September 11, which have had the side effect of
sensitizing many people to the reactions of society to catastrophic
To Al Harris from Oliver Morton
It seems to me that the costs may be higher than you suggest, and
that their spectrum in the sub-kilometre range may also be flatter
than you allow, depending on the basis on which they are calculated.
If the benefits are measured crudely as $1m for every person saved,
then the true costs are those of detection *and deflection* (or in
the case of very small objects, detection and evacuation). If you
don't pay the cost of deflection, you don't get the benefits of saved
How much does deflection cost? I can imagine it might conceivably be
as low as $1 billion (some NEAR-class missions including landing and
then finally nuke) but I'd suspect more like $5 billion (bells and
whistles including some sort of probe of internal structure, either
radar or seismic), rising to $50 billion and up if for some reason or
other it were decided that there had to be humans in the loop
(doesn't seem likely to be necessary, but the possibility it might be
either necessary or deemed necessary has to be considered; Story
Musgrove inspires more confidence than the people who brought you
Mars Climate Orbiter). To what extent to deflection costs change with
asteroid size? I'm not sure. I'd imagine that deflecting a large
asteroid would cost more than deflecting a small one, but whether it
would be just a tad more (some extra kilos of lithium deuteride) or a
great deal more (the difference between a manned and an unmanned
mission) is not clear.
Nor is it entirely clear to me how these costs should be incorporated
into the overall costs. One approach is to ignore the cost of
deflection, but this seems to me wrong: if you ignore that cost you
are not really paying for the benefits you lay claim to as a
justification for the detection. A more plausible approach might be
that since a detection without a deflection saves no one (especially
true at the high end), the cost of deflection has to be added to
*all* estimates. Since the cost of deflection would dominate total
costs down to below 300 metres that seems to flatten the spectrum. If
we say deflection costs $5 billion, then a billion for LSST on top of
$100m for achieving Spaceguard goals is an easily stomached
This approach seems to me to capture an important part of the cost
implications, in that it gives you a number you must be willing to
pay: if you detect an impactor you have to be willing to pay for
deflection (I think this actually the reason why most people ignore
the costs of deflection completely, which is probably fair in all
situations *except* a cost benefit analysis). But it fails to capture
the fact that more extensive surveys are more likely to put you in a
position where you have to pay for a deflection. Perhaps it is right
to leave that out. After all, if an impactor is detected, it is worth
deflection not on the basis of the annualised death toll from such
objects but on the basis of a specific number of people who will be
killed in a specific impact.
But - and I'd really appreciate guidance on this from someone who
thinks on such matters more clearly - it may be that the correct way
of doing the accounting is not to add the one-off cost of deflection
to the costs of detection, as in the "willingness to pay" approach,
but to add just some fraction of that cost which reflects the
likelihood of deflection being necessary in that size range. Thus the
costs at the 1km range would include a smaller fraction of the costs
of deflection than the costs at the 300 metre range. This would
restore a gentle underlying slope to the spectrum in that smaller
objects would attract a greater deflection cost as well as a greater
detection cost. It would also restore the implicit position in your
analysis, in which detection costs swamp deflection costs at the
For a Tunguska class impactor, then, we might imagine an annualised
cost of $20m for deflection ($5 billion once every 250 years). For
all larger impactors the costs would be smaller. With the death rates
offered in the UK report, this means deflection always works out at
$1m per fatality averted or less. Hooray! But as you point out, a
detection system that would allow such deflections would cost as much
or more than a single deflection: maybe $10 billion to launch a 12
metre schmidt into orbit, or whatever. Maybe more.
However, if it is fair to spread deflection costs out over time, is
it not also fair to spread detection costs out, too? Would not the
costs of tracking the 100m NEOs over a century be dominated by the
one off cost of launching the main acquisition system and doing the
initial survey? On these time scales, does not the detection
investment look more like $100m a year, and isn't that the right way
to see it?
Moreover if we imagine that smaller objects might be substantially
easier to deflect, or that the deflection of large objects, because
of its greater importance, might require a human presence that the
deflection of smaller objects would not, is it not possible that the
increased costs of the less likely deflections might come to outweigh
the increased frequency of the more likely ones. We might even find
that we got a cost-size spectrum that peaked with the smallest
objects that required a manned deflection mission and went *down*
towards the end as deflection costs dropped faster than detection
That is probably fanciful. But it does seem to me that cost benefit
analysis has to find some way to deal with the costs of deflection as
well as detection. In the "willingness to pay" approach, deflection
costs will dominate. In the annualised deflection costs approach,
detection costs have to be annualised too, rather than treated as a
one-off, and thus though they still dominate I think they do so less
overwhelmingly. I honestly don't know which of these approaches in
more intelectualy coherent and would welcome clarity.
Its quite possible that applying a proper discount rate would wipe
out all these distinctions and return us to your original analysis,
in that everything but the up-front detection costs gets wiped out.
But we do not necessarily apply discount rates to this sort of
calculation. Dealing with nuclear waste, we do not say that the lives
of people centuries hence are effectively valueless.
(An alternative way of seeing things is to say that a detection
programme simply buys a reduction in the risk of causalties due to an
impact by an *undetected* asteroid. I've often found this the
conceptually clearest way of dealing with the matter. But I'm not
sure that's entirely OK in this sort of discussion, since a detection
programme can also *increase* the risk of casualties due to an impact
by a detected asteroid, currently zero.)
On the benefits side, I recently read a rather good paper which a
consultant friend of mine, Chris Elliott, presented at a European
Science Foundation meeting on risk management. He points out that
while a ballpark figure of 1m euros to prevent death is widely used,
where deaths are likely to be concentrated in single events the
figure goes up as the concentration goes up.
There is general agreement in Europe that the VPF [value of
preventing a fatality] is around 1M euro. [However, a] higher VPF may
be needed to reflect the greater social impact of multiple deaths or
injuries from a single event, or where members of the public place
trust in industry. For example, in the railways a VPF in excess of
1.5M euro is used, rising to several million euro for major
accidents, and the airlines appear to use a VPF in excess of 10M
euro. The MEM [Minimum Endogenous Mortality -- a German term for not
allowing technological change that appreciably increases the risk of
death] approach weights the impact of each fatality or injury in a
major accident more heavily than it would be weighted if it occurred
Such an approach would inflate the "benefits" side of asteroid
detection and deflection significantly. Of course, there is no
asteroid industry analagous to railways and airlines to take this
approach. Maybe this type of scaling doesn't apply to natural
disasters. But airline fatalities are often used in the literature as
illustrations of the likelihood of asteroid fatalities, and by
airline standards $200m a year for 20 lives saved seems reasonable.
Hope this is helpful; it is far longer and more involved than the
message I set out to write, for which I apologise.
Best as ever,
To David Morrison from Duncan Steel
Thank you for circulating those recent comments regarding cost/benefit
ratios for NEO detection (and deflection), and possible implications
for any "Spaceguard goal". I have a few opinions to air, which I hope
will cause people to consider adopting changed viewpoints.
Before doing so, I start with a brief comment concerning the exchange
between Al Harris and Oliver Morton specifically pertaining to whether
the cost of deflection (for whatever sized object) should be explicitly
included in any assessment of the cost/benefit ratio involved in setting
some Spaceguard target. Unless I overlooked it somewhere, it appears to
me that Oliver missed the point that a Spaceguard project achieving a null
result (e.g. "we have discovered 90 percent of the NEAs larger than
x km in size and shown that none will hit the Earth within the next
century") provides a benefit entirely independent of any consideration of
deflection being necessary (or not, in that case). In that situation - the null
result at the 90 percent level of completeness - the estimate of the hazard
posed by NEAs larger than x km is reduced by a factor of ten, and that is a
benefit. Similarly, if it were to be possible to show that you will
certainly not have an accident in your car in y months over the next
year then the expectation of loss comes down, and with it your insurance premium
(e.g. I knew someone in Australia who had a veteran car which he only needed to
tax and insure to cover three outings/rallies per annum; the rest of the
time it was safely in the garage). Thus there is a benefit accruing from
*any* NEA discoveries shown not to be potential impactors. To give
another example, personal DNA testing has insurance implications because the
possession of a particular gene may indicate a propensity to develop a
certain type of cancer. It is not certain you will contract that cancer,
but it is more likely than the norm for the population as a whole.
Similarly, genetic screening showing you do *not* carry that gene has a
nominal benefit, and has the immediate effect of increasing your life
expectancy, despite the fact that you might fall to another type of
cancer. With each NEA discovered and shown not to be an Earth-impactor
within the interval of interest (the next century, say), a benefit has been delivered in
that the probability of individual A being killed by an asteroid impact has
There is another (real) benefit of NEO search programmes: future sources
of raw materials for space colonisation. When James Cook was sent to the
Pacific to watch the transit of Venus in 1769 the British government was
not necessarily thinking in terms of the benefits eventually coming from
setting up colonies in Australia and New Zealand.
Now turning to the main subject: the cost/benefit ratio of NEO detection
(and perhaps deflection) programmes. Let me again say what I have said
many times previously: that people elsewhere have no business saying
that "the US should be doing this" or "NASA should be doing that". The
American people bear the brunt of the cost of current research on NEOs, in
particular search programmes, and it is for others to try to do better in
their own countries if they feel that an upping of the game is required. Further,
I have long supported the party line that the first target should be the
larger-than-1-km NEAs, because (a) These dominate the impact hazard to
individuals/civilisation; and (b) They are the easiest and hence primary
targets for discovery.
Having written that, however, I continue to write that I believe that
the forms of cost/benefit analysis often propounded are incorrect in their
fundamental bases, and so are misguided. I believe that they grossly
underestimate the "benefit" (in terms of a needed avoidance of a
negative effect), and so grossly underestimate the expenditure that could
be justifiably incurred, because they mis-identify the *nature* of the
Now I explain what I mean. The calculations presented in the recent
exchange and indeed previously in many such discussions are based, it
seems to me, very much on a physicist's view of the world and economics:
calculate the long-term averaged annual number of deaths and apply a "value"
to each nominal life lost, and from that derive an annual expectation of loss
and hence a "justifiable" expenditure figure. Heck, I've done the same in
public talks many times. Fundamentally, however, the sum is wrong, in
that it does not represent the reality of the response of political
decision-makers around the world. There are two sides I'd like to highlight.
On the one side there is the reasoning (correct, so far as I can see) of
Geoff Sommer, one of your co-authors, on how the variety of stakeholders
view the NEO impact hazard. I take the lack of any adequately-funded
Spaceguard programme in the US, and the lack of any sensible programme
by any other nation at all, as proof positive that we have not understood
the (non-) response of decision-makers, and taken appropriate steps. If you,
or I, or any of the other members of the Spaceguard Committee in
1991-92, had been told that a decade hence we would still not have even one
dedicated NEO search telescope of aperture above two metres, I doubt
whether we would have believed it. As we often say, this is a no-brainer. So
why have we been singularly unsuccessful in getting the job done? That is
not a rhetorical question: I would dearly love to know why, as I simply
don't understand. Even more than that, the mass media don't understand. If
one tells journalists what is being done as compared to what needs to be
done, then they cannot see it either, even though they are habituated to
uncovering governmental shenanigans.
On the other side I would argue most strongly that the form of analysis
of the hazard presented by Al, and hence the cost/benefit treatment, is
centrally flawed. My belief is that the economic consequences of NEO
impacts are dominated not by nominal values for the human lives to be
lost in the next impact (whether this is a Tunguska-type event - i.e.
small - or a 1 or 2 km arrival causing a global catastrophe), but the
response of people at large by the next damaging impact no matter what its
magnitude, provided it's above some lower limit and also occurs in some
place which is easily accessible to the media. For the sake of argument I'd propose
an object having a 5 MT yield occurring anywhere over 50 percent of the
Earth's surface (I've just excluded Antarctica and the ocean centres).
Around a once-a-century event.
My belief is that, irrespective of deaths or physical damage caused, the
economic consequences would be above $100 billion, and so exceed Al's
estimates of (annual) costs for even the most ambitious search
programmes (going down to 100 metres).
The source of that loss/consequence is not deaths or property damage,
but the precipitous drop in confidence caused in people at large. Suddedly
it would be apparent that rocks from space delivering explosive yields in
excess of most operable nuclear warheads could arrive at any time,
randomly scattered across the Earth's surface and (here is the important
part) their government has done nothing to avoid this, nor is able to do so in the
near term. In short, we're naked against this threat. That would be so
damaging to people, psychologically, that a very substantial economic
hiccup would be inevitable.
It is unfortunate that we now live in times in which we have seen a
parallel event occur. The economic consequences of the appalling events
of September 11th are dominated not by the loss of the aircraft, nor of
the wing of the Pentagon, nor of the World Trade Center, nor of the
economic "values" of the human lives lost, nor the ongoing effects on their
friends and families, but by the effects upon other commerce not at first
sight having any great dependence upon the actual events. Things like airlines
going bust or laying off staff are obvious, but the many other
peripheral knock-on consequences will add up to far more. Central to the
response, not only in the U.S. but also in many other Western nations, is the
realisation that there is no simple solution, and our governments cannot
make us safe overnight.
Further, imagine applying Al's logic to the current anthrax panic. By
his analysis the benefit that might be accrued is three deaths avoided at
a few $M each and that should inform our response (I write "our" because
this scare has affected several other nations including the UK). The
reality is that the cost (money spent tackling the problem) is higher than
that by orders of magnitude, and of course there are many other deleterious
effects (Congress mail unopened for two weeks) not included in the
simplistic summation of "deaths times nominal human life value".
The reality here would be, I would guess, that more people have been
killed in the past two weeks by being hit by fullen-laden concrete mixer
trucks than by anthrax infection. Indeed I'd guess that more people die every
night by falling out of bed in their sleep. But I see no media furore,
nor popular panic, against concrete trucks. Nor demands for safer beds.
My bottom line, then, is that the real number to be put into
calculations of economic consequences of NEO impacts is much higher than Al
supposes, but that number is unquantifiable: the level of panic depends upon the
psychological mind-set of the people involved, and clearly the anthrax
panic has been heightened by events of September 11th and subsequent
developments. I regret needing to use this as an illustration, but it is
very pertinent to the argument.
What, then, should we (the developed world) be spending on tackling the
NEO impact hazard? The decision on that is not ours to make: it is a
job for politicians and the like. However, we should be presenting them
with a full summary of what we know (and indeed what we do *not* know).
One facet of what we know is that there is a better than one percent
chance that within a year an object from space will meet the Earth and
release more energy than the largest hydrogen bombs ever deployed in
missile warheads. As physical scientists we can say that. We could add
that this might well cause a panic on a level not seen even over the
past two months, but that they (the politicians) should consult psychologists
and the like to discover what the collective response might be. Counting
up deaths and their economic values is one approach, but I believe that
it is largely irrelevant to the overall hazard and its proper
Duncan Steel <D.I.Steel@salford.ac.uk>
To Oliver Morton from Clark Chapman:
Your essay is very interesting and stimulating. But I think the
issue is simpler than that. I am no economist, so my reaction
may be naive. But it seems to me that the problem is one that
should be divided into two parts: what happens *after* an
object is found that is definitely going to impact Earth, and
what we do about the impact hazard *before* such an (improbable)
discovery is made.
First, consider what happens afterwards. Certainly such an
object should be deflected, and -- in this case -- the cost of
deflection will surely be tiny compared to the potential death
and damage averted. (This might not be true at the smallest,
e.g. Tunguska, sizes but certainly applies to the >300 m objects
Prior to such a discovery, I see no reason to take the large costs
of deflection into account, since it is an on/off decision based
on whether an incoming object is detected. (Or should be: if one
is proposing to have a deflection system deployed *before* such
a discovery, then its costs would be included.) What should
be factored into the cost-benefit analysis are all the costs --
above and beyond what would be done anyway based on basic science
objectives -- of preparing for the possibility of deflection.
These include searches (say with the LSST, above and beyond the
astrophysical and solar system science that would otherwise be
done), studies of physical properties (including spacecraft missions
to assess surface and interior structure in a deflection context,
again above and beyond what would otherwise be justified as pure
science), research and development on deflection systems
(falling short of building and deploying them), and other research
on the impact hazard (e.g. sociological, environmental) and on
development of emergency management infrastructure.
I don't think it is trivial to distinguish what fraction of NEO
research is based on pure science versus "science in the public
interest". NASA testified to Congress in 1998 that its ongoing
small-bodies missions program (e.g. NEAR) addressed the impact
threat, yet most of these missions were approved strictly for
scientific reasons. Even "Deep Impact" had only a few sentences
in its proposal related to the impact threat, if I properly
understood what Mike A'Hearn told me a few days ago.
There are some grey areas in this argument -- how close of a
near-miss might engender deflection costs, the degree to which
partial development of deflection systems should be ramped up
prior to discovery of an object in order to be prepared for that
small fraction of cases where time would be of the essence, etc.
But fundamentally, there are *no* costs of deflection operations
until the impactor is found...and then there is no question of
making the expenditure.
Regarding your comments about circumstances where the value of
preventing a fatality may be larger (e.g. in the commercial
airline business): the events of Sept. 11th have certainly
changed my perception of the vulnerability of society to certain
kinds of death and destruction. As the work of Slovic (1987,
Science, 236, 280-285) demonstrated, human beings and society
may be expected to react much more strongly to risks that are
"unknown" and "dreaded" than to those risks that are common, humdrum.
He listed "satellite crashes" and "nuclear reactor accidents"
in the former category, but has subsequently said
that asteroid impacts are in the same category. Certainly
terrorist attacks are as well. This is exemplified by the
recent terrorism where the number of deaths normally accumulated
on US highways in two months has resulted in enormous economic
losses and damaged national psychology. Similarly we see the
federal government in partial shut-down after half-a-dozen
people die from anthrax whereas (see current issue of the
New Yorker) most of the 20,000 deaths expected from the flu
this winter could be prevented by a stepped up innoculation
To me it is becoming clear that the impact of a few-hundred-
meter-sized asteroid, or even a 50-meter asteroid, would have
societal repercussions far beyond what I had previously
imagined. And it might behoove politicians to consider the
potentail repercussions if we had chosen to do nothing about
such impacts when there were cost-effective things that we
could have done.
Clark Chapman <firstname.lastname@example.org>
To David Morrison & Al Harris from David Jewitt
The discussion about cost effectiveness is clearly
relevant to the NEO detection business, and there
is certainly a point of diminishing returns, as
Al mentions. I agree that this point hovers near
the 300-m NEO size mark.
The NUMBER ONE science driver of the LSST is the
search for NEOs down to the 300-m scale. The NEO
community should understand that LSST has a lot
of momentum, and that it will probably be built
whether they support it or not. What is at stake is
maintaining the LSST as PRIMARILY a solar system
survey telescope. If the NEO community does not
embrace LSST, rest assured that alternate science
drivers will float up from the astrophysics
community. Many of these are fascinating, of course
(supernova surveys, weak gravitational lensing) but
the solar system applications will surely be pushed
into the background, just as has happened with almost
every other large telescope at the national and
I would find this especially disappointing. The key
parameters of the LSST (the need to survey the visible
sky to mag 24 every week or faster) were set by the
NEO problem, because this is technically the most
challenging project for LSST. To lose this leverage
would mean that the LSST would not be operated as
an efficient NEO machine. It would become an
efficient supernova machine that happens to discover
some NEOs, but not in an optimised way. Right now,
LSST is an efficient NEO machine that will also do
some astrophysics. Why let the current, favorable
situation drift away?
As for the cost: LSST is budgetted at about $160M
for 10 years (not $1 Billion as suggested by
Al Harris). I think the cost is less the issue here than
is the question of whether the members of the NEO
community want to retain controlling influence over
the design and operation of LSST.
Dave Jewitt <email@example.com>
Draft from Chapter for Asteroids III book
DEALING WITH THE IMPACT HAZARD
David Morrison (NASA ARC), Alan Harris (JPL), Geoff Sommer (RAND),
Clark Chapman (SWRI, Boulder), Andrea Carusi (IAS, Roma)
.......While it is highly improbable that a large (diameter > 1 km)
NEA will hit the Earth within our lifetimes, such an event is
entirely possible. In the absence of specific information, such a
catastrophe is equally likely at any time, including next year.
Society needs to be prepared to deal with this eventuality. In the
meantime, however, the search for possible impactors will inevitably
lead to false positives, NEAs that appear for some time to be a real
threat. We need to consider the effect of such reports on society. As
we discuss in the final sections of this chapter, impact hazard
studies can be considered an applied science; that is, science
applied to tangible needs of society. In determining an optimum or
even advisable hazard mitigation strategy, the reaction of society to
scientific information on the hazard should be considered. The NEO
community has a social responsibility to ensure that its message is
not just heard but comprehended by society at large. Since the risk
knows no national boundaries, it also behooves us to seek solutions
that recognize the international constituency with a stake in impact
prediction and prevention........
........ While NEO research embodies classic scientific objectives,
impact hazard studies form an applied science that may be judged by
different criteria. In determining an NEO hazard mitigation strategy,
we must consider the reaction of society. Such considerations are
familiar to specialists in other fields of natural hazard, such as
meteorology (with respect to storm forecasts) and seismology. NEO
hazard specialists have the added difficulty of explaining a science
that is arcane (orbital dynamics) and beyond personal experience (no
impact disaster within recorded history). As the NEO community has
begun to realize, it has a social responsibility to ensure that its
message is not just heard but comprehended by society at large. The
adoption of the Torino Impact Scale (Binzel 1997, 2000) was a notable
first step toward public communication, although the unique aspects
of NEO detection and warning (particularly the evolution of
uncertainty) continue to cause communications difficulties (Chapman
Once it is accepted that the impact hazard is a social and not just
scientific problem, it is a short step to allow that considerations
of maximum social benefit may well constrain the scope and form of
scientific investigation. That is, while the scientifically optimum
level of uncertainty is zero, the socially optimum level is nonzero.
It is neither possible nor affordable to remove risk and uncertainty
entirely. This is not just a trite benefit-cost argument. Rather,
scientific information can have marginal disutility. As an example,
many might argue that society incurs a net cost for the science of
nuclear physics, since nuclear proliferation is facilitated thereby.
Nuclear test ban treaties rest upon a presumption of the disutility
of the scientific and technical information derived from the tests.
The inescapable conclusion is that if, despite its best intentions,
the NEO community levies a perceived cost to society through
mishandled or garbled communication, then society may well act to
remove that cost by choosing not to support NEO surveys and related
To date, international NEO survey programs have been conducted and
coordinated by an eclectic mix of state and non-governmental
organizations, operating within the scientific paradigm of openness.
The emphasis has been on generating discoveries. Some of these
discoveries indicate (initially at least) a non-zero possibility of a
future impact, raising the issue of whether to issue a warning. In
some cases (e.g., 1997 XF11, 2000 BF19) individuals or organizations
have made public warnings that were widely reported by the press,
only to be quickly withdrawn when additional data or more refined
calculations became available.
When and under what circumstances should public warnings be made? The
trigger threshold for a "confirmed warning" is a key parameter for
both NEO scientists and those (primarily science journalists) who
make decisions about what information to disseminate to the public.
The NASA NEO Program Office at JPL internally defines a confirmed
warning as the Earth being interior to a three-sigma error ellipsoid
projected into an Earth-impact target plane. The International
Astronomical Union (IAU) NEO Working Group adopts as a threshold the
"prediction of impacts with probability larger than one in a million
(10^-6) in the near future (less than 100 years)." This accords with
the Torino Scale threshold for a one-kilometer object to achieve
Level 1 (that is, to rise out of the background risk). The Torino
Scale "raises the bar" for smaller objects - for example, a 100 meter
object requires a one-in-ten-thousand (10^-4) collision probability
to reach Level 1. All of these guidelines are informal, and the IAU
leaves any decision about public release to the discoverer of the
threat. In practice, each case that has received wide publicity (1997
XF11, 1999 AN10, 2000 BF19, 2000 SG344) has had its own unique
nature, demonstrating both that guidelines must be flexible and that
it is impossible to control the behavior of either astronomers or the
media by fiat from above. Thus the recent historical average of
approximately one warning (or rumor thereof) per year may continue.
Although much thought has been applied to modeling the discovery rate
of survey programs (as discussed above), no researcher has attempted
to model warning rates, yet that question is of paramount interest to
policy makers. There have been no confirmed warnings to date that
have survived for more than 24 hours, so when the first such occurs,
society is in uncharted territory. Of course, most confirmed warnings
will become false alarms when new data are acquired, and it can be
expected that major astronomical facilities can be quickly turned to
NEO follow-up given sufficient priority to do so. It is an
interesting situation: since most warnings will be false alarms, it
would seem to make sense to raise the warning threshold, yet doing so
might result in less effort to make new observations and thereby
prolong the perception of a potential threat. The reason given for
most announcements has been to stimulate additional observations,
with warning to the public a secondary issue.
As discussed previously, the Spaceguard Survey will shortly
experience diminishing returns in its primary goal of discovering
NEAs larger than 1 km, a natural consequence of population sampling
without replacement. On the other hand, discoveries of smaller NEAs
will continue in proportion to sky coverage and aperture, as their
population has been barely sampled. There will be a natural incentive
to shift the survey goal down in size to the region of highest
return. There is already a "policy hook" for extending the search to
smaller objects, in the form of the Council of Europe's Parliamentary
Assembly Declaration 1080 "on the detection of asteroids and comets
potentially dangerous to humankind". This document called for
establishment of an "inventory of NEOs as complete as possible with
an emphasis on objects larger than 0.5 km in size." More recently,
the U.K. NEO Task Force (Atkinson et al. 2000) called for a "new 3
meter-class survey telescope for surveying substantially smaller
objects than those now systematically observed by other telescopes."
In the US, the National Research Council has recommended the
construction of a 6-8-meter Large-aperture Synoptic Survey Telescope
(LSST), with one goal to "catalog 90% of the NEOs larger than 300 m"
(NRC, 2001). Note that the smaller telescopes now used for follow-up
will be unable to keep up with the newer, larger aperture survey
telescopes in terms of limiting magnitude. Sometime soon, a
fundamental change will be required in the approach to follow-up
Mitigation of the NEA impact hazard can take two forms. The preferred
but more technically challenging option is to deflect the threatening
NEA, changing its orbit so that it will miss the Earth (Ahrens &
Harris 1992, 1994; Simonenko et al. 1994; Melosh et al. 1994;
Morrison & Teller 1994; Weissman 1994). Alternatively, we can follow
the example of other natural hazards such as earthquakes and severe
storms, focusing not on prevention but on dealing with the aftermath
on an impact (Garshnek et al. 2000). In practice, the two options are
complementary, depending primarily on whether or not there is a
long-lead warning of the threat. Note that a short lead time for an
NEA is extremely unlikely - we can expect either decades of warning
or none at all. But a warning of only a few months is possible in the
case of a comet, raising a somewhat different set of policy issues.
Consider the option of interception and deflection, which would
require new and expensive defensive systems. Should such systems be
developed now? From the standpoint of an allocator of society's
resources, an uncertain threat calls for adaptive policies, delaying
potentially costly action but informing later decision by investing
in uncertainty-reduction measures. In the context of the NEO impact
hazard, this means avoiding the costs of standing organizational
structures and capital expenditures until a threat materializes,
while continuing modest support for surveys. Measures to gather
information about the hazard (such as space missions to NEOs) could
also be supported, especially if they can be justified on other
In an organizational sense, planning for adaptivity entails
establishing a chain of responsibility prior to the materialization
of an emergency - that is, a shadow institution, the only type
possible where the usual incentives for institutional cohesion don't
It is illustrative to examine a hypothetical NEO emergency
organizational plan, using the U.S. government as an example. We need
an office in the Executive Branch of government, and the National
Science and Technology Council (NSTC, part of the White House Office
of Science and Technology Policy or OSTP) seems to have natural
purview over the impact hazard. NSTC has five multi-agency
committees, each of which pertains to an aspect of the impact hazard:
Environment and Natural Resources; International Science,
Engineering, and Technology; National Security; Science; and
Technology. The formal assignment of the NEO impact hazard to an NSTC
Committee would be accomplished by the drafting of a Presidential
Decision Directive (PDD/NSTC) or Presidential Review Directive
(PRD/NSTC). Little else would need to be done until there is
confirmed warning of a threat. In all likelihood the draft PDD has
remained in the desk drawer of an NSTC staffer. The President's
signature would be sought upon confirmation of the warning. In the
unlikely event that lead-time is short, the issue will be moved out
of the NSTC to the National Security Council (NSC). In that case, the
PDD can be issued as a joint NSC/NSTC document, for which there is
precedent in the National Space Policy of 19 September 1996 (PDD/NSTC
8 and PDD/NSC 49). Undoubtedly similar procedures exist in other
countries, and it may be that multinational organizations (including
the United Nations) would also wish to develop contingency plans.
The principles of adaptive planning in the face of uncertainty
fundamentally affect mitigation investment decisions. Civil defense
measures have the advantage that improvements can be gained due to
synergism with more mundane natural hazards. To date, very little
attention has been given to the demands that would be placed on
governmental and private disaster-response systems by even a small
(Tunguska-class) impact in a populated region (Garshnek et al. 2000).
However, consider the more challenging question of interception to
deflect a threatening NEO, and the expenses of a standing force of
anti-NEO launchers. It has been the position of most NEO researchers
that these expenses are best deferred, since in all likelihood there
will be sufficient warning time before impact to develop an
interception system from scratch. To this, the advocates of
interception systems reply that "in all likelihood" does not mean
"always", and then play the trump card of the cometary threat. An
example of a defense architecture oriented toward this most
challenging case is found in Gold (1999).
The diverse nature of the NEO population (particularly with regard to
mechanical strength and composition) has been used as an argument to
defer investment in interception capability until a specific target
object has been identified (e.g., Morrison et al. 1994, Harris et al.
1994, Sagan 1992, 1994, Sagan & Ostro 1994). On the other hand, in a
survey regime characterized by many false warnings, value can be
gained from a system that has uncertain effect, to the degree that it
reassures the population and prevents panic. For example, the US
rushed Patriot antiaircraft missile batteries to Israel during the
Desert Storm conflict with Iraq, for defense against ballistic
missiles, despite the fact that they were not designed to intercept
missiles. The Patriots proved militarily ineffective but politically
An adaptive planning approach could also accommodate the short
warning scenario associated with long-period comets, requiring that a
relatively low-cost generic interception system be built and tested,
then shelved. In the event of emergency, the system would enter surge
production, with industrial capacity commandeered from other
programs. In this manner, there would be a tailored response to the
threat, and operational flexibility would be enhanced. Salvoes could
be launched, and in many cases, shoot-look-shoot would be possible.
Uncertainty in threat composition could be countered by adding an
additional production margin.
It is facile but probably fallacious to imagine a scenario where an
NEO progresses in a step function from zero threat to Earth impactor.
The threat that stays a threat will experience an overall rise in
impact probability, as the error ellipse shrinks yet the Earth stays
within it. More threats than not, however, will suddenly see their
impact probability go to zero as the error ellipse shrinks to exclude
the Earth. This feature of the evolution of impactor uncertainty will
encourage those who wish to defer commitment to interception or who
just want to keep the public purse closed. The net effect is that the
system reaction time will need to be much shorter than the warning
time from point of confirmed threat. This already challenging
situation will only be worsened by failure to examine scenarios and
develop appropriate contingency plans. To date the NEO community has
not made much effort to pursue such options or enter into dialog with
government organs that deal with security issues.
Many of these issues were discussed by Parks et al. (1994). They
concluded that societies will not sustain indefinitely a defense
against an infrequent and unpredictable threat. Governments often
respond quickly to a crisis but are less well suited to remaining
prepared for extended periods. But these conclusions reflect a
history in which the less frequent threats are generally of less
consequence than those encountered more often. In contrast, the
greatest NEA impact hazard is from the very rare large impacts. Put
simply, each reader of this chapter has a greater chance of dying
within the next month from a globally catastrophic impact than from
any of the smaller more frequent impacts. It remains to be seen how
governments and other institutions of society will respond to this
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