The NASA Spaceguard Goal is to discover 90% of the NEAs larger than 1 km diameter by 2008. The 1 km size was selected because it is near the lower limit for an impact that would likely cause a global ecological catastrophe. Such "civilization threatening" impacts dominate the risk statistics. We are each at much greater risk from a potential impact by an asteroid 1 km or greater in diameter than by the cumulative risk of all smaller and more frequent impacts. Put simply, a global ecological catastrophe places us all at risk, while even the largest impact below the threshold for global catastrophe leaves most of the world unscathed.
More recently there has been growing interest in "raising the bar" by lowering the NEA size for which completeness is sought. Last year's UK NEO Task Group was among those advocating a shift toward smaller (hundreds of meter diameter) NEAs. Of course, we are finding more sub-km NEAs today than those above 1 km in diameter. We don't throw these small NEAs back, as we might when we catch small fish. But the present surveys will take a very long time to achieve completeness as 500 m or 300 m diameter.
Below is a story by reporter Rob Britt at Space.com that includes a discussion of shifting emphasis toward finding smaller NEAs. Following that is a comment on this issue from Al Harris of JPL. Harris argues that below the global catastrophe threshold (of 1-2 km), down to the atmospheric cut-off at about 50 m, there is no strong gradient in risk. That is, we are roughly at the same risk from 500 m NEAs as from 50 m NEAs. So there is no natural stopping point once we move our focus to smaller objects, except for one important thing. 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.
Two additional replies and comments conclude this news item, received from Oliver Morton and Duncan Steel in the UK.
Asteroid Discoveries May Outpace Ability to Assess Threat to Earth
By Robert Roy Britt
Senior Science Writer, Space.com
19 October 2001
After hunting asteroids for two centuries, astronomers achieved a minor milestone earlier this month when the tally of known space rocks whose orbits are well established surpassed 30,000, three times the total less than 3 years ago.
There was no press release. The people who do the counting are too busy for ballyhoo.
And soon they will be busier. The tally is expected to double in a matter of months and likely soar a startling six-fold or more within 3 years. And these are just the well-studied rocks. Roughly 150,000 more have been spotted but need further study before their orbits can be known well enough to put them in the books.
Why the bounty? Telescopes are getting bigger and better, and the high-tech electronic cameras that record the observations are able to see things that were invisible just a few years ago.
As a result, asteroids are being found at such a dramatically increasing rate that some astronomers say the discoveries may soon overwhelm the ability to properly catalogue the objects and do critical follow-up observations that could reveal if an asteroid is on a collision course with Earth.
Astronomers stress that there is almost no immediate threat that the planet will be hit.
Any large asteroids bearing down on Earth would likely be discovered decades in advance, experts say. But smaller objects often go undetected and could destroy a city. And no one can say if or when a surprise impact might occur.
For now, however, it is the data load that most worries some astronomers.
Several leading asteroid researchers interviewed by SPACE.com warned of a looming bottleneck in the worldwide network of computers and researchers who determine the future paths of thousands of asteroids that are detected every month. One crucial link in the process depends mostly on amateur astronomers who help to put together painstaking details of an asteroid's path after it has been first spotted.
"The increasing number of new asteroids will eventually overwhelm observers who do the follow-up," said Benny Peiser, an expert on the threat of asteroids at Liverpool John Moores University in the United Kingdom.
The flood of data also could overload the computers and staff of the primary international clearinghouse for asteroid information, the Minor Planet Center in Cambridge, Massachusetts.
The global asteroid monitoring system is a sometimes-loose collaboration of private institutions and government agencies, along with the amateur astronomers and several dozen professional asteroid researchers around the globe.
The amateur component has developed rapidly over the past decade, often on an ad hoc basis.
The professional side of things is marked by frequent disagreement between its most fervent and productive asteroid hunters. They argue over how much information should be provided to the public and how quickly it should be released. They debate definitions, procedures, and the fine points of risk assessment.
But the scientists all agree on one thing: Earth will one day be targeted by a potentially devastating asteroid, and they aim to avert disaster by spotting it in advance.
The most worrisome rocks are the big ones: Asteroids larger than 1 kilometer (0.62 miles) across suspected of hitting Earth every 100,000 to 300,000 years, says Michael Paine, a volunteer with Planetary Society in Australia. Paine tracks the varying estimates of asteroid impact risks made by several research groups. A collision from an object this large would rock the planet, disrupt the global climate for years and could render some species extinct.
Asteroids 100 meters (328 feet) across or larger slam into the planet every 1,000 to 3,000 years, Paine says. Such an event could eliminate a city or create a tsunami that might inundate shore communities and even large cities along multiple coastlines.
If and when such a calamity is foreseen, precautions could include evacuating areas to avoid any local disaster that might be rendered by a small asteroid. In the future, a spacecraft might be sent to destroy or deflect a larger incoming object, saving the planet Hollywood style.
The first priority of business on everyone's list is to find more space rocks. Lately, this is not a problem, and the success of asteroid hunters grows more stunning by the month.
The first asteroid, Ceres, was discovered in 1801. It took nearly 200 years -- until 1999 -- astronomers to find and number the next 9,999 cousins of Ceres. But with advances in telescope technology and additional human and optical resources devoted to the task, the count has tripled since then. On Oct. 2, it reached 30,716.
"I'm guessing we ought to be up to 200,000 in 2004," said Brian Marsden, who serves in a part-time capacity as director of the Minor Planet Center, "assuming we can physically keep up with it."
The center, which operates under the auspices of the International Astronomical Union, processes the world's asteroid observations and connects major observing programs to the individuals who do follow-up observations.
The center processes more than 70,000 observations on a busy day, Marsden said. The tremendous amount of data stems from this fact: Although a nearby asteroid can often be recognized from data on a single night, some must be observed over several months, even years, to determine their ultimate path or destination.
In September, a record 1,642 asteroids were officially numbered by the Minor Planet Center. But there are many more asteroids that have been spotted. In all, some 150,000 are already known. Most of these have not been observed well enough to determine their orbits precisely. Only when an asteroid's path is pinned down with certainty does it get an official number.
To keep up with the growing workload, Marsden and his two colleagues frequently put in 16-hour days and work six or seven days a week, he said.
Most asteroids pose no threat to Earth, traveling around the Sun between the orbits of Mars and Jupiter. But the gravity of Earth and other planets can cause an asteroid's path to change with each orbit. Of greatest concern are asteroids 1 kilometer or larger that stray close enough to our neck of the solar system that they could one day cross paths with the Earth.
Researchers disagree on how many of these Near Earth Asteroids there might be, but the leading estimates range from 700 to 1,200. Roughly 500 have been found, none of which poses a threat anytime in the next century.
But smaller objects are tougher to spot, and some are discovered just days before they pass by Earth.
On Oct. 8 of this year, for example, an asteroid thought to be between 50 and 100 meters in diameter zoomed by our planet at little more than twice the distance to the Moon -- a whisker by the standards of our solar system's size. The object was first detected just two days prior. Its path was determined only the day before the close encounter.
Search programs "red flag" such nearby objects, which move more quickly against the background of stars as compared to more distant asteroids. Scientists say it is critical to note these fast-movers and quickly do follow-up observations to make sure Earth is not in their sights.
Around 100 highly qualified but unpaid astronomers are often well equipped and are viewed as every bit as capable as professional astronomers. But there simply won't be enough of them as the ability of professionals to spot smaller asteroids improves and the data load grows.
"Sooner or later things are going to come to a crunch," Marsden says.
Marsden is confident that his small crew can keep an eye on things for now, but he says more funding is required to hire two more people, as well as someone to maintain the sometimes-glitchy computer system that processes asteroid data and supplies the follow-up observers with the data they need to go hunting.
Delays of 24 hours or more have occasionally occurred in the past, Marsden said, due either to computer problems or the fact that he and his two colleagues are putting in seven-day weeks in an effort to keep up.
The growing workload has begun to generate tension and cause workers to snap at each other -- something that never used to happen, Marsden said.
Meanwhile, the Minor Planet Center's funding is dropping. The bulk of its money has traditionally come from subscriptions to its publications of asteroid data. But with the transformation from printed to electronic publishing, fewer research institutions, libraries and individual astronomers are willing to pay for the data.
NASA provides about half of the center's budget (Marsden's own salary comes from his position with the Smithsonian Institution). Yet it is NASA that funds many of the major search programs that generate the data that pours into the Minor Planet Center.
David Morrison, an asteroid researcher and director of the space program at NASA's Ames Research Center in California, agrees that the increasing pace of discovery may overwhelm the cadre of amateur astronomers doing follow-up observations, no matter how well equipped they might be.
Morrison said that any asteroid aiming for Earth can be discovered decades before the impact, assuming it is bright enough to be picked up with current search telescopes.
"The problem is not that we won't see it soon enough," he said. "The problem is that we [might not] spot it at all."
But once an asteroid is discovered, Morrison said, there's no reason it can't be properly processed. Morrison thinks management of the growing bounty of data needs to extend beyond the Minor Planet Center.
"We'll need to distribute the workload among a number of international organizations that already have the capability to process these data," Morrison said.
At least three organizations around the world have the needed computer programs. One, in Arizona, is called the Lowell Observatory Near-Earth-Object Search (LONEOS). Lowell scientists used to process their own asteroid data, but a grant that funded the effort was not renewed. So now they are forced to forward mostly raw data to the Minor Planet Center.
Bruce Koehn, a research scientist who does the programming for the LONEOS effort, said his colleagues would prefer to process their own data. He said there are several reasons why distributing the workload is a good idea.
"The Minor Planet Center can easily become overwhelmed," Koehn said.
Equally important, he said, is that having multiple groups do the calculations provides a check and balance for overall accuracy. It also creates an environment where new and improved methods will be developed by one group and adopted by others.
Regardless, the future might not be as grim as others think, according to Koehn.
Most large asteroid survey programs do some of their own follow-up work, he said. As the pace of discovery increases, he thinks these survey teams will be forced to do more of their own follow-up. And other search projects currently in the planning stages have included follow-up observing as part of their strategy, he said.
In addition, two groups currently do some calculations that contribute to the overall knowledge base and double-checking. One team is at the University of Pisa and another at NASA's Jet Propulsion Laboratory.
Some researchers say the Minor Planet Center should remain the hub of asteroid information.
"The Minor Planet Center has the expertise, links and contacts," said Jonathan Tate, founder and director of Spaceguard UK, an group that advocates increased search efforts. "They are a central node, and to distribute this would produce further complications, both practical and organizational."
Tate agreed that more funding is needed to allow the center to keep pace.
One way to cope with the increasing workload is to fund and foster greater international cooperation and new search projects, several experts say. Efforts are under way to establish a multinational professional search program in Europe, says Peiser, the UK researcher. The idea was jumpstarted last year when a task force set up by the British government recommended sweeping changes to how governments should view the threat of asteroids.
"We suggest that the United Kingdom and other governments, together with the International Astronomical Union, NASA and other interested parties, seek ways of putting the governance and funding of the Minor Planet Center on a robust international footing, including the Center's links to executive agencies should a potential threat be found."
The Minor Planet Center has not yet benefited from the call to action.
Meanwhile, astronomers around the world grapple with a simple fact: They cannot see most asteroids smaller than 1 kilometer until they are relatively nearby. The technology exists, but it has not been devoted to the task. The reason goes back to decisions made roughly a decade ago.
Early discussions spearheaded by NASA resulted in the goal of finding 90 percent of all Near Earth Asteroids ['larger than 1 km diameter] in a decade's time. Scientists talked of setting the limit lower, to include objects down to 100 meters, but they knew that would have meant finding a lower percentage of the many more objects they'd be looking for. Several scientists involved in the discussions argued that the bulk of the danger rests with asteroids larger than 1 kilometer anyway.
As Morrison puts it: "Only these can create impacts that could have global consequences and perhaps end civilization as we know it."
So NASA funding for asteroid search programs today is driven primarily by the goal of finding objects 1 kilometer or larger. Many smaller objects are found in the course of these searches. But some researchers think it is time to begin focusing on the smaller rocks.
"We need bigger telescopes to come down to the 100-meter limit," Tate said. "There is a substantial risk from undetected 100-meter sized objects."
If Tate gets what he wants, then Marsden's prediction of 200,000 numbered asteroids by 2004 would later be superseded by quantities about which no one is willing to venture a guess. Millions of small asteroids are thought to exist.
"Then things will really start booming," Marsden said.
Which would, of course, greatly exacerbate the problem of follow-up observations.
"The amateurs doing follow-up are doing sterling work," Tate said, "but it is a bit odd that something as important as this is not a matter for official interest."
Copyright 2001, Space.com
Jet Propulsion Laboratory
Rob Britt's Space.com story stated: NASA funding for asteroid search programs today is driven primarily by the goal of finding objects 1 kilometer or larger. Many smaller objects are found in the course of these searches. But some researchers think it is time to begin focusing on the smaller rocks. "We need bigger telescopes to come down to the 100-meter limit," Tate said. "There is a substantial risk from undetected 100-meter sized objects." .......
The quote from Jonathan Tate is quite correct, as far as it goes, but fails to address the other side of the equation: the cost of detecting them. Deciding how small NEAs one should attempt to discover is simply a matter of cost-benefit analysis. One must weigh the cost of detection against the "benefit" in the form of ability to protect against a future impact.
The critics of NASA's chosen threshold of ~1 km are quite correct that the "benefit" side of the equation is nearly constant over a substantial size range extending down from 1 km diameter to less than 100-m diameter, or Tunguska-sized events. For example, Chapman and Morrison (Nature 367, 33, 1994, quoted in the British Task Force report) estimate an annual fatality rate of about 20/year from Tunguska-sized impacts (~5,000 fatalities every 250 years), and an identical rate from large subglobal events (~500,000 fatalities every 25,000 years). Indeed the uncertainties in both frequency and consequence of events over this size range make it hard to know even if the slope is up or down. That is, are the many smaller events more lethal on average than the fewer larger ones? To fair approximation, the "spectrum" is flat, so based only on this side of the equation it is hard to justify any particular cut-off of concern in the sub-km size range.
The other side of the equation, the cost of detection, is much steeper. The cost of detection for extinction level NEAs (10-km diameter range) is zero: we already know them all. We believe our current census is complete down to around 4 or 5 km, except perhaps for an odd "Damocloid" or two, plus long-period comets (that's a separate subject). Going down to 1-km diameter, the present surveys are progressing very well, and while they may fail to reach the "Spaceguard Goal" by 2008, they should get there in not much longer than that. The total cost including preparatory and ancillary work for this level of survey is between $25 M and $100 M, depending on how you do the arithmetic. The basic requirement of doing this survey is simply to scan the entire sky to about magnitude 20.5 continuously for ten years. If you want to estimate what it takes to go for smaller and smaller objects, you can just increase the threshold magnitude correspondingly. If you wish to choose 300-m as your completeness goal, you will need to go 2.5 magnitudes fainter, to around 23.0. This can be done, and in fact a plan to do this is a recommendation of the U.S. National Research Council "Decadal Survey of Astronomy and Astrophysics" report, in the form of an 8.4-m "Large Aperture Synoptic Survey (LSST) telescope". My guess is that the total lifetime cost of a project of this sort, for the 15 or so years to build and run it, will be of the order of $1 B. Going the next step to reach 100-m size objects requires another 2.5 magnitudes, or down to about 25.5. This is getting close to the magnitude threshold for HST, and beyond the range that is efficiently reachable from the ground due to background sky and limitations on resolution. Going into space with a small aperture doesn't do it either, because the targets are moving too fast (or actually, the telescope is moving too fast) for more than few-second exposures. So you have to go into space with really big telescopes of multi-meter aperture. Very, very expensive, I would guess many tens of billions of dollars, if not hundreds. Paving the tops of every suitable mountaintop in the world with multiple Keck telescopes might accomplish it from the ground, but the cost would be comparable.
With these numbers in hand, we can evaluate what is being done, and what is sensible to advocate with present technology. In the >1-km size range it's overwhelmingly favorable: the estimate is 1.5 billion fatalities every half-million years, or 3,000 deaths per year. That's weighed against a cost of several to perhaps as much as ten million dollars per year. Going down to the 300-m size range, the marginal gain (additional protection in lives-per-year) is about 100 per year (where here I have integrated the fatality/frequency function from 1 km down to 0.3 km). It might be as high as 300. On the cost side, it is probably a project of order $1B to do this, or $100M per year. In the usual crass currency of cost per life saved, it is usually considered a good buy to spend up to $1M per life. So extending the limit down to half a kilometer diameter as recommended by the British Task Force is a sensible recommendation. Continuing down to 100-m diameter, we are looking at an incremental value of about 100 lives per year, as in the previous half-decade of size, but here the cost jumps up to tens of billions of dollars, at least several billion of dollars per year. Thus the cost per life saved is in the range of tens of millions of dollars, generally considered to be unaffordable, even in the first world. If one does the equation in terms of only first-world lives (the ones paying the bills), the number of fatalities per year is an order of magnitude less but the cost is the same.
In summary, even though the "spectrum" of benefit is quite flat in the range from 1 km down to 0.1 km, the cost function is as steep as a stone wall, and the point of diminishing return is rather firmly defined at somewhere around 0.5 km diameter, maybe as small as 0.3 km. The "threshold of global catastrophe" at around 1.5 to 2 km makes a categorical difference in value, so NASA's stated goal of discovering most NEAs down to ~1 km is a sensible first goal. Continuing down to 0.5 km as recommended by the British task force is a sensible follow-on goal, using present technology. Extending significantly further down to smaller objects should await the development of more cost-effective technology. No doubt that will come naturally in time. And time is on our side. Even the present level of surveying stands a better than even chance of discovering the next Tunguska before it finds us. But it is likely to be more than ten years.
Oliver Morton, London
28 October 2001
Oliver Morton <firstname.lastname@example.org>
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 lives.
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 incremental 20%.
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 smallest scales.
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 costs rose.
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 intellectually 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 casualties 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 in isolation.
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.
To David Morrison from Duncan Steel
29 October 2001
Duncan Steel <D.I.Steel@salford.ac.uk>
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 been reduced.
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 hazard.
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. 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 (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 fully-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 assessment.