For recent news articles, including a new value for the rate of impacts and several coments on mitigation technology (one by Congressman Rohrabacher).

Following are four recent publications of interest.

(1) Paper published today in Nature by P. Brown (University of Western Ontario) and colleagues reporting on the flux of impacting objects in the 1-10 m size, derived from 8 years of surveillance satellite data on bolides entering the Earths atmosphere. They conclude that the average largest annual impact event is 5 kilotons, a value that falls almost exactly on the extrapolated impact-frequency diagram recently developed by Alan Harris of the Space Science Institute (and published in the review chapter by Morrison et al. of the forthcoming book Asteroids III). Based on the combination of rapidly increasing astronomical surveys of NEAs larger than 100 m together with the new ground truth data from atmospheric impacts, it appears that the average impact flux for NEAs from 1 m to 10 km in size is now reasonably secure. (This range covers a factor of 10^12, or a billion billion, in energy). The corresponding frequency for Tunguska-size impacts (10 megatons) is once per millennium, as concluded independently in both the Morrison et al. and the Brown et al. publications.

(2) Opinion (op-ed) article from Space News (21 Oct issue) by Representative Dana Rohrabacher, the Chair of the House Space Subcommittee of the U.S. Congress. Rohrabacher expresses concern that insufficient attention and funding are available for NEO studies, especially for work on possible mitigation technology.

(3) Article from the New York Times (19 Nov issue) discussing the broad issues of the NEO impact hazard and focusing on the events and discussion at the NASA mitigation meeting held in September.

(4) Article from Mercury Magazine (Nov/Dec issue) that also reports primarily on the recent mitigation discussions.

David Morrison

(1) THE FLUX OF SMALL NEAR-EARTH OBJECTS COLLIDING WITH THE EARTH

P. Brown, R.E. Spaulding, D.O. ReVelle, E.Taglioferri, and S.P. Worden.

Nature 420: 294-296 (21 November 2002)

SUMMARY (press release): In the past eight years, US Department of Defense satellites scanning the Earth for evidence of nuclear explosions have detected nearly 300 optical flashes caused by small (1-10 m) asteroids exploding in the upper atmosphere. This has provided a new estimate of the flux of near-Earth objects colliding with the Earth, which P. Brown of the University of Western Ontario, Canada, and colleagues publish in this week's Nature. The revised estimate suggests that Earth's upper atmosphere is hit once a year by objects that release energy equivalent to five kilotons of TNT. The object that exploded above Tunguska in June 1908 was a 'small' asteroid, yet big enough to flatten 2,000 square kilometres of Siberian forest. Brown and colleagues calculate that Tunguska-like (ten-megaton) events are likely to occur about once every 1,000 years. This is more encouraging than the previous estimate, from ground-based observations, of once every 200 to 300 years. This work "has linked the fields of meteor and comet/asteroid planetary astronomy in a manner which shows that they are not merely distant relatives," says Robert Jedicke of the University of Arizona, Tucson, in an accompanying News and Views article.

ABSTRACT: Asteroids with diameters smaller than 50-100 m that collide with the Earth usually do not hit the ground as a single body; rather they detonate in the atmosphere. These small objects can still cause considerable damage, such as occurred near Tunguska, Siberia, in 1908. The flux of small bodies is poorly constrained, however, in part because ground-based observational searches pursue strategies that lead them preferentially to find larger objects. A Tunguska-class event -- the energy of which we take to be equivalent to 10 megatons of TNT -- was previously estimated to occur every 200-300 years, with the largest annual airburst calculated to be about 20 kilotons TNT equivalent [reference to Shoemaker 1983]. Here we report satellite records of bolide detonations in the atmosphere over the past 8.5 years. We find that the flux of objects in the 1-10 m size range has the same power-law distribution as bodies with diameters greater than 50 m. From this we estimate that the Earth is hit on average annually by an object with about 5 kton equivalent energy, and that Tunguska-like events occur about once every 1000 years.

Additional notes added by D. Morrison from the Nature paper: The data are based on observations made by US Department of Defense and Department of Energy space-based systems in geostationary orbits& We corrected the number distribution based on the percentage coverage of the Earths surface, which varied from 60% to 80% & there are 300 bolides in our sample & energies were calculated based on a model 6000 K temperature for the flash & 13 calibrated examples provide best data & agrees with Spacewatch observations of very small NEAs (Rabinowitz et al. 2000) .. .infrasound acoustic data (for 19 events only) from ReVelle give a slightly higher value of 10 kton for the largest annual event & most impactors are asteroidal and not cometary judged from depth of penetration & average impact interval for 10 megaton body is 1000 (+800, -200) years & in agreement with recent work by Alan Harris (e.g., Morrison et al. 2003, review chapter in Asteroids III book).

(2) SPACE NEWS OP-ED: PRIORITIZING THE NEO THREAT

By Dana Rohrabacher 21 October 2002 This year NASA Administrator Sean OKeefe gave us his vision of NASAs mission for the future. Inclusive within that mission is understanding and protecting the Earths environmental resources and ecosystem.

Conspicuously absent from the administrators list, however, is the potential threat posed by Near-Earth Objects (NEOs). As chairman of the House Science space and aeronautics subcommittee, I heard disturbing testimony in 1998 that the NEO threat should be taken seriously.

Further, within the last several months, the media has reported three events involving asteroids that have Earth-crossing orbits with the potential for a close encounter or collision with the Earth. Although these asteroids passed the Earth within a distance several hundreds of thousands of miles, in astronomical terms they missed our planet by a hair.

Thus, the subject of NEOs is no longer considered to be just science fiction. Unfortunately, there is no U.S. government agency responsible for responding to the NEO threat or even how to mitigate that threat.

Planetary defense advocates have proposed a wide range of options for mitigating asteroids roughly one kilometer or more in size. An asteroid that size can cause enormous damage. The options that have been discussed range from establishing a Natural Impact Warning Clearinghouse, supported by military space-based surveillance satellites gathering data for possible international distribution, to using advanced propulsion technologies for getting us off the planet and possibly setting-up shop on the moon for the future preservation of humankind.

Despite these innovations, all agree that more information regarding the NEO population is needed before asteroid mitigation becomes credible.

In 1998, NASA initiated the Spaceguard Program. Its goal is to catalog by 2008 all Near-Earth objects, or at least 90 percent of those that are at least one kilometer in size. NASA has surveyed slightly more than 600 of these large asteroids thus far and their program appears to be on track with the use of ground-based telescopes. However, the surveillance of Near-Earth Objects does not appear to be a high priority at NASA.

Some have proposed that military space-based surveillance satellites play a role as part of an early warning of asteroid impacts, especially those that move toward the Earth from the sun. Last June, an asteroid roughly less than one kilometer in size and spotted three days after its flyby came within 75,000 miles of the Earth, where it went undetected due to the suns glare.

Space-based assets, however, should not be viewed as a panacea but rather as a possible complement to ground-based telescopes dedicated to the detection of NEOs of all sizes. Surveys of smaller asteroids with the potential to destroy cities, countries and global climate, should also be vigorously tracked. The National Research Council recommended, in a recently published report, that NASA partner with the National Science Foundation to design, build and operate a survey facility, such as the Large-Aperture Synoptic Survey Telescope, so as to accomplish the objective of assessing the population of NEOs down to 300-meters in diameter and providing a measure of the impact hazard. Ascertaining the relative critical nature of long-period comets also contributes to gauging the impact hazard to Earth.

The question now before us is what can be done today? One of the critical aspects of cataloging asteroids is keeping track of what has already been identified. Amateur astronomers can play a crucial role in this regard. They can help strengthen existing government capabilities for tracking natural space objects by encouraging private citizens to observe the heavens.

My bill H.R. 5303 ("Charles Pete Conrad Astronomy Awards Act") provides the vehicle for private citizens to take an active role in the governments efforts to conduct NEO surveys by encouraging amateur astronomers to discover new and track previously identified asteroids, particularly those that threaten close approach with the Earth. It also is my way to honor Pete Conrad, an explorer of the highest caliber, for his tremendous contribution to aerospace during the last nearly 40 years. He commanded Apollo 12, and during that mission became the third man to walk on the moon.

It should be noted that recent analysis of an orbiting object identified by an amateur astronomer suggests it is the remains of a Saturn 5 third stage most likely from Pete Conrads Apollo mission. I find no better way to honor Pete Conrad than to establish an annual astronomers award for future asteroid discoveries in his name.

The act contains three categories of awards to be presented annually to amateur astronomers who: Discover the largest new asteroid having a near-Earth orbit; discover asteroids using information derived from professional sources and track newly discovered asteroids; and provide the greatest service to update the Minor Planet Centers catalogue of known asteroids. At a time when we seek greater public interest and participation in the national space program, it is my hope that H.R. 5303 will bring greater attention to the NEO issue by prompting a new generation of Americans to pursue careers in engineering, science and astronomy.

Dana Rohrabacher is chairman of the space and aeronautics subcommittee, of the House Science Committee.

(3) ARMAGEDDON CAN WAIT: STOPPING KILLER ASTEROIDS

From The New York Times, 19 November 2002

By Henry Fountain

Sooner or later, it's bound to happen.

Sooner or later, scientists who study Earth-crossing asteroids say, astronomers will find one that has a significant chance of striking the planet.

Unlike several recently discovered asteroids that were first given very long odds for a collision, this time more precise orbital calculations won't eliminate the possibility. This one will be an asteroid "with our name on it," in the words of David Morrison, a scientist at the NASA Ames Research Center and one member of a small community of astronomers, physicists, engineers and other scientists who think a lot about such an unthinkable event.

It is not clear what would happen then, though Dr. Morrison and others are trying to awaken governments and the public to the need to at least think about developing a way to respond. "Eventually we will discover something," Dr. Morrison said, though maybe not in this century or even this millennium. "Society should start planning for that unexpected but potentially tragic possibility."

But it is becoming clear that a longtime assumption of many scientists - and of Hollywood filmmakers - that a nuclear weapon is the best way to save the planet from a threatening asteroid is no longer in such favor. Increasingly, those scientists who study asteroid hazards say that a subtler, quieter, slower approach might be called for. These scientists are turning T. S. Eliot on his head: it's not that the world will end with a whimper rather than a bang, they say. It's that it may not end that way.

A nuclear detonation, some scientists say, could break the asteroid into several large pieces, increasing, rather than eliminating, the threat. And a blast some distance from an asteroid, designed to shove it into a slightly different orbit, might not work either; the asteroid might soak up the energy like a sponge. "I'd say forget that," said Dr. Keith A. Holsapple, a professor at the University of Washington who studies the effects of simulated nuclear explosions.

By contrast, most of the alternative approaches would build up force gradually, gently nudging, rather than shoving, the asteroid. They would rely on the same basic Newtonian principle - that for every action there is an equal and opposite reaction - only written small, with tiny actions creating tiny opposite reactions that, given enough time, could shift an asteroid's orbit enough to change a hit into a close call.

Among the approaches being talked about are some that have been the stuff of science fiction for years: a mass driver, a sort of electromagnetic conveyor belt that would be planted on an asteroid and hurl dirt from its surface, or a solar concentrator, a parabolic mirror that would orbit the body and heat up the surface, creating a plume of vaporized material.

Perhaps the most intriguing idea - and one that may not be as far-fetched as it sounds - has been put forth by Dr. Joseph Spitale, a scientist at the University of Arizona. To move an asteroid, he says, just change its color.

This "paint it black" approach would change how much sunlight it absorbs, and how hot it gets. Heat radiating from an asteroid (in the form of thermal photons) creates a small force in the opposite direction - a phenomenon called the Yarkovsky effect, after I. O. Yarkovsky, a Russian engineer who first described it a century ago. Changing the amount of heat would change the force, affecting the orbit. The sun would move the asteroid, one photon at a time.

There are, of course, logistical problems with this and other alternative technologies - getting buckets of paint to an asteroid, for instance, is no sure (or inexpensive) thing. Many scientists acknowledge that in some circumstances a nuclear weapon may be the only option.

Few scientists are arguing that society should be developing an asteroid-deflection system, given the extremely low odds of an impact any time soon. "A major technological effort at this time is probably ill conceived because our children will be so much better at it," said Dr. Alan W. Harris of the Space Science Institute in Boulder, Colo.

Rather, most scientists say that any money available should go into detecting asteroids and investigating them to better understand the potential threat.

Improvements in detecting and understanding asteroids, in fact, are what is prompting the change of thinking toward a slow approach, which was exemplified by presentations at a NASA-sponsored workshop on asteroid hazards in September near Washington that "pretty much sent the nuclear weapon idea home packing," said Dr. Erik Asphaug, a professor at the University of California at Santa Cruz and one of the workshop's organizers.

There are several detection efforts under way, with a goal of meeting a Congressional mandate of finding 90 percent of objects larger than a kilometer in diameter by 2008. An asteroid of this size is thought to strike the Earth once every million years or so, but since it is capable of producing destruction on a regional scale or worse, in terms of potential loss of life over time it represents the biggest risk.

The best estimate is that there are perhaps 1,100 of these large asteroids, about half of which have been discovered and found to be harmless. The odds are extremely slight that any of the remaining large asteroids will prove threatening, either.

But any asteroid with a chance of hitting Earth would cross the planet's path many times before it actually hit, so it would probably be detected decades in advance.

There is no current detection program for smaller asteroids, of which there are perhaps half a million down to about 50 meters in diameter, the smallest size capable of penetrating Earth's atmosphere (and roughly the size of one that exploded over the Tunguska River in Siberia in 1908, destroying forests for hundreds of square miles). And there is no systematic survey for potentially hazardous comets, which come out of the astronomical equivalent of left field. "So we would either very likely have a lot of warning or none at all," said Dr. Clark Chapman of the Southwest Research Institute in Boulder.

No warning time means no options. A short amount, on the order of a decade or two, might leave a nuclear blast as the only choice. But with many decades of warning, there is room to investigate the asteroid first by sending a spacecraft to it, and then use a slow-acting method to divert it, one that wouldn't require launching a nuclear weapon. "We would want to seek out every alternative to a nuclear weapon before turning to that technology,'` Dr. Chapman said.

What makes some of these alternatives promising is what scientists have come to understand about asteroids. Many of them, the scientists say, are rather loose agglomerations of stony fragments that have stuck together over time in the cosmic rock tumbler that is the solar system. They are not giant solid boulders. "Maybe something like a popcorn ball is a better way to describe it," Dr. Holsapple said.

Such porous objects would be hard to obliterate or move with a nuclear blast, even one some distance from the surface, he said. "But pushing a little bit for a long time would work equally well whether an asteroid is porous or not," he added.

Porosity might prove to be a problem even for some of the alternative methods, however. A mass driver, for instance, would have to be firmly attached to an asteroid in order to work, as would a small rocket engine, another proposed method. It might not be possible to anchor such equipment to a popcorn-ball asteroid.

Dr. Spitale's idea would get around that problem, but it would not be without other difficulties. For one thing, a lot of paint would be required. (He has also suggested dumping a thin layer of dirt on the asteroid to change its color, and has estimated that dozens of rocket loads would be needed.) For another, because they are so small, asteroids have very little gravity, so it is unclear that paint or dirt would stay in place. One solution to that problem and to the problem of transporting large amounts of material, he says, would be to pepper an asteroid with small explosives, to remove the top layer and expose material that might have different thermal characteristics.

While generally saluting this kind of outside-the-box thinking, some other asteroid experts find Dr. Spitale's ideas largely impractical. "I guess I consider that approach kind of quirky," Dr. Chapman said.

"I'll be the first to confess that this isn't the last word in asteroid hazard mitigation," Dr. Spitale said. Still, he added, while it may not be easy, along with the nuclear option it is the only approach that appears technically feasible at this time. "If we were faced with the problem today," he said, "this is one of maybe two approaches where we could say, `Well, we could do this.' "

(4) ASTEROID IMPACT Mountains or Molehills: Sizing up the Impact Hazard

Mercury (Astronomical Society of the Pacific), Nov/Dec 2002

By Ivan Semeniuk

For Mike Belton the impact hazard wasn't a personal problem until he saw the numbers. As a former Kitt Peak planetary astronomer, Belton has long been aware of the infamous connection between the extinction of the dinosaurs 65 million years ago and a large comet or asteroid impact. But he also knows the odds are heavily stacked against such an event occurring again for millions of years.

Then Belton read a magazine article that changed his perspective. The article featured a graph showing how the likelihood of an impact increases as the size of the impactor decreases. What caught his eye were not the big dinosaur-killers at one end of the graph, but the far smaller and more numerous objects at the opposite end. These lesser rocks assault our planet at the rate of one every few millennia. Just one can deliver enough energy to destroy a region the size of New York state, killing tens of millions of people, or generate tsunamis that could devastate coastlines. Given their frequency, Belton realized there was a good chance -- maybe one in five -- that one would arrive within the next couple of generations of his family. "That kind of shook me up," he recalls.

Belton has since become an active member of the Near-Earth Object (NEO) community and advocates paying more attention to potential impactors of intermediate size. Objects in that range have diameters between about 100 meters and 1 kilometer. The upper limit is the approximate threshold for a worldwide catastrophe (the dinosaurs were done in by a 10-kilometer body). The lower limit is about twice the size of the object that exploded over the Tunguska region of Siberia in 1908, flattening 2,000 square kilometers of forest and killing entire herds of animals. The next time an impact rattles Earth, it's almost certain to come in near the bottom end of that range -- quite possibly before the end of this century. "Small impactors happen at rates which are of interest in human terms," says Belton. "I find that a compelling reason to learn more about them."

This past September, Belton co-chaired a NASA-sponsored workshop in Washington where he made his case for learning more and learning more quickly about the near-Earth objects (NEOs) that threaten us. Among the attendees were representatives from NASA, the Pentagon, the National Science Foundation, the aerospace industry, and most of the leading scientists involved in the NEO community. What emerged was a remarkably wide-ranging discussion -- one that reveals the impact hazard to be a much more complicated and subtle issue than was apparent a decade ago.

Size Matters

If there is one question that best sums up the current state of thinking about the impact hazard, it is this: At what size do we need to act? In the shooting gallery that is our solar system, everyone agrees we are the target of both cannonballs and BBs. The hard part is deciding where to drawn the line that separates them.

For practical reasons, that line is now set at 1 kilometer. Not only are objects of this diameter a global threat (no matter where they hit, we're all affected to some degree), they are also the easiest to spot. Under a mid-1990s congressional mandate, NASA currently funds search efforts to the tune of about $3.5 million per year, including MIT's Lincoln Near-Earth Asteroid Research (LINEAR) program, JPL's Near-Earth Asteroid Tracking (NEAT) program, the University of Arizona's Spacewatch survey, and the Lowell Observatory Near-Earth-Object Search (LONEOS). The explicit goal of the Spaceguard Survey is to find by 2008 90% of the estimated 1,000 to 1,500 NEOs 1 km or larger (about 630 had been found as of October 22, 2002). "The existing commitment to 1 km and larger is to retire the risk," says Tom Morgan, who heads NASA's NEO group. "By the end of this decade we'll be able to tell you if any of these objects presents a threat in the foreseeable future."

Within the NEO community there is little doubt this level of search is worth the effort. "When we first adopted 1 km as a goal, it was just a no-brainer," says Space Science Institute astronomer Alan Harris, formerly of the Jet Propulsion Laboratory. "Such objects can wipe out a fair percentage of Earth's population, and the cost of finding them all in a decade or so is about $50 million."

But as Harris points out, this lopsided cost/benefit ratio begins to level off when it comes to intermediate size objects. "If you go to smaller sizes, the amount of disaster you prevent gets less and the cost of actually finding them goes up," says Harris. "At some point it's going to crossover. You basically can't afford the insurance for what you're getting."

While NASA has not yet decided on that crossover point, Harris suspects it lies somewhere around 200 to 300 meters. The proposed Large Synoptic Survey Telescope (LSST), an 8.4-meter instrument with a whopping 7 square-degree field, would spot most of these intermediate-sized NEOs over the course of a few decades. Like most of his colleagues, Harris supports the $120 million LSST, which could be built by 2010, but he suggests that searching for 50- to 100-meter objects might require resources comparable to all the optical telescopes on Earth.

So, should the estimated 50,000 NEOs in the 200-meter category be ignored and left to fall where they may? Such an impactor could devastate a region as effectively as two or three hydrogen bombs, or it could trigger a nuclear war if it explodes over a nation like India or Pakistan. But from an actuarial perspective, these relatively small asteroids pose no greater risk than major earthquakes, hurricanes, and volcanic eruptions. In fact, they are probably much less of a hazard to humanity than we are to ourselves.

"In the real world we have a limited budget," says Colleen Hartman, NASA's director of solar system exploration. "The community needs to come up with a logical analysis for going below 1 km. Only then can we get a buy-in from the American taxpayer."

Taking Stock

The wide variety of opinions about NEOs grows wider still when the discussion turns to strategies for investigating them directly. While there have been successful comet and asteroid flybys -- and even NEAR-Shoemaker's impromptu landing on Eros -- Belton argues these encounters have not provided what scientists need most for impact prevention: detailed internal profiles of different types of NEOs. In Belton's view, the surest way to get that information is to create a new category of mission with its own dedicated budget.

Part of Belton's rationale stems from the increasingly obvious fact that NEOs are a diverse lot. Like meteorites, some are stone, some are iron, and others are made of a mishmash of material that predates the formation of the planets.

Even the physical structure of NEOs varies. While some asteroids are monoliths comprising single, solid pieces of rock, others are suspected of being loosely assembled rubble piles. Curiously, this difference appears to be size-dependent. An analysis of asteroid rotation rates reveals that almost all asteroids above 300 meters are spinning slower than the speed at which a rubble pile would fly apart. Meanwhile, asteroids of smaller size typically spin much faster. This suggests larger asteroids are fragile composites while smaller ones are uniform chunks. If so, it may explain why a surprising number of NEOs -- perhaps 5 to 10% -- come in pairs. Just passing near a planet may produce enough of a gravitational tug to split a large but weakly cemented asteroid in two.

With so much diversity, no single mission can hope to provide all the data needed to successfully prevent a major impact. Multiple missions to multiple targets are required. "The less you know about asteroids, the more likely it is you'll have to do something drastic to divert an incoming body," says Erik Asphaug of the University of California, Santa Cruz. "The scenario that really makes me nervous isn't being bonked on the head by an asteroid. It's preparing for it in the wrong way."

University of Michigan planetary scientist Dan Scheeres agrees: "Clearly you would want to have a diversity of missions to go out and look at different types of asteroid morphologies. If you have a plan for dealing with them, you want to make sure it will work across the spectrum."

Belton's preferred model for an asteroid mission is based on the multi-target approach. His proposed spacecraft, called Deep Interior, would rendezvous with a number of NEOs in succession, using radar tomography and seismic techniques to map them from the inside out. "The purpose of Deep Interior is to understand the physical structure of these small bodies so that you can do something about them," he says.

While developing the proposal Belton found such a mission would be difficult to achieve for $300 million. This places it beyond the upper limit for a low-cost Discovery-class mission such as Lunar Prospector or NEAR-Shoemaker. Hence Belton's proposal for a separate class of mission specifically aimed at gathering information for impact mitigation. It's an approach that flies in the face of current NASA practice, where NEO-related projects, like all space exploration missions, are justified on the basis of science alone.

"I think the public has this perception that the reason we're visiting NEOs is because they're hazardous," says Asphaug. "But from a science point of view, when you're proposing a mission, it has been the kiss of death to say that you're doing it because you want to save humanity."

Colleen Hartman doesn't see a conflict between science and impact mitigation as mission objectives. "If you do the intellectual experiment of asking what it is we need to know in order to begin down the path of mitigation, I think you'd be doing exactly what we're doing," she says.

Diversionary Tactics

The task of deflecting or destroying an incoming NEO makes a great premise for a Hollywood action flick, but as an engineering challenge it has at least as much potential to become a dark comedy. The comedy stems from the fact that it could be remarkably difficult to persuade an asteroid, particularly a rubble pile, to step aside. Like the Black Knight of Monty Python fame, such an asteroid can lose bits and pieces during our attempts to divert it, and still maintain a collision course with Earth.

Part of the problem is that the most powerful tools available are not particularly well suited to the task. A hydrogen bomb delivers a bigger burst of energy, pound for pound, than anything else we have at hand. But energy alone cannot budge a NEO. What is needed is an efficient way of using the energy of a nuclear explosion to produce momentum. Depending on its internal structure, an asteroid may absorb the push of a nearby explosion by deforming -- the same effect you get from punching a pillow. Even worse, an explosion could break up a NEO into several radioactive chunks. "If we're going to think about deflecting NEOs," says Jay Melosh of the University of Arizona, "we have to know how they're going to respond mechanically."

Recently a number of alternate solutions for moving NEOs have been proposed. They range from mass drivers that fling bits of rock away to create momentum, giant airbags that can shepherd a loose rubble pile into a new orbit, or focusing sunlight with giant mirrors to excavate jets of vaporized rock that push the asteroid in a desired direction. Some schemes even involve covering a threatening NEO with white chalk or metallic foil to enhance the tiny recoil it gains from reflecting sunlight. As Harris says, "We have matured from the idea of just building a bomb and nuking the thing."

For small, hard asteroids, nukes might do the job, although Asphaug finds the risk of space-borne nukes more sobering than the risk of small NEOs. For rubble piles, other methods will likely yield more fruitful results. But whichever solution proves to be the best, says Asphaug, "The main goal should be to start the process moving now so that when we need to do something we'll be prepared to do it with plenty of lead time." The lead time for a mitigation mission, according to a 1997 U.S. Air Force study, is about 15 years, with a cost of about $1.2 billion.

But as some presenters at the workshop pointed out, time may not be the only commodity needed to prevent an impact. In the years leading up to an anticipated collision with Earth, a hazardous NEO may be almost impossible to reach without expending vast amounts of fuel. "NEOs are the easiest things in the solar system to get to -- if you get to choose the target," says Alan Harris. "But nature may not be so kind when it's choosing the target for you."

As Harris explains, looking at the average orbital characteristics of nearby asteroids leads to a sobering conclusion. The total change in spacecraft velocity (a proxy for fuel use) required to reach and rendezvous with a typical NEO is in the neighborhood of 20 km per second. "That's just about what it would take to either land on Pluto or put a spacecraft in orbit around Pluto," says Harris.

To surmount the velocity barrier, future NEO missions may ultimately depend on a low thrust propulsion system, such as the ion drive used on the recently concluded Deep Space 1 mission, or more powerful nuclear-electric rockets. Harris lists the development of such systems as just one of the key precursor technologies that must be developed for successful impact mitigation. In addition, new propulsion technologies can also be applied to normal space exploration, particularly missions to the outer solar system.

Dodgeball

The comedian Emo Philips has a joke about asteroids. If an asteroid is coming toward you, he says, you don't have to blow it up the way they do in the movies. You just have to slow it down long enough for your country to rotate out of the way.

Humor lies in the unexpected, and the unexpected idea in Philips' punch line is that anyone facing an Armageddon-like impact would choose to pass the problem onto to someone else. Yet as the Washington workshop made clear, the real impact hazard is an issue loaded with choices -- and with opportunities to pass the problem along both to other players and to future generations.

Currently, many organizations and individuals have an interest in various aspects of the impact hazard, from searching to exploration to developing methods for deflection. But no single agency, military or civilian, assumes responsibility for the problem as a whole.

According to Jay Melosh, that many not be a problem. Melosh contends that some aspects of the impact hazard are overrated and that it's too early for a centralized Office of NEO Mitigation. The need may only arise if one of the current search programs finds a sufficiently large asteroid with a good chance of hitting Earth in the next few decades. But Melosh concedes, "the current situation is confusing."

From Belton's perspective, one of the main reasons for mounting the workshop was to get a good start on putting together a national program for dealing with NEOs. After all, he says, "You don't just ask someone to move a rock the size of a city that's traveling at 20 kilometers per second -- they have to learn how to do it."

But which rocks to move and which to leave alone? That question remains open after the workshop. It could well be the impactors we are most likely to encounter in the foreseeable future are so small its not worth the expense to try to stop them. They may even cost lives, but so will plenty of other natural hazards over the next century or two. On the other hand, just one city-buster exploding over North America could swing public and political opinion from complacency to concern in the same way that terrorism became a high priority in the aftermath of 9/11.

In many ways, 9/11 offers an apt comparison for scientists working on the impact hazard. That's because it may not be possible to prepare society for a hazard for which there is no historic memory. And once it occurs, it is difficult to put the event in perspective alongside other more familiar hazards.

For many in the NEO community, perspective was the most valuable outcome of the Washington workshop. "It's very interesting to see all aspects of the problem brought together in one place at one time," says Scheeres. "When that happens you learn to view all components of the risk relative to each other, and relative to other risks."

Scheeres remarks are a reflection of a new era in the history of the impact hazard. In less than a generation we're gone from almost no awareness of the rocks that bombard our planet to treating the discovery of potential impactors as headline news. In the future, what it more likely is continued maturing of the issue. The impact risk is real and deserves our attention, but it must also become part of the way we think about survival on all fronts.