SIMON P. WORDEN, BRIGADIER GENERAL, USAF
Deputy Director for Operations
United States Space Command
Peterson AFB, CO
July 10, 2002
The opinions and concepts expressed are those of the author and do not necessarily represent the position of the Department of Defense or the United States Space Command
A few weeks ago the world almost saw a nuclear war. Pakistan and India were at full alert and poised for a large-scale war - which both sides appeared ready to escalate into nuclear war. The situation was defused - for now! Most of the world knew about this situation and watched and worried. But few know of an event over the Mediterranean in early June of this year that could have had a serious bearing on that outcome. U.S. early warning satellites detected a flash that indicated an energy release comparable to the Hiroshima burst. We see about 30 such bursts per year, but this one was one of the largest we've ever seen. The event was caused by the impact of a small asteroid - probably about 5-10 meters in diameter on the earth's atmosphere. Had you been situated on a vessel directly underneath the intensely bright flash would have been followed by a shock wave that would have rattled the entire ship and possibly caused minor damage.
The event of this June caused little or no notice as far as we can tell. But had it occurred at the same latitude, but a few hours earlier, the result on human affairs might have been much worse. Imagine that the bright flash accompanied by a damaging shock wave had occurred over Delhi, India or Islamabad, Pakistan? Neither of those nations have the sophisticated sensors we do that can determine the difference between a natural NEO impact and a nuclear detonation. The resulting panic in the nuclear-armed and hair-trigger militaries there could have been the spark that would have ignited the nuclear horror we'd avoided for over a half-century. This situation alone should be sufficient to get the world to take notice of the threat of asteroid impact.
I've just relayed the aspect of the near-earth objects (NEO) that should worry us all. As more and more nations acquire nuclear weapons - nations without the sophisticated controls and capabilities build up by the United States over the 40 years of Cold War - we must first and foremost ensure that the 30-odd impacts on the upper atmosphere are well understood by all to be just what they are.
A few years ago those of us charged with protecting this nations vital space system, such as the Global Positioning System (GPS) became aware of another aspect of the NEO problem. This was the Leonid meteor storm. This particular storm occurs every 33 years. It is caused by the debris from a different type of NEO - a comet. When the earth passes through the path of a comet, it can encounter the dust thrown off by that comet through its progressive passes by the Sun. This dust is visible on the Earth as a spectacular meteor storm. But our satellites in space can experience the storm as a series of intensely damaging micrometeorite strikes. We know about many of these storms and we've figured out their parent comet sources. But there are some storms arising from comets that are too dim or spent for us to have seen that can produce "surprise" events. One of these meteor storms has the potential of knocking out some or even most of our earth-orbiting systems. If just one random satellite failure in a pager communications satellite a few years ago seriously disrupted our lives, imagine what losing dozens of satellites could do!
Most people know of the Tunguska NEO strike in Siberia in 1908. An object probably less than 100 meters in diameter struck over Siberia releasing the equivalent energy of up to 10 megatons. It leveled a forest 50 miles across. But most people don't know that we have evidence of two other strikes during last Century. One occurred over the Amazon in the 1930s and another over central Asia in the 1940s. Had any of these struck over a populated area, thousands and perhaps hundreds of thousands might have perished. Experts now tell us that an even worse catastrophe that a land impact of a Tunguska-size event would be an ocean impact near a heavily populated shore. The resulting tidal wave could inundate shorelines for hundreds of miles and potentially kill millions. There are hundreds of thousands of objects the size of the Tunguska NEO that come near the earth. We know the orbits of but a handful.
Finally, just about everyone knows of the "dinosaur killer" asteroids. These are those objects a few kilometers across that strike on timescales of tens of millions of years. While the prospect of such strikes grab people's attention - and make great catastrophe movies - too much focus on these events has in my opinion been counterproductive. In my organization, the Department of Defense, I have tried to raise our concern and interest in addressing the very real threats outlined above. However I get the predictable response. "General, if this threat only hits every 50 million years, I think we can focus our attention of more immediate threats!" In short the "giggle factor" in the professional scientific and national security community has meant that we have gotten little done on this problem.
What Should We Do?
First and foremost we must know when an objects strikes the earth exactly what it is and where it hit. Fortunately our early warning satellites already do a good job of this task. And our next generation system, the Space-Based Infrared System (SBIRS) will be even better. The primary difficulty here is that this data is also used for vital early warning purposes and its detailed performance is classified. However, in recent years the U.S. DoD has been working to provide extracts of this data to nations potentially under missile attack with cooperative programs known as "Shared Early Warning." Some data about asteroid strikes has also been released to the scientific community. Unfortunately this data takes several weeks to get released. Thus my first recommendation is that the United States DoD make provision to assess and release this data a soon as possible to all interested parties - exercising proper cautions of course to ensure that sensitive performance data is safeguarded.
We have begun to scope what an NEO warning center might look like. We believe adding a modest number of people, probably less than 10 all told, to current early warning centers and supporting staffs within Cheyenne Mountain could accomplish this. A Natural Impact Warning Clearinghouse has been scoped to do this job.
Perhaps the most urgent mid-term task has already been begun. This is the systematic observation and cataloguing of close to all potentially threatening NEOS. We are probably about halfway through cataloging "large" NEOS (greater than a kilometer in diameter). It's interesting to note that the most effective sensor has been the MIT Lincoln Lab LINEAR facility in New Mexico. This is a test bed for the next generation of military ground-based space surveillance sensors. But this ground-based system, however effective, can only really address the "large", highly unlikely threats. We find out every few weeks about "modest" asteroids a few hundred meters in diameter. These are often caught as they sail by the earth, often closer than the Moon, unnoticed until they have nearly passed. Most recently the object 2002MN had just this sort of near miss - this time only a few tens of thousands of kilometers from the earth! Moreover, ground-based systems such as LINEAR are unable to detect one of the potentially most damaging classes of objects, those such as comets that come at us from the direction of the sun. New space-surveillance systems capable of scanning the entire sky every few days are what's needed.
New technologies for both space-based and ground based surveys of the entire space near the earth are available. These technologies could enable us to completely catalog and warn of objects as small as the Tunguska meteor (less than 100 meters in diameter). The LINEAR system is limited primarily by the size of its main optics - about 1 meter in diameter. By building a set of three-meter diameter telescopes equipped with new large-format CCD-devices, the entire sky could be scanned every few weeks. But more important the follow-up observations necessary to accurately define orbits, particularly for small objects could be done.
The most promising systems for wide-area survey - particularly to observe close to the sun to see objects coming at up from that direction - are space-based surveillance systems. Today the only space-based space surveillance system is the DoD's "MSX" Satellite. This was a late 1990s missile defense test satellite and most of its sensors have now failed. However one small package weighing about 20 kg and called the "SBV" sensor is able to search and track satellites in Geosynchronous orbit using visible light. This has been a phenomenally successful mission having lowered the number of "lost" objects in GEO orbit by over a factor of two. MSX is not used for imaging asteroids, but a similar sensor could be. The Canadian Space Agency, in concert with the Canadian Department of National Defense is considering a "microsatellite" experiment with the entire satellite and payload weighing just kg. This Near-Earth Surveillance System (NESS) would track satellites in GEO orbit, as MSX does today. However, it would also be able to search the critical region near the sun for NEOs that would be missed by conventional surveys.
The U.S. DoD is planning a constellation of somewhat larger satellites to perform our basic satellite-tracking mission. Today our ground-based radars and telescopes, and even MSX only track objects that we already know about. These systems are not true outer-space search instruments as the LINEAR system is. However, the future military space surveillance system would be able to search the entire sky. As an almost "free" byproduct it could also perform the NEO search mission. Corresponding, larger aperture ground based systems could then be used to follow up to get accurate orbits for the NEOs discovered by the space-based search satellites. Again, I believe there is considerable synergy between national security requirements related to man-made satellites and global security related to NEO impacts.
Regardless of how well we know NEO orbits and how well we can predict their impacts the fact remains that today we have insufficient information to contemplate mitigating an impact. We do not know the internal structure of these objects. Indeed, we have reason to believe that many, if not most are more in the nature of "rubble piles" than coherent objects. This structure suggests that any effort to "push" or divert a NEO might simply fragment it - and perhaps turn a single dangerous asteroid into hundreds of objects that could damage a much larger area.
What are needed are in-situ measurements across the many classes of NEOs, including both asteroids and comets. This is particularly the case of small (100meter) class objects of the type we would most likely be called upon to divert. Until recently missions to gather these data would have taken up to a decade to develop and launch and cost 100s of millions of dollars. However, with the rise of so-called "microsatellites" weighing between 50-200 kg and which are launchable as almost "free" auxiliary payloads on large commercial and other flights to GEO orbit, the situation looks much better. These missions can be prepared in one-two years for about $5-10M and launched for a few million dollars as an auxiliary payload. Such auxiliary accommodation is a standard feature on the European Ariane launched and should be, with proper attention, here in the United States on our new EELV launcher systems.
With a capable microsatellite with several kilometers per second "delta-V" (maneuver capacity) launched into a GEO transfer orbit (the standard initial launch orbit for placing systems into GEO) the satellite could easily reach some NEOs and perform in-situ research. This could include sample return and direct impact to determine the internal structure and potential to physically move a small object. Indeed, NASA is planning several small satellite missions. The key point here, however, is that with missions costing $10M each, we can sample many types of objects in the next decade or so to gain a full understanding of the type of objects we face.
There is an interesting concept to consider. If we can find the right small object in the right orbit we might be able to nudge it into an orbit "captured" by the earth. This would make a NEO a second natural satellite of earth. Indeed, there is at least one NEO that is close to being trapped by the Earth now, 2002 AA29. If such an object were more permanently in earth orbit it could not only be more closely studied but might form the basis for long-term commercial exploitation of space. Moreover, a very interesting next manned space flight mission after the Space Station would be to an asteroid, maybe even one we put into earth's gravity sphere.
The key of each of these proposed actions on developing the ability to mitigate NEO impacts is that they are all items our national security community and we in the United States are likely to do for other reasons. If these efforts can be adapted to the NEO threat problem, this would add minimal additional expense.
One of the most important aspects of NEO mitigation is often overlooked. Most experts prefer to focus on the glamorous "mitigation" technologies - diverting or destroying objects. In fact, as the military well knows the much harder part is what we call "command and control." Who will determine if a threat exists? Who will decide on the course of action? Who will direct the mission and determine when mission changes are to be made? Who will determine if the mission was successful? And there are hosts more.
These command and control issues are those that the military has long struggled with. The NEO community has not faced this essential issue. Indeed, the United States Space Command has just completed a concept of operations for the first step in NEO mitigation - a Natural Impact Warning Clearinghouse. This operation is a command and control function. It would be able to catalog and provide credible warning information on future NEO impact problems as well as rapidly provide information on the nature of an impact.
Much discussion has been expended suggesting that any NEO impact mitigation should be an international operation. I would respectfully disagree. International space programs such as the International Space Station fill many functions. An NEO mitigation program would have only one objective. In the latter case a single responsible nation and organization would have the best chance of a successful mission. Moreover, the nation responsible would not need to worry about giving up national security sensitive information and technology as it would build and control the entire mission itself. For as pointed out the means to identify threats and mitigate them overlap considerably with other national security objectives.
It does, however make considerable sense that the data gathered from surveys and in-situ measurements be fully shared among all. This will maximize the possibility that the nation best positioned to perform a mitigation mission would come forward. One of the first tasks of the Natural Impact Warning Clearinghouse noted above would be to collect and provide a distribution point for such data.
NEO Mitigation is a topic whose time has come. Various aspects related to NEO impacts, including the possibility than an impact would be misidentified as a nuclear attack, are critical national and international security issues. The focus of NEO mitigation efforts - both in finding and tracking them and in exploring and moving some should shift to smaller objects. Not only are the near-term threats much more likely to come from these "small" objects (100 meters in diameter or so), but we might also be able to divert such objects without recourse to nuclear devices.
After a suitable class of NEOs are found, microsatellite missions to fully explore and perhaps perform test divert operations should commence. The technologies for low-cost NEO missions exist today.
The necessary command and control, sensor and space operations technologies and equipment are all "dual use" to the military. We have similar, and in some cases almost identical requirements. It thus stands to reason that strong military involvement and even lead in the decades ahead on NEO mitigation is in order. As the U.S. Government considers how to proceed on this critical issue, the major role that the military and the technologies it controls should be carefully integrated into our overall national work.
=============================================== ON IMPACT FREQUENCY AND NEED FOR A WARNING CENTER
In his written statement (above) Pete Worden mentions three large impacts during the 20th century, and in his oral testimony he called all three of these 100-m class impacts. He wrote: "Most people know of the Tunguska NEO strike in Siberia in 1908. An object probably less than 100 meters in diameter struck over Siberia releasing the equivalent energy of up to 10 megatons. It leveled a forest 50 miles across. But most people don't know that we have evidence of two other strikes during last Century. One occurred over the Amazon in the 1930s and another over central Asia in the 1940s. Had any of these struck over a populated area, thousands and perhaps hundreds of thousands might have perished". Others have made similar comments, sometimes also including the dramatic Sikhote-Alin iron meteorite fall of February 12, 1947.
Of these four events, the Tunguska impact (June 30, 1908) of an asteroidal object nominally 60 m in diameter was by far the most dangerous, producing an airburst releasing 5-15 megatons energy. Sikhote-Alin was well observed and studied, and more than 40 tons of iron were recovered from multiple craters, but the estimated diameter of the projectile was no more than 3 meters. The Amazon impact in the 1930s has been discussed but is based on scattered human reports with no supporting physical evidence, and most researchers suspect that this impact is spurious. I have not heard anything about the Kazakastan impact of the 1940s, and I suspect that is spurious also.
Thus by my count for the 20th century we have one confirmed 60-m impactor (Tunguska) and no evidence of anything else approaching this size (although of course we would miss most impacts since they would occur in the ocean; absence of evidence in this case is not evidence of absence). For comparison, the latest estimated frequency of impact of 60-meter projectiles is only about once per millennium, rather lower than the older estimates of once every couple of hundred years.
Call for an NEO Warning Center
Several participants in the NEO Roundtable called for establishing a NEO coordination and warning center. In the summaries by the panelists this was a nearly unanimous recommendation. Worden wrote above that "We [USAF Space Command] have begun to scope what an NEO warning center might look like. We believe adding a modest number of people, probably less than 10 all told, to current early warning centers and supporting staffs within Cheyenne Mountain could accomplish this. A Natural Impact Warning Clearinghouse has been scoped to do this job."
It would be interesting to me to understand better what is meant by such a warning center. I think everyone can share Worden's concern about misidientification of meteors that hit the atmosphere and explode with kiloton-scale energies. I certainly support his proposal that this information be disseminated more widely and quickly. However, these are not what I call "warnings" -- they are timely reports on events that have already happened and been observed from space.
The only warnings I know of would concern asteroids or comets discovered to be on possible impact trajectores. Over the past 6 years there have been several short-lived "warnings" of possible future impacts that were quickly withdrawn as new data and/or better orbital calculations became available. Today with multiple international centers for calculating orbits and improved data sharing, it is likely that there will be fewer such public warnings. In fact, the only legitimate warning (if you want to call it that) on the books today is NEA 1950DA, with a nominal chance of 1 in 300 of an impact in March 2880.
As the NEA surveys increase in power, there will almost certainly be additional cases of newly-discovered NEAs that appear for a short time to have a possibility of colliding with the Earth. These will all be predictions for far in the future, probably at least several decades. Some will be reported in the press, but most will be quietly checked out and their orbits refined without the glare of publicity. Astronomers in several countries today have this computational capability. I therefore wonder what is the purpose of the proposed warning center, and just what sort of warnings it anticipates issuing?
Perhaps it is worth repeating that none of the proposed surveys is designed to look for any NEA on its final plunge to collision with the Earth. Indeed, it would be very difficult and non-cost-effective to try to design such a "last minute warning" system. The approach first articulated a decade ago is to survey the sky, discover NEAs, determine their orbits, and predict their future paths. Any potential impactor should be picked up decades (or more) in advance. We can do this because orbital dynamics is an exact science, and asteroids do not change orbits capriciously. This approach will apply as well to the smaller NEAs that are discovered in the future as it does to those being found today. "Warning" is a word that conveys the wrong impression: In my opinion, what we should be talking about are long-term predictions, based on a comprehensive survey of NEAs.