Following are three documents:
- Article from Space.com discussing open letter from concerned citizens
- Open Letter to Congress on Near Earth Objects
- Summary of NEO issues from Science, by Andrea Milani
Concerned Citizens Ask for Congressional Action on NEOs
Leonard David, Space.com, 9 July 2003
A distinguished group of Americans joined together to send a unique request to Congressional leaders Wednesday -- a request that preparations be made to deal with the prospect of Earth being slammed by an asteroid or comet.
In an "Open Letter to Congress on Near Earth Objects," the communication underscores the danger our planet faces from near Earth objects, also termed NEO's.
The letter has been sent to President Bush and his cabinet, the Secretary General of the United Nations and to leaders around the globe.
Included among those that urged action on the NEO issue were: Apollo 17 Astronaut, Harrison Schmitt; Neil Tyson, Director of the Hayden Planetarium; Freeman Dyson, Professor Emeritus of Princeton University; Lucy Ann McFadden, NEO scientist at the University of Maryland; New York University professor and author, William Burrows; John Lewis, a scientist at the University of Arizona, Tucson; and Thomas Jones, former astronaut and veteran of four shuttle missions.
Potentially Devastating Threat
"We write to you today as concerned citizens, convinced that the time has come for our nation to address comprehensively the impact threat from asteroids and comets," the letter begins.
The overall aim of the Open Letter is start a process to educate national leadership about the real threat posed by worrisome comets and asteroids that can approach Earth:
"A growing body of scientific evidence shows that some of these celestial bodies, also known as Near Earth Objects (NEOs), pose a potentially devastating threat of collision with Earth, capable of causing widespread destruction and loss of life. The largest such impacts can not only threaten the survival of our nation, but even that of civilization itself."
Three Step Effort
The letter urges U.S. lawmakers to take a series of three steps, thereby shaping a coordinated program to deal with the impact threat:
Step 1: NEO Detection - Expand and enhance this nation's capability to detect and to determine the orbits and physical characteristics of NEOs.
Step 2: NEO Exploration - Expand robotic exploration of asteroids and Earth-approaching comets and direct that U.S. astronauts again leave low-Earth orbit ... this time to further explore certain NEOs in deep space for information required to develop an effective capability to deflect an NEO should we learn that one threatens life on Earth.
Step 3: NEO Contingency Planning - Initiate comprehensive contingency planning for deflecting any NEO found to pose a potential threat to Earth. In parallel, plan to meet the disaster relief needs created by an impending or actual NEO impact. U.S. government/private sector planning should invite international cooperation in addressing the problems of NEO detection, potential hazards and actual impacts. This step also advocates establishment of an Interagency NEO Task Force to address the NEO Impact Threat. This Task Force should be composed of senior representatives from appropriate government agencies.
Resources committed to the NEO work have been very modest, an enclosure to the Open Letter declares, "and not commensurate with the potential threat." What is warranted is additional investment in search programs, deemed by the letter's supporters as both "appropriate and prudent."
A dramatic improvement in the rate at which asteroids and comets are discovered would likely result if the United States were to increase the current level of funding, now at about $3.5 million per year, to at least $20 million annually, the letter's enclosure explains.
The Open Letter concludes: "For the first time in human history, we have the potential to protect ourselves from a catastrophe of truly cosmic proportions."
"We cannot rely on statistics alone to protect us from catastrophe; such a strategy is like refusing to buy fire insurance because blazes are infrequent. Our country simply cannot afford to wait for the first modern occurrence of a devastating NEO impact before taking steps to adequately address this threat."
A leader in scripting the NEO Open letter is former shuttle astronaut, Thomas Jones. He is a veteran space traveler of shuttle missions, STS-59, 68, 80, and 98.
Contacted by SPACE.com, Jones said he is hopeful that the Open Letter stirs Congress to take action. But he is also realistic.
"It may very well take an impact to shake things up and make the government act," Jones said. "But since it's a basic responsibility of government to provide for the common defense, and since that mission is spread over many agencies, we thought that Congress is the right body to address the hazard, and to direct a joint approach."
If Congress takes no action, Jones said that he and the other supporters hope the President will act in response.
"It seems no one agency desires to take the lead on this, but since many have roles to play, from Homeland Security to Defense to NASA, our hope is that Congress can direct a concerted plan of action," Jones told SPACE.com.
"We already devote taxpayer funds to disaster preparedness in advance of other natural hazards, and so we call for a similar, prudent approach to studying and countering the impact hazard," Jones concluded.
An Open Letter to Congress on Near Earth Objects
Re: The Imperative to Address the Impact Threat From Near Earth Objects (NEOs)
July 8, 2003
Dear Members of Congress:
We write to you today as concerned citizens, convinced that the time has come for our nation to address comprehensively the impact threat from asteroids and comets. A growing body of scientific evidence shows that some of these celestial bodies, also known as Near Earth Objects (NEOs), pose a potentially devastating threat of collision with Earth, capable of causing widespread destruction and loss of life. The largest such impacts can not only threaten the survival of our nation, but even that of civilization itself.
Although we are genuinely concerned about the NEO threat, none of us is an alarmist. We know of no Near Earth Object currently on a collision course with Earth, but science's limited knowledge of the NEO population cannot rule out that possibility. Base on current information, a crisis response to these potential threats is not warranted. That being said, however, based upon evidence of past impacts and recent asteroid observations as well as the possible consequences from just one relatively "small" NEO impact, "business as usual" regarding this threat is simply no longer a responsible or sensible course of action.
Studies indicate that, with the commitment of modest resources, NEO impacts can likely be predicted and, with adequate warning, steps taken to prevent them. Thanks to scientific advances and increased awareness, we now have a historic opportunity to deal comprehensively and effectively with the NEO threat. Doing so, however, will require determined and coordinated action by Congress, the Executive Branch, and the private sector to direct effective use of our nation's substantial scientific and technological capability.
U.S. and international academic conferences, as well as Congressional hearings, have served to illuminate some aspects of the NEO impact hazard. Here, we build upon this background and outline a recommended course of action for Congress.
To address this potential threat, we strongly urge that each of you take steps within your respective committee jurisdictions to implement immediately the following recommendations (each is discussed in more detail in the enclosure):
1. NEO Detection: Expand and enhance this nation's capability to detect and to determine the orbits and physical characteristics of NEOs.
2. NEO Exploration: Expand robotic exploration of asteroids and Earth-approaching comets. Obtain crucial follow up information on NEOs (required to develop an effective deflection capability) by directing that U.S. astronauts again leave low-Earth orbit . . . this time to protect life on Earth.
3. NEO Contingency and Response Planning: Initiate comprehensive contingency and response planning for deflecting any NEO found to pose a potential threat to Earth. In parallel, plan to meet the disaster relief needs created by an impending or actual NEO impact. U.S. government/private sector planning should invite international cooperation in addressing the problems of NEO detection, potential hazards and actual impacts.
Overview of Confirmed NEO Impacts and Recently Detected NEOs
Sixty-five million years ago, a trillion-ton comet or asteroid only about six miles across struck what is now Chicxulub on Mexico's Yucatan Peninsula. That impact resulted in the extinction of at least 75% of Earth's species, including the dinosaurs.
Thirty-five million years ago, a comet or asteroid only approximately 3 miles in diameter struck Earth in Chesapeake Bay, about 120 miles southeast of Washington, D.C. That impact created a crater some 50 miles wide, changed the courses of many modern rivers and caused changes in ground-water aquifers that are still evident today.
Fifty thousand years ago, an asteroid just 150 feet in diameter, weighing approximately 300,000 tons, and traveling at 40,000 miles per hour struck Earth in what is today Arizona. Today, the crater from that impact, even after weathering, is still nearly a mile wide and 570 feet deep.
About a hundred years ago, on June 30, 1908, an object from space appeared in the morning sky over western China. It plunged through the atmosphere, glowing at a temperature of over 5,000 degrees F. Streaking over central Russia, the object's passage produced a deafening roar, preceded by a supersonic blast wave that leveled trees and houses in its path. As reported in the newspaper Sibir, this impact occurred "early in the ninth hour of the morning." Near the Stony Tunguska River, the object exploded in mid-air with an energy greater than a 10-megaton nuclear blast. The explosion devastated a region some 40 miles across, two-thirds the size of Rhode Island. Only a few people were killed in this sparsely populated region, but the story would have been very different if the object had hit a few hours later over Europe instead of the Siberian forest. The death toll in major cities such as St. Petersburg, Helsinki, Stockholm or Oslo might have reached 500,000.
In 1947, also in Russia, in the Sikhote-Alin Mountains, northeast of Vladivostok, a small meteor traveling at 31,000 miles per hour struck Earth's atmosphere, creating a fireball witnesses said was brighter than the sun. One of the fragments left an impact crater 85 feet across and 20 feet deep.
In 1994, the world witnessed the devastating effects that a large NEO impact could inflict on Earth. Astronomers who had observed the breakup of comet Shoemaker-Levy 9 then tracked its headlong crash into Jupiter, where it generated an explosion with an energy equivalent to a billion megatons of TNT. The resulting dust cloud in Jupiter's atmosphere swelled larger than our own Earth; a similar impact here would have destroyed our civilization and devastated life on this planet. Shoemaker-Levy 9 was discovered just sixteen months before it hit Jupiter, and its spectacular demise was a shot across our bow - a reminder that comets also can strike Earth. Comets, though less frequent visitors to Earth's vicinity than asteroids, strike with much greater kinetic energy, and comprise a small but significant part of the impact threat to Earth.
On January 7, 2002, the asteroid 2001 YB5 missed our planet by a little more than twice the distance to the Moon. If this 300-yard-wide, stadium-sized object, discovered only 12 days before its closest approach, had hit the Earth's continental landmasses, it would have destroyed nearly everyone and everything in an area about the size of New England. An ocean impact would also have spawned huge tsunamis, with the potential for damage to coastal areas beyond anything in historical experience.
The modest search efforts sponsored by the National Aeronautics and Space Administration (NASA) and the Department of Defense have detected a steady stream of close encounters. On June 14, 2002, asteroid 2002 MN, an object about 100 yards in diameter, passed within just 75,000 miles of Earth at a speed of over 23,000 miles per hour. 2002 MN was detected by astronomers at the Lincoln Near Earth Asteroid Research (LINEAR) search facility in New Mexico three days after its closest approach to Earth. Had this object struck Earth, it would have exploded with energy about equal to that of the 1908 Siberian impact near Tunguska.
On July 5, 2002, the LINEAR astronomers discovered another object, designated 2002 NT7, estimated to be over a mile in diameter. And in November 2002, astronomers discovered 2002 VU94, an NEO estimated to be over two miles across. While both objects pose no danger to Earth in the coming centuries, their recent discovery and large size emphasize the fact that many large NEOs remain undiscovered.
Scientists have realized for some time that Earth travels amid a "sea" of similar objects, large and small. NASA stated last year in Congressional testimony that we have detected only a little more than half of all NEOs larger than a kilometer in diameter. Prudence dictates that more be done to identify NEOs, and to obtain the scientific information necessary to divert any sizable NEO found to be on a collision course with Earth.
The NEO Threat
The latest NEO close approaches are typical of the two dozen such encounters known to have occurred in the 20th Century. These are only a small fraction of the actual number that have occurred; most have gone completely undetected. Such approaches are commonplace in our part of the solar system. The late planetary geologist Eugene Shoemaker put it succinctly: "Earth exists in an asteroid swarm."
We know that since 1937, at least 22 asteroids have approached Earth more closely than did 2001 YB5, which missed by just twice the distance to the Moon. Five of those objects were larger than 100 yards in diameter. According to NASA, there may be as many as 100,000 NEOs with diameters of 100 yards or larger. Of those asteroids larger than 150 yards in diameter, about 250 are today estimated to be potentially hazardous. The United States has very limited capability to detect these smaller NEOs, which can nevertheless inflict substantial damage upon striking Earth. There is a significant probability (20%) of such an object colliding with the Earth during the next century.
Although the annual probability of a large NEO impact on Earth is relatively small, the results of such a collision would be catastrophic. The physics of Earth's surface and atmosphere impose natural upper limits on the destructive capacity of natural disasters, such as earthquakes, landslides, and storms. By contrast, the energy released by an NEO impact is limited only by the object's mass and velocity. Given our understanding of the devastating consequences to our planet and its people from such an event, (as well as the smaller-scale but still-damaging effects from smaller NEO impacts), our nation should act comprehensively and aggressively to address this threat. America's efforts to predict, and then to avoid or mitigate such a threat, should be at least commensurate with our national efforts to deal with more familiar terrestrial hazards.
If space research has taught us anything, it is the certainty that an asteroid or comet will hit Earth again. Impacts are common events in Earth's history: scientists have found more than 150 large impact craters on our planet's surface. Were it not for Earth's oceans and geological forces such as erosion and plate tectonics, the planet's impact scars would be as plain as those visible on the Moon.
Potential Misinterpretation of NEO Impacts
Even small NEO impacts in the atmosphere, on the surface, or at sea create explosions that could exacerbate existing political tensions and escalate into major international confrontations. For example, an atmospheric impact in 2002 produced a large, highly visible burst of light in the sky during the height of war tensions between nuclear-armed countries India and Pakistan. That high-altitude explosion happened to occur over the Mediterranean, just a few thousand miles from their disputed border region. Had that NEO impact occurred less than three hours earlier, it would have detonated over southern Asia, where its misinterpretation as a surprise attack could have triggered a deadly nuclear exchange. With military and diplomatic tensions at their peak in other areas of conflict in the world, the potential for a mistake is even greater today.
For the first time in human history, we have the potential to protect ourselves from a catastrophe of truly cosmic proportions. All of us remember vividly the effect on our nation of terrorist strikes using subsonic aircraft turned into flying bombs: thousands of our citizens dead, and our economy badly shaken. Consider the ramifications of an impact from a relatively small NEO: more than a million times more massive than an aircraft, and traveling at more than thirty times the speed of sound. If such an object were to strike a city like New York, millions would die. In addition to the staggering loss of life, the effects on the national and global economy would be devastating. Recovery would take decades.
We cannot rely on statistics alone to protect us from catastrophe; such a strategy is like refusing to buy fire insurance because blazes are infrequent. Our country simply cannot afford to wait for the first modern occurrence of a devastating NEO impact before taking steps to adequately address this threat. We may not have the luxury of a second chance, for time is not necessarily on our side. If we do not act now, and we subsequently learn too late of an impending collision against which we cannot defend, it will not matter who should have moved to prevent the catastrophe . . . only that they failed to do so when they had the opportunity to prevent it.
Our nation, our families, and others around the globe deserve our best efforts to protect against the NEO impact threat. We urge the Congress to call on this nation's ready supply of talents and energies to responsibly address this threat. Our international partners also should be called upon to help meet this challenge, but the United States has a compelling responsibility to lead the way. Preventing an NEO impact is a vital mission for our nation's space program and for the American people. For the first time since Apollo, our astronauts should once again leave low-Earth orbit and journey into deep space, this time to protect life on our home planet.
We strongly recommend your prompt attention and action to address this too-long-ignored threat to the security of America and to the world. The accompanying recommendations are prudent and concrete steps each of you can now take to safeguard our nation. Your timely and effective response can protect the people of the United States and the world from the real threat posed by Near Earth Objects.
Dr. Harrison H. Schmitt
Dr. Carolyn S. Shoemaker
David H. Levy
Dr. John Lewis
Dr. Neil D. Tyson
Dr. Freeman Dyson
Dr. Richard P. Hallion
Dr. Thomas D. Jones
Bruce Joel Rubin
Dr. Lucy Ann McFadden
Erik C. Jones
William E. Burrows
Impact Hazard Summary by Andrea Milani
Science, 20 June 2003
Asteroid and comet impacts on the Earth did happen many times in the past. Raise your eyes to the Moon: what you see is a surface saturated by craters. The largest lunar impacts, however, happened before 3,800 million years ago. Over a time scale comparable to human life the largest impact was at Tunguska, Siberia, in 1908, by an asteroid <70 meters in diameter. A recent reassessment of the Tunguska-class impacts suggests that the average frequency for impacts of this size is only one in about 1,000 years (1).
The rate of impacts is a function of the impactor size. Large impacts, forming craters tens of kilometers in diameter, are very rare events over the time scale of human life and civilization, frequent events over geological times. The most energetic impacts could have triggered some of the transitions between geological eras. 65 million years ago an impact by a ~10 kilometer diameter asteroid generated a ~100 million Megaton explosion and excavated the 180 kilometer wide Chicxulub crater in Mexico. Its ash layer covers the entire surface of the Earth and marks the transition between the Cretaceous and the
Tertiary eras, across which most dinosaurs became extinct. The mechanisms of the extinctions are not established, but the association with the crater cannot be a coincidence.
For a given impact energy the physical effects can be modeled with the methods developed for nuclear weapons. These effects are local: the Tunguska impact flattened a few thousands square kilometers of desert taiga. The loss of life due to a Tunguska-class impact is unlikely to exceed that of other natural disasters (2). For a much larger impact the most damaging effects could be the global ones. An explosion of thousands of Megatons could change atmospheric chemistry and world climate, could damage the biosphere and affect mankind through global failure of food crops. This chain of consequences is hard to model in a quantitative, predictive way.
What, then, is the frequency of impacts that could kill one billion people, or even result in extinction of the human species? The uncertainties exceed one order of magnitude. A Chicxulub-class impact, expected only once in 100 million years, may result in extinction of mankind. An impact of ~1,000 Megatons, expected to occur every ~60,000 years (3), should not have global effects, although the local effects might include Tsunami waves affecting an entire ocean basin. Somewhere in between, at few kilometers in diameter and at once every few million years, lies the frequency of the unknown critical sized impactors for global effects.
The Spaceguard Goal
The cause of the impact risk is the population of Earth-crossing asteroids and comets. Most asteroids orbit around the Sun between Mars and Jupiter, while most comets have orbits beyond Neptune. As a result of planetary, stellar, or non gravitational perturbations, a small fraction of both classes ends up in unstable orbits and close approaches to our planet become possible. To compile a catalog of all these Near Earth Objects (NEO) is the first goal; to scan the entire sky became possible with the development of CCD cameras and of the necessary software (4). This made feasible the so called "Spaceguard goal": to discover 90% of all the NEO with diameter 1 kilometer or larger.
The choice of targeting the surveys to objects above 1 km is optimal, provided that this coincides with the critical size for global effects. This can be shown by a probabilistic computation of the expected damage, in human lives lost per year (5). The onset of global effects is believed to increase the number of casualties by as much as an order of magnitude. If this is true, then the expected casualty rate is approximately the number of casualties just above the global effect threshold times the frequency of such an impact (6). Thus, if the casualties are 1 billion and this happens once in a million years, the expected damage is ~1,000 deaths per year (7). The contribution of the more frequent Tunguska-class impacts is minor: the expected damage is a few tens of thousands casualties divided 1,000 years, or a few tens per year. Thus to target for discovery the objects just above the global effects threshold is the most "cost effective" way to decrease the risk.
This "insurance approach" to the problem of impact risk appears questionable to many, including myself. Two other rational arguments can be used. We could agree on a level of damage we consider unacceptable: an impact by a 1 km asteroid, with energy comparable to a global thermonuclear war, could be considered unacceptable even if it might not trigger global effects. The other argument is the technological limit: there is no point in setting a goal we cannot achieve in the foreseeable future. At the beginning of the 90's both arguments supported the Spaceguard goal, that could be achieved in few decades, by using telescopes of limited size (8). In conclusion, there was no mathematical theorem proving that the Spaceguard goal was optimal, but it was a reasonable approximation. Being a simply stated and achievable goal, it has contributed significantly in spreading the discussion outside of the scientific community.
In 1998 NASA, upon request from the US Congress, accepted the task of achieving the Spaceguard goal in 10 years. With support from NASA (also from scientific institutions and the US military) there has been a very encouraging progress in the NEO searches. From 1998 essentially all the observable sky became covered to a depth corresponding to apparent magnitude 18.5. As a result, the number of NEO discovered has grown very rapidly: now (9) we know 569 Near Earth Asteroids (NEA) with absolute magnitude brighter than 17.65, corresponding to 1 km diameter for average albedo (10). The number of NEA with 1 km diameter and larger is now estimated at about 1000, with a comparatively small uncertainty (3). Thus NASA has been able to claim that the Spaceguard goal is now more than half achieved, but the number of NEA of a given size remaining to be discovered decreases exponentially. The current survey simulations predict that a significant upgrade in the survey telescopes is required to achieve the goal by 2008; this upgrade is taking place.
The Spaceguard goal does not completely account for the risk of impacts by comets. Short periodic comets can be detected with the same techniques used for NEA, but a long periodic comet can have a close approach to our planet only a few months after becoming observable. The probability of an impact by a single long periodic comet is very small (~1/1,000,000,000 per perihelion passage). To assess the risk of impacts with a given energy by long periodic comets we would need to have much more information on the mass of these objects. A very rough estimate indicates that the risk from long periodic comets is at least one order of magnitude less than that from NEA, for the same impact energy range. Still, this implies that it may not be possible to go much beyond the 90% Spaceguard goal for risk reduction without taking into account long periodic comets.
Is Observation Enough?
Can we assume that, as soon as a NEO is observed, its contribution to the total impact risk becomes zero? To detect the asteroid/comet signal in a CCD frame is not enough to discover a NEO. A number of observations are needed to compute an orbit: e.g., Apollo was detected in 1930, but discovered, with a computed NEO orbit, only in 1932. Main belt asteroids are detected by the hundreds for each NEO detection, but the current survey systems aiming to discover NEO cannot follow up all the detections. Unfortunately, it is not always easy to discriminate a NEO from the main belt detections. An object moving at a fast angular rate must be a NEO, a slower moving one is most likely main belt. But a fraction of NEO are detected while moving at main belt rates. Thus the requirement of following up only the NEO is inconsistent.
The progress in NEO discovery has been slowed down by the failure to appreciate that all the asteroid/comet data are also data on NEO. The only way to be sure that a given detection is not a NEO is to identify it with a main belt asteroid with a known orbit. Because the asteroid identification problem is a mathematically difficult one (11), all the detection data, from all surveys, should be available for competitive research of asteroid identifications. In the past this did not happen, also because of obsolete "discovery credit rules" discouraging publication of data until a full discovery could be announced. There are now encouraging indications that the observers have understood that to extract as much information as possible from the raw data is in the best interest of the scientific community.
Virtual and Real Impactors
Even after a NEO has been observed long enough to compute an orbit, its future position belongs to an "uncertainty region" that grows with time. At some time in the future such region could touch the Earth. Is it possible to establish whether a NEO, with a given set of observations, can or cannot impact the Earth in the next, say, 100 years? This issue was raised forcefully in March 1998, when an ambiguously worded statement about the NEA 1997 XF11 issued by the Minor Planet Center resulted in a media storm. Scientists in this field have learned that making a public announcement on a possible impact is a difficult task. But the main issue was not a PR problem. As of 1998 nobody knew how to solve the mathematical problem of detecting possible future impacts.
Although the orbit of an asteroid is perfectly deterministic, we need to describe its future position in a probabilistic way, to quantify our ignorance of where the object really is. We can describe this as a swarm of virtual asteroids (VA): only one of them is the real one, but we do not know which one. If one of the VA has a very close approach to the Earth at some time in the future, in such a way that a minute modification of the orbit, still compatible with the observations, results in an impact, then there is a Virtual Impactor (VI). Each VI is associated with a non-zero probability of impact. If we only want to detect the VI with large probability, then only a few VA orbits must be computed. If the risk is one of a catastrophic impact we are interested in knowing about minute probabilities (1/1,000,000 and less). In this case a brute force approach would require to compute millions of VA orbits, and this task is beyond the current generation of computers (12). This problem can be overcome if a set of VA are assembled into a geometric object, such as a string. With this approach (13), VI for the asteroid 1999 AN10 could be detected at a probability level of one in a billion, using only ~1000 VA.
Since 1999, the impact monitoring robot CLOMON at the University of Pisa has used this approach to monitor each newly discovered asteroid, scanning the possible evolution of the orbit for the next 80 years to look for VI to probability levels 1/1,000,000 and below. In 2002 the second generation monitoring system Sentry (14) went on line at Jet Propulsion Lab, California, and CLOMON2 replaced the first robot in Pisa (15). By comparing the output of the two systems, we have reached very high levels of reliability.
If no VI are found, the asteroid is safe. If VI are found, the two robots send alarm messages to the human operators. The observers, coordinated by the Spaceguard Central Node (SCN) (16), then keep track of the object until new observations, decreasing our ignorance of the future orbit, force the probability of the impact down to negligible values. This procedure has now been used dozens of times, and all VIs, including those of the 2 km-diameter 2002 NT7, have been eliminated by observation within a day to several months.
In an increasingly connected world, the sudden death of many people is considered less acceptable, thus actions to prevent it should meet with increasing support. As new technology for CCD chips and arrays, faster computers and telescopes in the 4-m to 6-m class become available, a more ambitious goal than Spaceguard should be formulated.
A post-Spaceguard goal could be to discover 90% of the NEO down to 300 meters in diameter and to increase the completeness for those with diameter >1 km to, say, 97%, with provisions for a better understanding of the risk due to long periodic comets. Such a goal, to be achieved within the next 10 to 20 years, is realistic. The issue is who should provide the resources for achieving it.
It might be argued that the discovery of NEO is not pure scientific research, but rather a civil defense task. As such, it could be responsibility of other agencies, both civil and military, different from the ones funding science. In the USA, the US Space Command has expressed interest, whereas NASA does not seem keen to take responsibility beyond the Spaceguard goal. Other countries have funded theoretical research (such as the one done in Italy), some interesting initiatives such as the Bisei center in Japan, and some public relations exercises (such as the one going on in the UK); but overall, support for NEO searches has been negligible. If this does not change the international community, in particular the scientific one, might not play a major role in handling the NEO impact risk.
The more an issue is critical for the safety of mankind, the less it should be entrusted to a bureaucratic and secretive organization. Would you like to know that maybe an asteroid is on collision course toward the Earth, but some organization is taking care of it without public discussion? Our experience with the asteroid observational data confirms that the involvement of military organizations implies enormous difficulties in implementing the open data policy that, as outlined above, is necessary. On the contrary, the scientists are committed to publish the results of their research, because scientific knowledge remaining secret has no long term value and generates no credit. The NEO impact risk assessment should therefore remain the responsibility of the international scientific community (17).
If the basic technology required is available, such a task is within the level of effort and resources normally available for scientific research. However, NEO searches are seen by many astronomers as less fundamental than other scientific goals, such as elucidating the origin and large scale structure of the universe. This way of thinking neglects two important points. First, the relevance of a scientific discovery for humankind depends also on its practical implications: knowledge about what could destroy us should have some priority. Science should pursue knowledge for the sake of knowledge, but this does not imply that knowledge without practical application is "more fundamental" (17).
Second, no planetary system can form without comets and asteroids. There is indirect observational evidence that such small bodies exist, for example as the steady state source of the transient dust belts around many stars. Spectroscopic data have shown that comets and asteroids impact the star Beta Pictoris (18) in the same way as our Sun (19). Small bodies thus exist around other stars and have similar dynamical behavior. Their collisions with each other and with planets are a universal phenomenon, and should be included in all astrophysical and exobiological models of the evolution of extra-solar planetary systems.
In my opinion, the scientific community should take upon itself the duty to investigate the NEO population at the level of knowledge necessary to identify all possible impactors, down to the size compatible with available technology and with the public perception of acceptable risk. In the next decades, this should go well beyond the Spaceguard goal, with the help of sky surveys by large telescopes such as LSST and Pan-STARRS (17).
Observations and computations to date have not discovered a likely impactor. It is unlikely that a serious threat is discovered in our lifetime (20). But what should be done if such an impactor is identified? We cannot justify the effort to discover it unless we can safeguard of our planet even in this worst case.
The effort necessary to deflect an asteroid to avoid a collision goes well beyond the level of resources available to the scientific community and cannot be prepared before the need arises. On the contrary, the know how necessary for such a task should be gathered in advance. The space agencies, such as NASA and ESA, have the necessary capabilities and are interested in including this goal in their mission.
This year NASA has convened a workshop to identify the scientific and technological knowledge which would be necessary for an asteroid deflection (21). One such requirement is to understand the internal structure of an asteroid, otherwise an attempt at deflection may result in disruption, with loss of control on the pieces. The European Space Agency (ESA) has funded several studies of innovative NEO missions. One of them, called Don Quijote, would involve two spacecrafts. The Hidalgo probe should impact a small asteroid (~500 meter in diameter) at a relative velocity >10 km/s, while the Sancho spacecraft orbits the same asteroid. This allows to study the internal structure of the asteroid by seismology and to test a non-nuclear deflection technique, by measuring the amount of deflection achieved. If this mission is found to be technically and economically feasible, the knowledge necessary to face the worst case will become available. (22)
Notes & References
(1) Harris, A. Bull. Amer. Astron. Soc. 34, 020 (2002)
(2) Even now the average density of the human population on the
surface of the Earth is only ~12 per square kilometer. Tunguska-class
impacts on major cities are less likely than impacts by significantly
(3) Morbidelli et al., Icarus, 158, 329 (2002)
(4) Carusi, A. et al., in Hazards due to comets and asteroids,
T. Gehrels et al. eds, (Univ Arizona press, Tucson, 1994), pp 127-148.
(5) It is obtained formally by an integral of the product of the
yearly probability for an asteroid of a given size times the estimated
casualties from such an impact.
(6) Morrison, D. et al., in Hazards due to comets and asteroids,
T. Gehrels et al. eds., (Univ. Arizona Press, Tucson, 1994) pp. 59-92;
Chapman, C.R. and Morrison, D., Nature, 367, 33 (1994).
(7) The size distribution of the NEO is steep, with objects 10 times
larger a few hundred times less numerous, thus Chicxulub-class
impacts, being much rarer, contribute little to the total risk.
(8) Not more than 2 meters in diameter; such telescopes have limited use
for competitive research in other fields of astronomy and are
(9) NEODYS, January 2003; available at http://newton.dm.unipi.it/neodys/
(10) Chesley, S.R. et al., Icarus, 159, 423 (2002).
(11) Milani,A., Icarus, 137, 269, (1999).
(12) Milani, A. et al., in Asteroids III, R. Binzel et
al. eds. (Univ. Arizona Press, Tucson, 2003).
(13) Milani,A. et al, Astron. Astrphys., 346, L65. (1999).
(14) Sentry http://neo.jpl.nasa.gov/risk/
(15) The output of CLOMON2 is included in the same web site as NEODyS,
(16) SCN http://spaceguard.ias.rm.cnr.it/spaceguard/
(17) While in the rest of the paper I am summarizing the scientific
consensus, in these paragraphs I am expressing a very personal
(18) Beust, H., et al., Astron. Astrophys. 310, 181, (1996).
(19) Farinella, P. et al., Nature, 371, 314 (1994).
(20) E.g., there is a probability of 1 in 630 that a 1000 megaton
impact will occur in the next 100 years.
(21) NASA Mitigation Workshop, extended abstracts volume:
(22) The author has been assisted, in preparing this article, by Nanni
Riccobono (Tumbling Stone) and Julia Fahrenkamp-Uppenbrink (Science).