The United States spends about $4 million annually searching for near-Earth objects (NEOs), according to NASA.1 The goal is to detect those that may collide with Earth. The funding helps to operate several observatories that scan the sky searching for NEOs, but, as explained below, it is insufficient to detect the majority of NEOs that may present a tangible threat to humanity. A smaller amount of funding (significantly less than $1 million per year) supports the study of ways to protect Earth from such a potential collision (“mitigation”).
Congress established two mandates for the search for NEOs by NASA. The first, in 1998 and now referred to as the Spaceguard Survey, called for the agency to discover 90 percent of NEOs with a diameter of 1 kilometer or greater within 10 years. An object of this limiting size is considered by many experts to be the minimum that could produce global devastation if it struck Earth. NASA is close to achieving this goal and should reach it within a few years. However, as the recent (2009) discovery of an approximately 2- to 3-kilometer-diameter NEO demonstrates, there are still large objects to be detected.
The second mandate, established in 2005, known as the George E. Brown, Jr. Near-Earth Object Survey Act,2 called for NASA to detect 90 percent of NEOs 140 meters in diameter or greater by 2020. As the National Research Council’s (NRC’s) Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies noted in its August 2009 interim report (NRC, 2009):
Finding: Congress has mandated that NASA discover 90 percent of all near-Earth objects 140 meters in diameter or greater by 2020. The administration has not requested and Congress has not appropriated new funds to meet this objective. Only limited facilities are currently involved in this survey/discovery effort, funded by NASA’s existing budget.
Finding: The current near-Earth object surveys cannot meet the goals of the 2005 George E. Brown, Jr. Near-Earth Object Survey Act directing NASA to discover 90 percent of all near-Earth objects 140 meters in diameter or greater by 2020.
THE SURVEY AND DETECTION OF NEAR-EARTH OBJECTS
The charge from Congress to the NRC committee was stated as two tasks (see the Preface for the full statement of task). The first asks for the “optimal approach” to completing the George E. Brown, Jr. Near-Earth Object Survey. The second asks for the same approach to developing a capability to avert an NEO-Earth collision and for options that include “a significant international component.”
The committee concluded that there is no way to define “optimal” in this context in a universally acceptable manner: there are too many variables involved that can be both chosen and weighted in too many plausible ways. Recognizing this fact, the committee first took a broad look at all aspects of the hazards to Earth posed by NEOs and then decided on responses to the charge. The body of this report contains extensive discussions of these many issues. This summary concentrates on responses to the charge and at the end provides a few comments on some of the other main conclusions drawn from the report.
Regarding the first task of its charge, the committee concluded that it is infeasible to complete the NEO census mandated in 2005 on the required time scale (2020), in part because for the past 5 years the administration has requested no funds, and the Congress has appropriated none, for this purpose. The committee concludes that there are two primary options for completing the survey:
Finding: The selected approach to completing the George E. Brown, Jr. Near-Earth Object Survey will depend on nonscientific factors:
If the completion of the survey as close as possible to the original 2020 deadline is considered more important, a space mission conducted in concert with observations using a suitable ground-based telescope and selected by peer-reviewed competition is the better approach. This combination could complete the survey well before 2030, perhaps as early as 2022 if funding were appropriated quickly.
If cost conservation is deemed more important, the use of a large ground-based telescope is the better approach. Under this option, the survey could not be completed by the original 2020 deadline, but it could be completed before 2030. To achieve the intended cost-effectiveness, the funding to construct the telescope must come largely as funding from non-NEO programs.
Multiple factors will drive the decision on how to approach completion of this survey. These factors include, but are not limited to, the perceived urgency for completing the survey as close as possible to the original 2020 deadline, the availability of funds to complete the survey, and the acceptability of the risk associated with the construction and operation of various ground- and space-based options.
Of the ground-based options, the Large Synoptic Survey Telescope (LSST) and the Panoramic Survey Telescope and Rapid Response System, mentioned in the statement of task, and the additional options submitted to the committee in response to its public request for suggestions during the beginning of this study, the most capable appears to be the LSST. The LSST is to be constructed in Chile and has several science missions as well as the capability of observing NEOs. Although the primary mirror for the LSST has been cast and is being polished, the telescope has not been fully funded and is pending prioritization in the astronomy and astrophysics decadal survey of the NRC that is currently underway.
Unless unexpected technical problems interfere, a space-based option should provide the fastest means to complete the survey. However, unlike ground-based telescopes, space options carry a modest launch risk and a more limited lifetime: ground-based telescopes have far longer useful lifetimes and could be employed for continued NEO surveys and for new science projects. (Ground-based telescopes generally have an annual operating cost that is approximately 10 percent of their design and construction costs.)
The committee notes that objects smaller than 140 meters in diameter are also capable of causing significant damage to Earth. The best-known case from recent history is the 1908 impact of an object at Tunguska in the Siberian wilderness that devastated more than 2,000 square kilometers of forest. It has been estimated that the size of this object was on the order of approximately 70 meters in diameter, but recent research indicates that it could have been substantially smaller (30 to 50 meters in diameter), with much of the damage that it caused being due to shock waves from the explosion of the object in Earth’s atmosphere. (See, e.g., Chyba et al., 1993; Boslough and Crawford, 1997, 2008.) The committee strongly stresses that this new conclusion is preliminary and must be independently validated. Since smaller objects are more numerous than larger ones, however, this new result, if correct, implies an increase in the frequency of such events to approximately once in three centuries.
All told, the committee was struck by the many uncertainties that suffuse the subject of NEOs, including one other related example: Do airbursts from impactors in this size range over an ocean cause tsunamis that can severely damage a coastline? This uncertainty and others have led the committee to the following recommendation:
Recommendation: Because recent studies of meteor airbursts have suggested that near-Earth objects as small as 30 to 50 meters in diameter could be highly destructive, surveys should attempt to detect as many 30- to 50-meter-diameter objects as possible. This search for smaller-diameter objects should not be allowed to interfere with the survey for objects 140 meters in diameter or greater.
In all cases, the data-reduction and data-analysis resources necessary to achieve the congressional mandate would be covered by the survey projects themselves and by a continuation of the current funding of the Smithsonian Astrophysical Observatory’s Minor Planet Center, as discussed in the report.
CHARACTERIZATION AND THE ARECIBO AND GOLDSTONE OBSERVATORIES
Obtaining the orbits and the physical properties of NEOs is known as characterization and is primarily needed to inform planning for any active defense of Earth. Such defense would be carried out through a suitable attack on any object predicted with near certainty to otherwise collide with Earth and cause significant damage. The apparently huge variation in the physical properties of NEOs seems to render infeasible the development of a comprehensive inventory through in situ investigations by suitably instrumented spacecraft: the costs would be truly astronomical. A spacecraft reconnaissance mission might make good sense to conduct on an object that, without human intervention, would hit Earth with near certainty. Such a mission would be feasible provided there was sufficient warning time for the results to suitably inform the development of an attack mission to cause the object to miss colliding with Earth.
In addition to spacecraft reconnaissance missions as needed, the committee concluded that vigorous, ground-based characterization at modest cost is important for the NEO task. Modest funding could support optical observations of already-known and newly discovered asteroids and comets to obtain some types of information on this broad range of objects, such as their reflectivity as a function of color, to help infer their surface properties and mineralogy, and their rotation properties. In addition, the complementary radar systems at the Arecibo Observatory in Puerto Rico and the Goldstone Solar System Radar in California are powerful facilities for characterization within their reach in the solar system, a maximum of about one-tenth of the Earth-Sun distance. Arecibo—which has a maximum sensitivity about 20-fold higher than Goldstone’s but does not have nearly as good sky coverage as Goldstone—can, for example, model the three-dimensional shapes of (generally very odd-shaped) asteroids and estimate their surface characteristics, as well as determine whether an asteroid has a (smaller) satellite or satellites around it, all important to know for planning active defense. Also, from a few relatively closely spaced (in time) observations, radar can accurately determine the orbits of NEOs, which has the advantage of being able to calm public fears quickly (or possibly, in some cases, to show that they are warranted).
Finding: The Arecibo and Goldstone radar systems play a unique role in the characterization of NEOs, providing unmatched accuracy in orbit determination and offering insight into size, shape, surface structure, and other properties for objects within their latitude coverage and detection range.
Recommendation: Immediate action is required to ensure the continued operation of the Arecibo Observatory at a level sufficient to maintain and staff the radar facility. Additionally, NASA and the National Science Foundation should support a vigorous program of radar observations of NEOs at Arecibo, and NASA should support such a program at Goldstone for orbit determination and the characterization of physical properties.
For both Arecibo and Goldstone, continued funding is far from assured, not only for the radar systems but for the entire facilities. The incremental annual funding required to maintain and operate the radar systems, even at their present relatively low levels of operation, is about $2 million at each facility (see Chapter 4). The annual funding for Arecibo is approximately $12 million. Goldstone is one of the three deep-space communications facilities of the Deep Space Network, and its overall funding includes additional equipment for space communications.
“Mitigation” refers to all means of defending Earth and its inhabitants from the effects of an impending impact by an NEO. Four main types of defense are discussed in this report. The choice of which one(s) to use depends primarily on the warning time available and on the mass and speed of the impactor. The types of mitigation are these:
Civil defense. This option may be the only one feasible for warning times shorter than perhaps a year or two, and depending on the state of readiness for applying an active defense, civil defense may be the only choice for even longer times.
“Slow-push” or “slow-pull” methods. For these options the orbit of the target object would be changed so that it avoided collision with Earth. The most effective way to change the orbit, given a constraint on the energy that would be available, is to change the velocity of the object, either in or opposite to the direction in which it is moving (direct deflection—that is, moving the object sideways—is much less efficient). These options take considerable time, on the order of decades, to be effective, and even then they would be useful only for objects whose diameters are no larger than 100 meters or so.
Kinetic impactors. In these mitigation scenarios, the target’s orbit would be changed by the sending of one or more spacecraft with very massive payload(s) to impact directly on the target at high speed in its direction, or opposite to its direction, of motion. The effectiveness of this option depends not only on the mass of the target but also on any net enhancement resulting from material being thrown out of the target, in the direction opposite to that of the payload, upon impact.
Nuclear explosions. For nontechnical reasons, this would likely be a last resort, but it is also the most powerful technique and could take several different forms, as discussed in the report. The nuclear option would be usable for objects up to a few kilometers in diameter.
For larger NEOs (more than a few kilometers in diameter), which would be on the scale that would inflict serious global damage and, perhaps, mass extinctions, there is at present no feasible defense. Luckily such events are exceedingly rare, the last known being about 65 million years ago.
Of the foregoing options, only kinetic impact has been demonstrated (by way of the very successful Deep Impact spacecraft that collided with comet Tempel-1 in July 2006). The other options have not advanced past the conceptual stage. Even Deep Impact, a 10-kilometer-per-second impact on a 6-kilometer-diameter body, was on a scale far lower than would be required for Earth defense for an NEO on the order of 100 meters in diameter, and it impacted on a relatively large—and therefore easier to hit—object.
Although the committee was charged in its statement of task with determining the “optimal approach to developing a deflection capability,” it concluded that work in this area is relatively new and immature. The committee therefore concluded that the “optimal approach” starts with a research program.
Struck by the significant unknowns in many aspects of NEO hazards that could yield to Earth-based research, the committee recommends the following:
Recommendation: The United States should initiate a peer-reviewed, targeted research program in the area of impact hazard and mitigation of NEOs. Because this is a policy-driven, applied program, it should not be in competition with basic scientific research programs or funded from them. This research program should encompass three principal task areas: surveys, characterization, and mitigation. The scope should include analysis, simulation, and laboratory experiments. This research program does not include mitigation space experiments or tests that are treated elsewhere in this report.
NATIONAL AND INTERNATIONAL COOPERATION
Responding effectively to hazards posed by NEOs requires the joint efforts of diverse institutions and individuals, with organization playing a key role. Because NEOs are a global threat, efforts to deal with them could involve international cooperation from the outset. (However, this is one area in which one nation, acting alone, could address such a global threat.) The report discusses possible means to organize, both nationally and internationally, responses to the hazards posed by NEOs. Arrangements at present are largely ad hoc and informal here and abroad, and they involve both government and private entities.
The committee discussed ways to organize the national community to deal with the hazards of NEOs and also recommends an approach to international cooperation:
Recommendation: The United States should take the lead in organizing and empowering a suitable international entity to participate in developing a detailed plan for dealing with the NEO hazard.
One major concern with such an organization, especially in the area of preparing for disasters, is the maintenance of attention and morale, given the expected exceptionally long intervals between harmful events. Countering the tendency to complacency would be a continuing challenge. This problem would be mitigated if, for example, the civil defense aspects were combined in the National Response Framework with those for other natural hazards.
RECENT NEAR-EARTH-OBJECT-RELATED EVENTS
The U.S. Department of Defense, which operates sensors in Earth orbit capable of detecting the high-altitude explosion of small NEOs, has in the past shared this information with the NEO science community. The committee concluded that this data sharing is important for understanding issues such as the population size of small NEOs and the hazard that they pose. This sharing is also important for validating airburst simulations, characterizing the physical properties of small NEOs (such as their strength), and assisting in the recovery of meteorites.
Recommendation: Data from NEO airburst events observed by the U.S. Department of Defense satellites should be made available to the scientific community to allow it to improve understanding of the NEO hazards to Earth.
In 2008, Congress passed the Consolidated Appropriations Act3 calling for the Office of Science and Technology Policy to determine by October 2010 which agency should be responsible for conducting the NEO survey and detection and mitigation program. Several agencies are possible candidates for such a role.
During its deliberations the committee learned of several efforts outside the United States to develop spacecraft to search for categories of NEOs. In particular, Canada’s Near-Earth-Object Surveillance Satellite, or NEOSSat,
and Germany’s AsteroidFinder are interesting and capable small-scale missions that will detect a small percentage of specific types of NEOs, those primarily inside Earth’s orbit. These spacecraft will not accomplish the goals of the George E. Brown, Jr. Near-Earth Object Survey Act of 2005. However, they highlight the fact that other countries are beginning to consider the NEO issue seriously. Such efforts also represent an opportunity for future international cooperation and coordination in the search for potentially hazardous NEOs. In addition, the committee was impressed with the European Space Agency’s early development of the Don Quijote spacecraft mission, which would consist of an observing spacecraft and a kinetic impactor. This mission, though not funded, would have value for testing a mitigation technique and could still be an opportunity for international cooperation in this area.
Finally, the committee points out a current estimate of the long-term average annual human fatality rate from impactors: slightly under 100 (Harris, 2009). At first blush, one is inclined to dismiss this rate as trivial in the general scheme of things. However, one must also consider the extreme damage that could be inflicted by a single impact; this presents the classic problem of the conflict between “extremely important” and “extremely rare.” The committee considers work on this problem as insurance, with the premiums devoted wholly toward preventing the tragedy. The question then is: What is a reasonable expenditure on annual premiums? The committee offers a few possibilities for what could perhaps be accomplished at three different levels of funding (see Chapter 8); it is, however, the political leadership of the country that determines the amount to be spent on scanning the skies for potential hazards and preparing our defenses.
Boslough, M., and D. Crawford. 2008. Low-altitude airbursts and the impact threat. International Journal of Impact Engineering 35:1441–1448.
Boslough, M.B.E., and D.A. Crawford. 1997. Shoemaker-Levy 9 and plume-forming collisions on Earth. Near-Earth Objects, the United Nations International Conference: Proceedings of the International Conference held April 24-26, 1995, in New York, N.Y. (J.L. Remo, ed.). Annals of the New York Academy of Sciences 822:236–282.
Chyba, C.F., P.J. Thomas, and K.J. Zahnle. 1993. The 1908 Tunguska explosion—Atmospheric disruption of a stony asteroid. Nature 361:40–44.
Harris, A.W., Space Science Institute. 2009. The NEO population, impact risk, progress of current surveys, and prospects for future surveys, presentation to the Survey/Detection Panel of the NRC Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies, January 28–30, 2009.
NRC (National Research Council). 2009. Near-Earth Object Surveys and Hazard Mitigation Strategies: Interim Report, The National Academies Press, Washington, D.C., available at http://www.nap.edu/catalog.php?record_id=12738, pp. 1-2.