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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies 3 Survey and Detection of Near-Earth Objects Congress has established for NASA two mandates addressing near-Earth object (NEO) detection. The first mandate, now known as the Spaceguard Survey, directed the agency to detect 90 percent of near-Earth objects 1 kilometer in diameter or greater by 2008. By 2009, the agency was close to meeting that goal. Although the estimate of this population is continually revised, as astronomers gather additional data about all NEOs (and asteroids and comets in general), these revisions are expected to remain. The 2009 discovery of asteroid 2009 HC82, a 2- to 3-kilometer-diameter NEO in a retrograde (“backwards”) orbit, is, however, a reminder that some NEOs 1 kilometer or greater in diameter remain undetected. The second mandate, the George E. Brown, Jr. Near-Earth Object Survey section of the NASA Authorization Act of 2005 (Public Law 109-155), directed that NASA detect 90 percent of near-Earth objects 140 meters in diameter or greater by 2020. However, what the surveys actually focus on is not all NEOs but the potentially hazardous NEOs. It is possible for an NEO to come close to Earth but never to intersect Earth’s orbit and therefore not be potentially hazardous. The surveys are primarily interested in the potentially hazardous NEOs, and that is the population that is the focus of this chapter. Significant new equipment (i.e., ground-based and/or space-based telescopes) will be required to achieve the latter mandate. The administration did not budget and Congress did not approve new funding for NASA to achieve this goal, and little progress on reaching it has been made during the past 5 years. The criteria for the assessment of the success of an NEO detection mandate rely heavily on estimates that could be in error, such as the size of the NEO population and the average reflectivity properties of an object’s surface. For many years, the average albedo (fraction of incident visible light reflected from an object’s surface) of NEOs was taken to be 0.11. More recent studies (Stuart and Binzel, 2004) determined that the average albedo was more than 25 percent higher, or 0.14, with significant variation in albedo present among the NEOs. The variation among albedos within the NEO population also contributes to the uncertainties in estimates of the expected hazardous NEO population. This difference implies that, on average, NEOs have diameters at least 10 percent smaller than previously thought, changing scientists’ understanding of the distribution of the NEO population by size. Ground-based telescopes have difficulty observing NEOs coming toward Earth from near the Sun’s direction because their close proximity to the Sun—as viewed from Earth—causes sunlight scattered by Earth’s atmosphere to be a problem and also poses risks to the telescopes when they point toward these directions. Objects remaining in those directions have orbits largely interior to Earth’s; the understanding of their number is as yet very uncertain. In addition, there are objects that remain too far from Earth to be detected almost all of the time. The latter include
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies Earth-approaching comets (comets with orbits that approach the Sun at distances less than 1.3 astronomical units [AU] and have periods less than 200 years), of which 151 are currently known. These represent a class of objects probably doomed to be perpetually only partly known, as they are not likely to be detected in advance of a close Earth encounter. These objects, after the completion of exhaustive searches for NEOs, could dominate the impact threat to humanity. Thus, assessing the completeness of the NEO surveys is subject to uncertainties: Some groups of NEOs are particularly difficult to detect. Asteroids and comets are continually lost from the NEO population because they impact the Sun or a planet, or because they are ejected from the solar system. Some asteroids have collisions that change their sizes or orbits. New objects are introduced into the NEO population from more distant reservoirs over hundreds of thousands to millions of years. The undiscovered NEOs could include large objects like 2009 HC82 as well as objects that will be discovered only months or less before Earth impact (“imminent impactors”). Hence, even though 85 percent of NEOs larger than 1 kilometer in diameter might already have been discovered, and eventually more than 90 percent of NEOs larger than 140 meters in diameter will be discovered, NEO surveys should nevertheless continue, because objects not yet discovered pose a statistical risk: Humanity must be constantly vigilant. Finding: Despite progress toward or completion of any survey of near-Earth objects, it is impossible to identify all of these objects because objects’ orbits can change, for example due to collisions. Recommendation: Once a near-Earth object survey has reached its mandated goal, the search for NEOs should not stop. Searching should continue to identify as many of the remaining objects and objects newly injected into the NEO population as possible, especially imminent impactors. THE SPACEGUARD EFFORT Recognizing that impacts from near-Earth objects represent a hazard to humanity, the United States, the European Union, Japan, and other countries cooperatively organized to identify, track, and study NEOs in an effort termed “Spaceguard.” From this organization, a nonprofit group named the Spaceguard Foundation was created to coordinate NEO detection and studies; it is currently located at the European Space Agency’s (ESA’s) Centre for Earth Observation (ESRIN) in Frascati, Italy. The United States input to this collective effort comprises three aspects: telescopic search efforts to find NEOs, the Minor Planet Center (MPC) at the Harvard-Smithsonian Center for Astrophysics, and the NASA NEO Program Office at the Jet Propulsion Laboratory. Existing, retired, and proposed telescopic systems for the U.S. NEO searches are detailed below. Other telescopic survey, detection, and characterization efforts are conducted worldwide and work synergistically with U.S. telescopic searches (e.g., Asiago-DLR Asteroid Survey, jointly operated by the University of Padua and the German Aerospace Center [DLR], Campo Imperatore Near-Earth Object Survey at Rome Observatory; and the Bisei Spaceguard Center of the Japanese Spaceguard Association). To date, the U.S. search effort has been the major contributor to the number of known NEOs. The functions of the two U.S. data- and information-gathering offices, the MPC and the NEO Program Office, are complementary. A European data- and information-gathering office, the Near-Earth Objects Dynamic Site (NEODyS) is maintained at the University of Pisa in Italy, with a mirror site at the University of Valladolid in Spain. These three services are described below. Minor Planet Center The MPC serves as the clearinghouse for positional information from the observers of minor planets (including all asteroids) from all observatories around the world. The MPC is charged with processing and publishing every single positional measurement made, worldwide, of asteroids, comets, and outer satellites of the Jovian planets. Its efforts are sanctioned by the International Astronomical Union (IAU), the international professional society for astronomers. The IAU provides guidance but currently only minor financial support for the MPC. Current MPC efforts are supported mostly by NASA’s Near-Earth Object Program, with a much smaller contribution from the Smithsonian Institution.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies As of December 2008, the MPC had a database of 59 million observations of more than 435,000 small bodies, with a second database of more than 10 million observations of objects having no or incomplete orbital information. The MPC receives daily observations of small bodies. The MPC first identifies new observations with known objects or determines that the object is new. All orbits of identified objects are updated and improved daily. Most, but not all, MPC processing is now automated. Observations of NEOs are made available to the public in less than 24 hours after they are acquired; comet observations can require up to a week to process and are largely not automated (Spahr, 2008). All incoming observations from NEO surveys are checked routinely for potential NEOs. This process is now automated: on the basis of its orbit, any new discovery is assigned a probability code of being an NEO. New possible NEOs are posted on the Web NEO Confirmation Page (NEOCP) in order to facilitate follow-up observations within minutes of posting. Updating of the NEOCP is 95 percent automatic; data and calculated orbits are publicly available. Recent upgrades to computer equipment allow the MPC to calculate tens of thousands of orbit improvements per day. Access has also been established to a 1,000+ node supercluster run by the Smithsonian Institution, and the MPC is purchasing nodes for this computer. The MPC is currently able to handle the large volumes of data expected in the near future from NEO discovery programs using larger telescopes. Near Earth Object Program Office The Near Earth Object Program Office operates at the Jet Propulsion Laboratory (JPL) for NASA; it is charged with coordinating the NEO observations program for NASA. This office is fully funded by NASA and maintains Web-accessible information about NEOs, including their close approaches to Earth as well as NEO discovery statistics. The NEO Program Office also maintains the automated Sentry software, a collision-monitoring system that continually scans the most current asteroid orbit data for objects that could hit Earth in the next 100 years. When a potential impactor is detected, its future orbit is calculated along with its uncertainty, and the results are published in the Sentry Risk Table on the NEO Program Office Web site. Near-Earth-Objects Dynamic Site The NEODyS maintains Web-accessible information about NEOs including orbits, an information database sorted by individual NEOs, and risk assessment of possible impact. The NEODyS is maintained at the University of Pisa, Italy, with a mirror image site at the University of Valladolid, Spain, to ensure that information is always accessible to users. PAST NEAR-EARTH-OBJECT DISCOVERY EFFORTS The survey and discovery effort for NEOs has advanced through several phases. Significant initial progress in the effort to identify the NEO population benefited greatly from the seminal efforts at many different telescope systems. The size of NEOs that can be detected is, however, related to the sizes of telescopes and their optics, cameras, and detection software, as well as to the observing strategy of the teams performing the searches. In recent years, some previous NEO survey programs have ended or are being phased out of operation because surveys more capable of finding smaller-diameter NEOs have become operational, and the emphasis on detection has shifted to objects with increasingly smaller diameters. These previous surveys, the Lowell Observatory Near-Earth-Object Search (LONEOS) and the Near-Earth Asteroid Tracking (NEAT) Program, are described below. Lowell Observatory Near-Earth-Object Search The LONEOS, operated by the Lowell Observatory, had the capability to scan the entire sky accessible from Flagstaff, Arizona, every month. The 0.6-meter-diameter telescope could record objects about 100,000 times
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies (12.5 magnitudes) fainter than can be seen with the naked eye. The project, funded by NASA, began in 1993 and was concluded at the end of February 2008. LONEOS discovered 288 NEOs. Near-Earth-Asteroid Tracking The Near-Earth Asteroid Tracking Program began in 1995 and was initially a collaborative effort among NASA, the Jet Propulsion Laboratory, and the U.S. Air Force. This program originally converted a Ground-based Electro-Optical Deep Space Survey (GEODSS) 1-meter-diameter telescope on Haleakala, Maui, Hawaii, to the world’s first fully automated asteroid-search telescope. Operations on the GEODSS telescope ended in 1999. In 2000, the NEAT program completed both the conversion of the Maui Space Surveillance System 1.2-meter-diameter telescope on Haleakala, and the conversion of the 1.2-meter-diameter Oschin telescope at Mount Palomar, California, to become fully automated and to search for NEOs. NEAT ceased operations in 2007 after detecting over approximately 20,000 objects, about 430 of which were NEOs. PRESENT NEAR-EARTH-OBJECT DISCOVERY EFFORTS In 2005, five NEO detection programs were operational: Catalina Sky Survey (CSS); the Lincoln Near-Earth Asteroid Research (LINEAR) program; and Spacewatch, as well as LONEOS and NEAT. Today, only CSS, the LINEAR program, and Spacewatch remain operational. These three NEO detection programs primarily address the congressional charge to detect 90 percent of NEOs down to 1 kilometer in diameter. Catalina Sky Survey Of the three search programs currently in operation, the CSS discovers NEOs at the highest rate. CSS is a system of three telescopes, located at the Mount Lemmon Observatory in Arizona, the Catalina Observatory also in Arizona, and the Siding Spring Observatory in Australia (all funded by NASA). The Mount Lemmon Observatory is the largest and most productive of these telescopes, having a 1.5-meter-diameter mirror and 1.2-square-degree field of view, enabling it to detect asteroids as faint as M = 22 (i.e., 22nd absolute magnitude in the visual band; see Appendix E). The Siding Spring facility has a 0.5-meter-diameter telescope for discovery. The Catalina Observatory houses the original CSS telescope, which has a 0.7-meter-diameter mirror. These telescopes work together to carry out sustained, highly productive searches for NEOs. Because two of these observatories are operating on the opposite side of Earth from the third, same-night follow-up on a newly discovered object can usually be accomplished, facilitating the rapid determination of its orbit and thus an evaluation of the hazard posed by the object. Indeed, this follow-up technique allowed the CSS to both discover the asteroid 2008 TC3, and to determine that it would impact the Sudan within 19 hours. In analyzing observations, the CSS employs a human operator who can spot faint moving objects that current versions of automated software may miss. The CSS has discovered more than 2,400 NEOs. Lincoln Near Earth Asteroid Research Program The LINEAR program at the Massachusetts Institute of Technology Lincoln Laboratory is funded by the U.S. Air Force and NASA and was the most successful NEO search program from 1997 until 2004. The goal of LINEAR is to demonstrate the application of technology originally developed for the surveillance of Earth-orbiting satellites to the discovering and cataloguing of NEOs. LINEAR consists of a pair of GEODSS telescopes at the Lincoln Laboratory’s Experimental Test Site at White Sands Missile Range in Socorro, New Mexico. These two 1-meter-diameter telescopes were eventually joined by a third telescope used for the confirmation of NEO orbits and were able to detect asteroids as faint as M = 20. LINEAR has discovered 2,210 NEOs and accounted for more than 50 percent of all NEO discoveries from 1998 to 2004. In 2005, the rate of discoveries by the Catalina Sky Survey increased substantially and overtook that of LINEAR.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies Spacewatch Spacewatch was one of the first NEO discovery systems, established in 1981 and run by the University of Arizona. Routine detections of asteroids and comets started in 1984 with a 0.9-meter-diameter telescope on Kitt Peak, Arizona, and a relatively small charge-coupled device (CCD) (see Appendix E) imaging array. Upgrades in 1989 enlarged the field of view and resulted in Spacewatch’s first detection of an NEO. Automated software to identify and discover NEOs was implemented in 1990; this was the first time that automated, real-time software was used for the detection of moving cosmic objects, and it proved the efficiency of such software. In 2001, a second telescope, 1.8 meters in diameter, was added to the program. The smaller Spacewatch telescope typically detects NEOs brighter than M = 21 over its field of view of 2.9 square degrees, whereas the larger telescope can potentially detect NEOs as faint as M = 23 over a field of view of 0.7 square degrees. The larger telescope is primarily used for recoveries of previously discovered, fainter NEOs, to confirm their orbits; the smaller telescope was used primarily for NEO discovery surveys. Spacewatch has discovered more than 700 NEOs. The Spacewatch program is anticipating the transition from conducting discovery observations to a recovery and characterization role as more powerful surveys come online. CURRENT SPACE-BASED DETECTION EFFORTS No nation has had or currently operates a space-based observatory dedicated to the discovery and/or characterization of NEOs. Space-based observatories are, however, planned for launch that will help to discover and/or characterize NEOs, especially because of the sensitivity of the observatories’ telescopes to infrared light, as explained below. Asteroids in orbits that bring them close to Earth are especially menacing if they are dark and have evaded detection by ground-based surveys in visible light. Also, since the assumed albedo might not be representative of a dark object, the calculated diameter could be misrepresented as smaller than the object’s true diameter. But dark objects are especially detectable in infrared light. The bias against lower-albedo (darker) asteroids is reduced through the use of infrared observations in space: At the temperatures and albedos that dominate the solar system inside the orbit of Mars, the diameters computed from infrared signals are more accurate than those derived from visible-light reflections from asteroids and comets. Thus, the detections of potentially hazardous NEOs by an infrared telescope (one sensitive to infrared light) will result in a more accurate size-frequency distribution for these objects. Additionally, the background from other astronomical sources is about 100 times lower at infrared wavelengths of 10 microns (a micron is one-millionth of a meter) than at visible wavelengths, since most stars emit far less infrared light than visible light. This difference reduces the chance for interference from other strong astronomical sources. Combined with visible-light data, the albedos of NEOs detected in the infrared can also be derived. This derivation of albedos offers insight into composition and surface properties. The Wide-field Infra-red Survey Explorer for Near-Earth Objects (NEOWISE), a U.S. mission (see below), will leverage this infrared advantage. Canada and Germany are both building spacecraft (see below) that could contribute to the discovery of NEOs, especially those whose orbits are partially or fully inside Earth’s orbit. These NEOs are less able to be observed by ground-based telescopes because they are so close to the Sun, as seen from Earth. Searching for NEOs from orbits in which spacecraft can be positioned to observe objects while the spacecraft is not pointed toward the Sun is an advantage for observing NEOs with orbits largely inside Earth’s orbit. Neither mission, however, will detect fainter or smaller objects than those detected by ground-based telescopes. Wide-field Infrared Survey Explorer for Near-Earth Objects (NEOWISE) The Wide-field Infrared Survey Explorer (WISE) is a NASA spacecraft mission launched in December 2009. WISE will produce a high-sensitivity imaging survey of the entire sky in four infrared wavelength bands centered at 3.3, 4.7, 12, and 23 microns. It will deliver a catalog of sources and a calibrated, position-registered image atlas. Using a cooled 0.4-meter-diameter aperture telescope and always looking 90 degrees from the Sun, WISE will
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies conduct an all-sky survey for 6 months. Imaging is obtained simultaneously in the four bands, and every location on the sky will be imaged at least 8 times. NASA has funded an enhancement to the baseline WISE mission, called NEOWISE, to facilitate solar system science. NEOWISE is expected to discover hundreds of new NEOs with sizes as small as about 100 meters in diameter. The advantage of this infrared-detected sample is that it will be inherently less biased against the discovery of low-albedo objects than optical surveys are, and, combined with ground-based visible observations of the same NEOs, it can be used to determine asteroid diameters with errors of only a few percent. Canada’s Near-Earth-Object Surveillance Satellite The Near-Earth-Object Surveillance Satellite (NEOSSat) is currently being constructed in Canada as a joint venture between the Canadian Space Agency (CSA) and Defense Research and Development Canada, an agency of the Canadian Department of National Defense. NEOSSat is based on a previous satellite, Microvariability and Oscillation of Stars, launched in 2003, which remains operational long after the completion of its initial mission. Set to launch in mid-2010, NEOSSat is scheduled to operate continuously for at least 1 year and should operate considerably longer. NEOSSat will conduct two simultaneous projects during its operational lifetime: High-Earth Orbit Surveillance System (HEOSS), which will monitor and track human-made satellites and orbital debris; and Near-Earth Space Surveillance (NESS), which will discover and track NEOs. NEOSSat will be the first satellite to be built on Canada’s Multi-Mission Microsatellite Bus; it will be roughly the size of a large suitcase, with a mass of approximately 75 kilograms. It will have a 15-centimeter-diameter mirror. This microsatellite will operate in a “Sun-synchronous” orbit at an altitude of approximately 700 kilometers. NEOSSat will be the first dedicated space platform designed to obtain observations on both human-made and natural objects in near-Earth space. The NESS project will focus primarily on discovering NEOs interior to Earth’s orbit. NEOSSat will expand the overall knowledge of potentially hazardous asteroids, monitor NEOs for cometary activity, perform follow-up tracking of newly discovered NEOs, and explore the synergies between ground-based and space-based facilities involved in NEO detection. Germany’s AsteroidFinder The German Aerospace Agency has selected AsteroidFinder as the first payload to be launched under its new national compact satellite program. Currently, the spacecraft is planned to be launched sometime in 2012, with a 1-year baseline-mission duration with the possibility of an extension; it is funded through the development stage. It will be equipped with a 30-centimeter-diameter telescope mirror. The satellite will operate in low-Earth orbit. Its primary goals are to estimate the population of NEOs interior to Earth’s orbit, their size distribution, and their orbital properties. AsteroidFinder will thus aid in the assessment of the impact hazard due to NEOs. ADDRESSING THE 140-METER REQUIREMENT: FUTURE GROUND- AND SPACE-BASED NEAR-EARTH-OBJECT DISCOVERY EFFORTS The NASA Authorization Act of 2005 ordered NASA to “plan, develop, and implement a Near-Earth Object Survey program to detect, track, catalogue, and characterize the physical characteristics of near-Earth objects equal to or greater than 140 meters in diameter in order to assess the threat of such near-Earth objects to Earth. It shall be the goal of the Survey program to achieve 90 percent completion of its near-Earth object catalogue (based on statistically predicted populations of near-Earth objects) within 15 years after the date of enactment of this Act.”1 1 National Aeronautics and Space Administration Authorization Act of 2005 (Public Law 109-155), January 4, 2005, Section 321, George E. Brown, Jr. Near-Earth Object Survey Act.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies The 140-meter-diameter requirement was based on the modeling presented in the 2003 NASA Science Definition Team near-Earth object study. An impacting object with a 140-meter-or-greater diameter, which could cause major regional destruction on Earth, occurs on average every approximately 30,000 years. To detect 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter, a telescope must be able to reach a limiting magnitude of M = 24. With the magnitude limitations discussed above, CSS, LINEAR, and Spacewatch are incapable of meeting the goal of discovering 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter or greater by 2020 or any later date. FUTURE TELESCOPE SYSTEMS FOR SURVEYS OF NEAR-EARTH OBJECTS The pursuit of NEOs as small as 140 meters in diameter requires that more advanced telescope systems be constructed and used to detect these objects. Required, for ground-based telescopes for example, are larger-diameter telescope mirrors to increase light-gathering power in order to observe smaller (therefore fainter at a given location) objects; imaging instruments with larger fields of view on the sky in order to maximize sky coverage for the surveys; more advanced observing strategies for optimizing NEO detection in the areas of the sky that are searched; faster operating detectors; and large data-storage capabilities. Because of the rate of motion of asteroids across the sky, exposures are limited to about 30 seconds. A telescope needs to be able to gather sufficient light from dim objects in that short time in order to achieve the goal—a smaller telescope using longer exposures to reach that magnitude will not suffice. Multiple smaller telescopes imaging the same field to make up the aperture will work, but smaller telescopes imaging fields nonsimultaneously will not. There are cost, schedule, and technical performance risks involved with the construction of any large-diameter mirror or large detector, although the risk for such ground-based telescopes is less than that for space-based telescopes. The new systems described below are examples of ones that could contribute significantly to the detection of NEOs that could impact Earth in the future. Such systems thus could support efforts required to meet the mandated goal. Large Synoptic Survey Telescope The Large Synoptic Survey Telescope (LSST) is a survey project under development, sponsored by a large consortium, centered around a telescope with an 8.4-meter-diameter mirror having a 9.6-square-degree field of view. This survey would scan the entire sky accessible from its planned location on El Pachon, a developed site in Chile. The survey plan is to scan the visible sky twice per night every 3 to 4 days in five visible and near-infrared wavelength bands. The LSST can reach a limiting magnitude of M = ~25.1 for detecting NEOs. The major science goals for LSST include cataloging and characterizing all classes of moving objects in the solar system, and hence identifying NEOs. By building a telescope with a wide field of view to cover the sky quickly, coupled with a large mirror to detect faint objects, the LSST expects to use the same images to fulfill most of its science goals. Each area of sky observed in one night will include two back-to-back 15-second image exposures, combined to become one 30-second exposure. The output of the survey will include very large multi-color, multi-epoch catalogs of asteroids and comets, with precisely calibrated sky location and brightness. Simulations of LSST operations (cf. Ivezič, 2009) show that typical NEOs will have hundreds of observations spaced across the lifespan of the survey (10 years under “normal” operations), and often more than 50 observations during 6 months, allowing for better characterization of the NEOs. The Moving Object Processing System (MOPS) developed for Panoramic Survey Telescope and Rapid Response System (PanSTARRS 1; see below) is also under further development by the LSST team, for use in detecting and determining orbits for all moving objects. All data produced by LSST will be publicly available. Within 60 seconds of acquisition of an image at the telescope, real-time data processing will identify moving sources (e.g., NEOs) and forward the data to MOPS. Images will then be transmitted to the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, for permanent storage and to multiple Data Access Centers, which are designed for public queries of the LSST data and include additional data-processing software.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies According to the LSST project, LSST will be capable of detecting 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter in about 17 years under normal (non-NEO-optimized or -dedicated) operations (cf. Chesley, 2008). The LSST project’s simulations using the LSST operations simulator and an NEO model supplied by PanSTARRS in MOPS (based on the Bottke et al.  NEO distribution) show that by optimizing operations for NEO detection, the required time could be reduced to about 12 years to detect 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter (Chesley, 2008). These optimizations include exposing for longer time intervals in the area of the sky within ±10 degrees of the plane of Earth’s orbit to observe fainter objects and detect NEOs at larger distances, limiting observations to only those three wavelength bands in which NEOs have the strongest signals, and adding observations targeted to locations at 60- to- 90-degree angles away from the Sun, and within 10 degrees of the plane of and inside Earth’s orbit, thus maximizing the observing of the surface of the NEO illuminated by the Sun. An LSST telescope dedicated solely to searching for NEOs could complete the survey in about 9 years of operation at much greater expense (see below). Design and development for LSST has been ongoing for more than 4 years, but construction funding is still pending. A total budget for construction and 10 years of operations of approximately $800 million are estimated by the project to be necessary for the basic LSST telescope (Ivezič, 2009). Several project management milestones have been passed, including a National Science Foundation (NSF) Conceptual Design Review. The mirror is being ground and polished (see Figure 3.1), and first science operations are hoped for in 2016. FIGURE 3.1 The Large Synoptic Survey Telescope 8.4-meter-diameter mirror after being successfully cast at the University of Arizona Mirror Laboratory. SOURCE: Courtesy of Howard Lester, LSST Corporation.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies Optimizing the system for NEO detections requires approximately 15 percent additional cost to compensate for extended observations specific to NEO detection but not useful to meet other goals. The LSST project estimates that $125 million of additional funding is required for this optimization (Chesley, 2008). Even if dedicated to the NEO issue and completed in 2015, LSST alone could not meet the 2020 deadline for detecting 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter. However, simulations show that the LSST could reach this goal before 2030, as indicated above. (Note: Not all NEOs will come in view in the southern sky, although most will eventually. The LSST observational strategy focuses on rapidly scanning the entire visible sky, including NEO “sweet spots” where many objects will pass at some point in their orbits.) Panoramic Survey Telescope and Rapid Response System 4 PanSTARRS 4 (PS4) is the planned development of the PanSTARRS survey project. The U.S. Air Force-funded PanSTARRS 1 (PS1), the prototype 1.8-meter-diameter mirror telescope with its 7-square-degree field of view and 1.4-billion-pixel camera. PS1 has been constructed and partially tested but has not yet started science operations. (See Figure 3.2.) The PS1’s major advance is its very-large-field-of-view camera and its sophisticated software for detecting moving objects—MOPS. The PS4 would take the completed PS1 and add three more (not yet built) identical or nearly identical telescopes, for a total of four 1.8-meter-diameter telescopes. All four telescopes pointing at the same area of sky and observing the same wavelength bands at the same time could then achieve limiting magnitude in its most sensitive band of 23.5, that is, approximately twice as sensitive as PS1. Major goals for PS4 include identifying and cataloging potentially hazardous NEOs, with follow-up to be done on other telescopes. The observing plan for PS4 is unavailable; however, if PS4 operates under the same observing schedule as those for PS1, exposures will range from 30 seconds to 60 seconds, covering a large portion of the visible sky twice per night every 5 to 10 nights in five wavelength bands. Observations would concentrate on the same areas of the sky that the LSST observations concentrate on (see above). Large numbers of observations of individual NEOs would potentially yield rates of rotation and optical surface properties for a substantial fraction of the NEOs. The MOPS developed for PS1 will be further developed for PS4 (as well as for LSST), to allow for the greater computational burden required by the ability to detect fainter objects. PS4 will produce a catalog of NEOs precisely calibrated in location and brightness. The NEO discoveries will be released to the public through the MPC. The PS1 prototype telescope is completed but is being reexamined owing to a problem with achieving its expected performance. A second telescope is currently in the initial phases of construction. For PS4, three telescopes similar to the prototype must be completed, as well as the housing structure for all four telescopes. PS2, that is, PS1 and the second telescope—will be located on Haleakala in Maui. The planned site for PS4 is Mauna Kea, Hawaii; PS2 would be moved to Mauna Kea. Additional clusters of telescopes could be added at other locations. The PS1 telescope was funded through the U.S. Air Force. Most of the original funding for PS4 has been spent building PS1. Funding for completion of PS4 has not been identified. PanSTARRS 4, even if completed and used on an “optimistic” schedule, could not alone meet the 2020 deadline, or any date, for detecting 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter. Catalina Sky Survey Binocular Telescopes The CSS University of Arizona’s team of astronomers proposes a series of three binocular telescopes fully dedicated to discovering NEOs (Beshore, 2009). The proposal is based on using six existing 1.8-meter-diameter primary telescope mirrors, an existing observing site and other equipment, commercially available off-the-shelf hardware and software components, and established detection methodologies. Two developed observatory sites are currently being considered for the location of the telescopes: San Pedro Martir, Mexico, and Mount Hopkins, Arizona. The six 1.8-meter-diameter mirrors composing the original Multiple Mirror Telescope’s (MMT’s) primary mirrors would be used. (See Figure 3.3.) Two mirrors would be placed in tandem to create one binocular telescope,
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.2 The PanSTARRS 1 telescope on Haleakala, Maui, Hawaii. SOURCE: Courtesy of Brett Simison, Institute for Astronomy, University of Hawaii. having an equivalent mirror diameter of 2.4 meters. The individual binocular telescope can detect objects to a limiting near-infrared magnitude of R = 22.6. Each binocular telescope could survey independently; images obtained simultaneously from any combination of these telescopes could be added together. Three binocular telescopes operated together would produce an equivalent mirror diameter of 4.2 meters and could detect objects to a limiting diameter of R = 23.2 (Beshore, 2009). A commercially available 100-million-pixel camera would be used in each telescope. The images acquired in one binocular telescope would cover 4 square degrees of the sky.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.3 The six 1.8-meter-diameter mirrors that until 1999 composed the primary mirrors in the (old) Multiple Mirror Telescope (MMT). These mirrors now in storage are proposed for use in the CSS+ (see text). SOURCE: Courtesy of the MMT Observatory. The CSS+ would have capability unique among the proposed NEO survey telescopes to acquire spectra of the sunlight reflected from a target NEO across the broad wavelength range of 0.4 to 2.4 microns. Small mirrors would be installed in the instrument attached to a binocular telescope that could switch between the instrument’s imaging mode to a pair of low-resolution spectrographs. The wavelength range would cover many absorption features caused by the presence of materials on the object’s surface, allowing the system to discern part of the surface composition of the object.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies On-site processing of data would take place initially, including the detection of moving objects and the calculation of their precise sky locations and brightness. The currently proposed coverage strategy includes obtaining three to four exposures of the same area of sky with binocular telescopes surveying independently in locations 45 to 90 degree angles away from the Sun, and a four-exposure search of locations 60 to 90 degrees away from the Sun. Follow-up observations would be conducted on the same night. For observations covering locations ≥20 degree latitudes from the plane of Earth’s orbit, two binocular telescopes would conduct independent four-exposure searches, with follow-up to be provided by a third telescope using two or three exposures. Three binocular telescopes would survey independently in Earth’s plane with observations repeated on the next night, allowing new discoveries to be made by correlation between observations on more than one night. Follow-up observations would be made on subsequent nights. Consistent with the existing CSS technique, an examination of images with the human eye would also be conducted. This technique has allowed the CSS to identify additional interesting objects. The detection of 2008 TC3 was partially due to identification by the human eye. The system would aim to discover and characterize NEOs in a fashion complementary to that of the LSST and PS4 systems. As a dedicated facility, it would also retain the choice to vary or adjust the survey strategy as needed during operations. The CSS+ is currently not funded. The six 1.8-meter-diameter mirrors and mirror cells are currently in storage at Mount Hopkins, Arizona, and the sites both at Mount Hopkins and at San Pedro Martir have power and support buildings in place. Assuming that site negotiations are completed and arrangements for the use of the mirrors is established before start-up, the project team estimates that the time required to complete one binocular combination is 28 months, with full operation of three telescope combinations in 40 months (Beshore, 2009). The resulting observations from this development have not yet been simulated by the NEO Program Office. However, CSS+ could not alone detect 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter, as its limiting magnitude is not sufficient to reach the faintest NEOs. Discovery Channel Telescope The 4.2-meter-diameter mirror Discovery Channel Telescope (DCT) is a collaborative effort between Lowell Observatory and Discovery Communications. The telescope is being constructed on a cinder cone at Happy Jack, Arizona, southeast of Flagstaff. (See Figure 3.4.) It is designed overall to contribute to multiple astronomical search projects, including searches for NEOs. The DCT’s camera is planned to have a 2.3-square-degree field of view. The nominal search method is designed to obtain four exposures per night on a specific area of sky. These exposures would be repeated on two additional nights per month, providing follow-up observations. NEO search observations would be conducted over a wide wavelength range. For detecting NEOs in one night, the limiting VR magnitude (a combination of V [visible] and R [near-infrared] magnitudes) is 23.8. Data from the focal plane would be delivered to control and reduction computers housed in the telescope building. This initial storage of the data will be handled by DCT. For the NEO search, data processing would be based on the methodology used by LONEOS. Data reduction would encompass two techniques. A traditional source-detection technique would be used, and data for all NEOs identified would be immediately reported to the MPC. A “frame-subtraction” technique based on existing Lowell Observatory routines would also be used. All NEOs discovered with the frame-subtraction technique would be made public immediately. All frames are to be archived at Lowell Observatory. Construction of the housing structure and the telescope mount for DCT was completed in fall of 2009. The primary mirror was constructed by Corning and will be coated by the Department of Optical Sciences, University of Arizona. First light (not requiring the camera) is expected in early 2011. Project estimates of the time required to build the camera are approximately 4 years. Schedule risk, construction risks, and technical risks are low for the overall project. The DCT telescope construction has been entirely privately funded through the Discovery Channel and private donors; however, the approximately $14.5 million for the camera is not yet funded. DCT is an outgrowth of the LONEOS NEO detection system (see above) run by the same astronomers at the Lowell Observatory. It is expected that DCT can contribute significantly to the NEO search, but it could not alone meet the 2020 deadline for detecting 90 percent of all potentially hazardous NEOs larger than 140 meters
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.4 The Discovery Channel Telescope under construction. SOURCE: Courtesy of Lowell Observatory. in diameter, as its limiting magnitude is not sufficient to reach the faintest NEOs. The DCT could, however, be a valuable follow-up asset for NEOs detected at other locations. Space-Based Detection Techniques The 2003 NASA NEO Science Definition Team Study concluded that an infrared space telescope is a powerful and efficient means of obtaining valuable and unique detection and characterization data on NEOs (Stokes et al., 2003). The thermal infrared, which denotes wavelengths of light from about 5 to 10 microns, is the most efficient color regime for an NEO search. An orbiting infrared telescope that detects these wavelengths and has a mirror between 0.5 and 1 meter in diameter is sufficient to satisfy the goal of detecting 90 percent of potentially hazardous NEOs 140 meters in diameter or greater. Also, locating an NEO-finding observatory internal to Earth’s orbit is preferable for identifying NEOs with orbits mostly or entirely inside Earth’s orbit. Specific advantages to space-based observations include the following: A space-based telescope can search for NEOs whose orbits are largely inside Earth’s orbit. These objects are difficult to find using a ground-based telescope, as observations risk interference from the Sun when pointing to the areas of the sky being searched; Thermal-infrared observations are immune to the bias affecting the detection of low-albedo objects in visible or near-infrared light, by observing the thermal signal from the full image of the NEO, providing more accurate albedo measurements (see the discussion above); Space-based searches can be conducted above Earth’s atmosphere, eliminating the need to calibrate the effects introduced by the atmosphere on the light from an NEO; and Observations can be made 24 hours a day. Two concepts for space-based infrared telescopes are discussed here, as illustrations of means to satisfy the congressional mandate to identify 90 percent of all potentially hazardous NEOs larger than 140 meters in diameter.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies 0.5-Meter-Diameter Infrared Space Telescope There is a Discovery-class mission proposal from the Jet Propulsion Laboratory, estimated by JPL to cost slightly under $500 million, designed to complete the George E. Brown, Jr. Near-Earth Object Survey (Mainzer, 2009). This is a proposal for a 0.5-meter-diameter infrared telescope that would be placed inside Earth’s orbit, on the Earth-Sun line at the so-called Earth-Sun L1 Lagrangian point (see Appendix E) to survey for NEOs. It would survey nearly continuously in the regions where NEOs are predicted to be orbiting the Sun (Chesley and Spahr, 2004). In its 5-year baseline mission, the telescope could discover about 75 percent of all NEOs larger than 140 meters in diameter; after 10 years, 90 percent completeness could be achieved (Chesley and Spahr, 2004). In combination with a suitable ground-based telescope or telescopes, these times to completion could be accelerated (see Figures 3.7 and 3.8 later in this chapter for examples in which the spacecraft is modeled as the 0.5-meter infrared telescope at the L1 Lagrangian point). Sixteen-million-pixel detectors covering a single infrared wavelength band spanning 6 to 10 microns would be used. The proposal draws its heritage from the very successful Spitzer Space Telescope and from WISE (Mainzer, 2009). NEO Survey Spacecraft The NEO Survey is a spacecraft mission proposal from Ball Aerospace and Technologies Corporation, estimated by Ball to cost about $600 million, designed to complete the George E. Brown, Jr. Near-Earth Object Survey (Reitsema, 2009). The NEO Survey would have a 0.5-meter-diameter infrared telescope in a Venus-trailing orbit. The NEO Survey design allows observations over slightly more than the entire anti-Sun hemisphere. It should complete its mission of detecting more than 90 percent of all potentially hazardous NEOs larger than 140 meters in slightly under 8 years. With the addition of a suitable ground-based telescope system (see Figures 3.7 and 3.8 below in this chapter, in which the NEO Survey is modeled as the 0.5-meter infrared telescope at Venus orbit; Chesley and Spahr, 2004), the NEO Survey could complete this mission in under 5 years of operations. The NEO Survey draws its heritage from Spitzer Space Telescope and Kepler (Reitsema, 2009). Figure 3.5 shows the basic concept of operations for the NEO Survey and illustrates the greatly expanded search region available from a Venus-like orbit compared to any Earth-based option. The depicted orbits are to scale, and the red ellipse is the nominal Venus-like orbit having a radius of 0.65 AU with an orbital period of approximately 206 days. The Venus-like orbit distinguishes the NEO Survey operations concept because it is the spacecraft’s orbit in general that is important, not the spacecraft’s location along the orbit. The results are not sensitive to the orbit’s final details as long as the final orbit falls within a distance from the Sun of between 0.8 AU and 0.6 AU. SURVEY AND DETECTION SCHEDULES Table 3.1 summarizes the relative merits of various possible survey techniques. Their performance and efficiency can be parameterized by means of a number of criteria, including the number of NEOs discovered, how fast the 90 percent goal is reached, the estimated development time, additional characterization information recovered, and general programmatic and technical risks. The first column of Table 3.1 describes the various projects, including the current Spaceguard systems (CSS and LINEAR) as well as planned or proposed projects in both visible and infrared wavelengths. Only those projects that either currently exist or have a “reasonable” probability of existing are included. Facilities that could only negligibly contribute to the survey goal (e.g., the Hubble Space Telescope or the James Webb Space Telescope) are not assessed here. The second column shows the number of years required for the various projects to reach 90 percent completeness for potentially hazardous objects larger than 140 meters in diameter. This time interval represents time doing the survey; development time is excluded. Programs that take in excess of two decades to reach 90 percent completeness are denoted by “N/A” in this column, as any program taking longer than two decades is deemed by the committee to be an unworkable solution. The third column describes the percentage completeness for NEOs larger than 140 meters in diameter at 10 years after start of the projects’ survey operations. The fourth column gives the projects’ own estimates of the development time: that is, the time from the start of the preliminary
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.5 The region of the sky observed by the NEO Survey (see text). NOTE: NEO, near-Earth object; AU, astronomical unit; IR, infrared; FOR, field of regard. SOURCE: Courtesy of Ball Aerospace and Technologies Corporation and NASA. design phase to the beginning of survey operations. For projects already under development, the time given is the estimated time remaining (from the date of this report) before survey operations could begin. The fifth column describes any ancillary characterizations enabled by the particular survey program, such as those discussed in Chapter 4 (g, r, i, Z, and Y refer to various filters used to view specially designated bands of wavelengths of light). The sixth column describes programmatic risks, if any; it also encapsulates the risk that projects whose primary purpose is not the search for NEOs might not, in fact, carry out the NEO survey over the lifetime of the project.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies TABLE 3.1 Comparison of Various Options for Achieving the Near-Earth Object (NEO) Survey Goals Project Years to 90% Completeness for 140 meter NEOsa Percentage Completeness for >140-meter NEOsb (%) Project’s Own Estimated Development Time (years)c Characterization Scienced Programmatic Risk Technical Risk DCT N/A 50 ? (camera only) VR Not fully funded; primary purpose not NEO discovery Technology development PS1 N/A 5–10 for 3.5 yrs 1 gri Fully funded Technology development PS4 N/A ~75 5 gri, light curve Not fully funded; primary purpose not NEO discovery Technology development LSST 17 12 81 90 (shared) 7 ugriZY, light curve Not fully funded; primary purpose not NEO discovery Technology development CSS N/A 8 N/A, already exists V None (completed) None CSS+ N/A ~75 3.3 0.3-3.2 µm spectra Not funded; primary purpose is NEO discovery and study Technology development LINEAR N/A 8 N/A, already exists VR None (completed) None 0.5-m IR at L1/L2 11 88 5 6-10 µm, IR light curve Not funded; primary purpose is NEO discovery and study 2% launch losse 2-m visible at L1/L2 16 83 6 VR visible light curve Not funded; primary purpose is NEO discovery and study 2% launch losse 0.5-m IR at Venus 7.5 95 5 6-10 µm, IR light curve Not funded; primary purpose is NEO discovery and study 2% launch losse 2-m visible at Venus 7 94 5 VR visible light curve Not funded; primary purpose is NEO discovery and study 2% launch losse 0.5-m IR at Venus; −2 bandpass 7.5 ~95 5 3–5.5, 6–10 µm, IR light curve Not funded; primary purpose is NEO discovery and study 2% launch losse Combined systems: 0.5-m IR at Venus + PS1 5.5 97 5f gri, 6–10 µm light curves in visible and IR Requires ground and space facilities to be funded and operated 2% launch losse Combined systems: 0.5-m IR at Venus + LSST 3–4 98 7g ugriZY, 6–10 µm light curves in visible and IR Requires ground and space facilities to be funded and operated 2% launch losse
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies NOTE: See discussion of this table in the text. DCT, Discovery Channel Telescope; CSS, Catalina Sky Survey; IR, infrared; L1, Lagrangian point 1; L2, Lagrangian point 2; LINEAR, Lincoln Near Earth Asteroid Research; LSST, Large Synoptic Survey Telescope; N/A, not applicable; PS1, PanSTARRS 1; PS4, PanSTARRS 4. a “N/A” if more than 20 years total. Estimates derived from earlier figures, except for CSS+ (Stephen Larson, personal communicaton). b For fixed start date, and fixed operations interval = 10 years. Estimates derived from earlier figures, except for CSS+ (Stephen Larson, personal communication). c Estimated by representatives of the individual projects. d The notations V, R, u, g, r, i, Z, and Y refer to the various filters that would be used with these telescopes. These observations could derive optical colors, albedos, spectra, IR color temperatures, etc., yielding information characterizing the NEOs. eTypical failure rate for Delta or Atlas. f Dominated by IR telescope development time. gSet by LSST development time. The seventh column captures any technical risks unique to a particular project, such as the risks associated with a launch vehicle; the descriptions given in this column are based on each project’s current predicted survey style. The numbers in several of the columns have intrinsic uncertainties since (1) many projects are in their planning stages and have not settled on an observing mode, and (2) there are still substantial uncertainties in the estimated number of NEOs larger than a given size. Figures 3.6 through 3.10 show the relative times to completion for various types of combined space-based and ground-based systems for the detection of NEOs with limiting diameters of 140, 50, and 30 meters. (The importance of the 50-meter and 30-meter objects is discussed later in this chapter.) These plots should be viewed as sliding scales, with the survey portion only beginning at the year 0 (i.e., programmatic and construction lead time is not included). These plots are based on the modeling and assumptions by Chesley included in the 2007 NASA study Near-Earth Object Survey and Detection—Analysis of Alternatives (NASA PA&E, 2007) unless otherwise noted in the figure caption. The completeness percentages are considered by Chesley to be accurate to ±2 percent for results near 90 percent completeness. The plots are made with an assumption of an average albedo for NEOs of 0.11. Thus, they represent a lower limit to the number of objects detected in those size ranges. They therefore could be used with more confidence for the relative differences of detection systems for a given condition. Finding: The mandated survey to locate 90 percent of near-Earth objects 140 meters or greater in diameter has not yet been funded by the federal government. Because the survey requires several years for budgeting and for the building of new equipment and then for conducting the search, completion by 2020 is not realistic. Figure 3.8 compares the estimated ability of the proposed largest ground-based telescope and ground- and space-based telescope combinations to complete the survey of NEOs. Including the developmental lead time required, a dedicated or shared LSST is the only ground-based system currently proposed that could complete the survey of 90 percent of the potentially hazardous 140-meter-diameter objects within 20 years of the start of observations. In contrast, the survey can be completed within 20 years including the estimated 5-year development period by infrared space-based options and visible space-based options in Venus-type orbit (Figure 3.7). Combinations of space-based infrared and ground-based telescopes can accelerate the completion of the survey (Figure 3.8). Extending the search to smaller-diameter objects (Figure 3.9) demonstrates that the ground-based LSST cannot reach a detection of 90 percent of the 50-meter and 30-meter populations within 30 years of beginning operations. Combining LSST with a 0.5-meter space-based infrared telescope (Figure 3.10) allows a detection of 90 percent of the potentially hazardous NEOs down to 50-meter-diameter NEOs but is still not adequate to detect 90 percent of those down to 30 meters in diameter in 30 years of operation. Detecting 90 percent of the smallest NEOs that might cause significant damage on impact is thus a very difficult task.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.6 Years to 90 percent completion for the detection of potentially hazardous near-Earth objects (NEOs) 140 meters in diameter or larger with various ground-based telescopes. NOTE: LSST, Large Synoptic Survey Telescope; PanSTARRS, Panoramic Survey Telescope and Rapid Response System. SOURCE: Courtesy of Steve Chesley, Jet Propulsion Laboratory. NEO-optimized LSST numbers courtesy of LSST project. FIGURE 3.7 Years to 90 percent completion for the detection of potentially hazardous near-Earth objects (NEOs) 140 meters in diameter or larger with various space-based telescopes. NOTE: IR, infrared; L1, Lagrangian point 1. SOURCE: Courtesy of Steve Chesley, Jet Propulsion Laboratory.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.8 Years to 90 percent completion of the congressionally mandated survey for the detection of potentially hazardous near-Earth objects (NEOs) 140 meters in diameter or larger for combinations of space-based 0.5-meter infrared (IR) telescopes and ground-based telescopes. NOTE: LSST, Large Synoptic Survey Telescope; PanSTARRS, Panoramic Survey Telescope and Rapid Response System; L1, Lagrangian point 1. SOURCE: Courtesy of Steve Chesley, Jet Propulsion Laboratory. FIGURE 3.9 Years to completion for a shared Large Synoptic Survey Telescope (LSST) for near-Earth objects (NEOs) with diameters greater than or equal to 30, 50, and 140 meters. The shared LSST achieves 90 percent completion of the survey for potentially hazardous 140-meter-or-greater-diameter NEOs within 10 years of start of operations. SOURCE: Courtesy of Steve Chesley, Jet Propulsion Laboratory.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies FIGURE 3.10 Years to completion for a 0.5-meter infrared telescope in a Venus-like orbit plus a dedicated Large Synoptic Survey Telescope (LSST) for near-Earth objects (NEOs) with diameters greater than or equal to 30, 50, and 140 meters. Note that 90 percent completion is never achieved within 30 years for NEOs with diameters down to 30 meters in diameter. SOURCE: Courtesy of Steve Chesley, Jet Propulsion Laboratory. For different size regimes, some overarching conclusions can be drawn: Ninety percent completeness for the detection of potentially hazardous NEOs 140 meters in diameter or larger—In theory, this goal could be achieved by 2020. Experience suggests, however, that the congressional goal cannot be met by 2020. Most options could complete this survey within 20 years, including those involving only ground-based telescopes. Ninety percent completeness for the detection of potentially hazardous NEOs 50 meters in diameter or larger—All space-based or combination space-based and ground-based options could complete this survey, although not all in 20 years. No currently planned ground-based-only option is able to complete this survey. Ninety percent completeness for the detection of potentially hazardous NEOs 30 meters in diameter or larger—No combination of telescope systems discussed above can complete this survey within 20 years, although significant progress could be made. Combined ground- and space-based surveys have a number of advantages. Such surveys discover more NEOs of all sizes, including a substantial number smaller than 140 meters in diameter. These combined surveys also provide more characterization data about the entire NEO population. With both infrared and visible data for most targets, it would be possible to obtain accurate diameter estimates for all objects, as well as measurements of their albedos and their surface and thermal properties. These high-value characterization data could help to guide mitigation campaign studies. Additionally, a dual survey provides much information on the population of objects smaller than 140 meters in diameter. 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
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies 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 on the basis of non-NEO programs. As noted above, neither Congress nor the administration has requested adequate funding to conduct the survey to identify ≥90 percent of the potentially hazardous NEOs by the year 2020. Multiple factors will drive the decision on how to approach this survey in the future. These include but are not limited to the perceived urgency for completing the survey of 140-meter-diameter NEOs as close to the original 2020 deadline as feasible and the availability of funds to provide for the successful completion of the survey. The combination of a space-based detection mission with a large ground-based telescope could complete the survey in the shortest time, that is, closest to the original 2020 deadline. A space-based mission alone could complete the survey only 2 to 4 years later than a survey conducted with both a space-based telescope and a large ground-based telescope. The cost of optimizing the LSST for NEO detection observations was estimated in 2007 to be an increment of approximately $125 million to the cost of the basic telescope system (Ivezič, 2009), becoming the most cost-effective means to complete the survey. (Note that the annual operating cost of a ground-based telescope is approximately 10 percent of the development and construction costs.) The completion date would be extended. The decision to extend this date requires the acceptance of the change in risk over that time. Low-Altitude Airburst NEOs: Advance Warning Increasing concern with the possibility of smaller NEOs resulting in low-altitude airbursts has led the committee to raise the question of the identification of hazardous NEOs that have a diameter smaller than 140 meters. The ability to detect objects having diameters of greater than 50 meters and greater than 30 meters was therefore also compared among the modeled telescope systems. Finding: It is highly probable that the next destructive NEO event will be an airburst from a smaller-than-50-meter object, not a crater-forming impact. 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. Imminent Impactors: NEOs on Final Approach to an Earth Impact With the discovery of NEO 2008 TC3, found within 19 hours of impact into the Sudan desert, the committee discussed the question of an increasing capability to detect imminent impactors on their final approach to Earth. Optimizing the detection of imminent impactors requires a different observing strategy than the approaches discussed above designed to discover hazardous NEOs with long lead times before impact. The existing CSS (which found 2008 TC3) is configured such that with a change in observing sequence, it could discover up to 50 percent of the imminent impactors (i.e., bodies smaller than 1 kilometer in diameter that could impact in hours or weeks). Likewise, the Discovery Channel Telescope could make a significant contribution toward identifying imminent impactors. Other types of systems designed specifically to detect such objects could be built but were not considered by the committee. The imminent impactors represent the next level of survey and detection efforts, as their discoveries contribute to gains in the knowledge of NEO properties and their prompt discovery would allow for civil defense measures to be instituted in a timely manner.
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Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies REFERENCES Beshore, E. 2009. Surveying for Near-Earth Objects with Small Binocular Telescopes. Presentation to the Survey/Detection Panel of the Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies, April 20–22, Tucson, Arizona. Bottke, W.F., A. Morbidelli, R. Jedicke, J. Petit, H.F. Levison, P. Michel, and T.S. Metcalfe. 2002. Debiased orbital and absolute magnitude distribution of the near-Earth objects. Icarus 156:399–433. Chesley, S. 2008. NEO Surveying with the Large Synoptic Survey Telescope. Presentation to the Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies, December 10, Washington, D.C. Chesley, S.R., and T.B. Spahr. 2004. Earth impactors: Orbital characteristics and warning times. Pp. 22–37 in Mitigation of Hazardous Comets and Asteroids (M.J.S. Belton et al., eds.). Cambridge University Press, Cambridge, Mass. Ivezič, Z. 2009. LSST’s NEO Survey Capabilities. Presentation to the Survey/Detection Panel of the Committee to Review Near–Earth Object Surveys and Hazard Mitigation Strategies, January 28–30, Washington, D.C. Mainzer, A. 2009. Space–Based Infrared NEO Observation Platforms. Presentation to the Survey/Detection Panel of the Committee to Review Near–Earth Object Surveys and Hazard Mitigation Strategies, January 28–30, Washington, D.C. NASA PA&E (NASA Office of Program Analysis and Evaluation). 2007. Near Earth Object Survey and Deflection Analysis of Alternatives. Report to Congress. March. NASA, Office of Program Analysis and Evaluation, Washington, D.C. Reitsema, H. 2009. The NEO Survey Concept. Presentation to the Survey/Detection Panel of the Committee to Review Near–Earth Object Surveys and Hazard Mitigation Strategies, April 20–22, Tucson, Arizona. Spahr, T., Minor Planets Center, Harvard–Smithsonian Center for Astrophysics. 2008. NEOs and the Minor Planet Center in the next generation of surveys. Presentation to the Committee to Review Near–Earth–Object Surveys and Hazard Mitigation Strategies, December 10, Washington, D.C. Stokes, G., D. Yeomans, W.F. Bottke, S. Chesley, J.B. Evans, R.E. Gold, A.W. Harris, D. Jewitt, T.S. Kelso, R. McMillian, T. Spahr, and S.P. Worden. 2003. A Study to Determine the Feasibility of Extending the Search for Near Earth Objects to Smaller Limiting Magnitudes. Report Prepared at the Request of NASA Headquarters Office of Space Science’s Solar System Exploration Division. Stuart, J.S., and R.P. Binzel. 2004. Bias–corrected population, size distribution, and impact hazard for the near–Earth objects. Icarus 170:295–311.