From: Senate Committee on Commerce, Science, and Transportation
Posted: Wednesday, April 7, 2004
Given at a Senate Science, Technology, and Space Hearing: Near Earth Objects (NEO) Wednesday, April 7 2004 - 2:30 PM - SR-253
The Testimony of Dr. Wayne Van Citters, Division Director, Division of Astronomical Sciences, National Science Foundation
Chairman Brownback, Ranking Member Breaux, and distinguished members of the Subcommittee. Thank you for the opportunity to present the position of the National Science Foundation on the important subject of Near Earth Objects. In responding to the questions that the Committee has presented to us, I will present a picture of NSF's support of research into the nature and origin of these objects, as well as potential important contributions that NSF-supported instrumentation and techniques could make to an expanded discovery and characterization effort.
Background and Context
The Division of Astronomical Sciences supports basic research in astronomy covering a very wide range of subjects - from studies of objects in our own solar system to investigations of the beginning of the universe, including the very nature of matter and energy. In planning and conducting its programs, the Division benefits from the advice of the scientific community in many ways, including the recently established Astronomy and Astrophysics Advisory Committee (AAAC, jointly advising NSF, NASA, and DOE). The establishment of the AAAC recognizes the value of an integrated strategy to address national efforts to answer questions about our origins and our future. The number and nature of NEOs are clearly fundamental questions about both our origins and our future. In their March 15, 2004 report the AAAC recommended a coordinated implementation effort to ensure timely development of the Large Synoptic Survey Telescope, calling it a key facility for the detection of potentially hazardous earth-intersecting objects as small as 300 meters. Current Activity A number of awardees in our Planetary Astronomy Program are investigating Near Earth Objects (NEOs). The proposals funded by our program are determined by the interest of the research community, as reflected in the number and subject matter of proposals that we receive, and the results of our merit review of these proposals. As one example, Dr. Derek Richardson at the University of Maryland will be modeling the tidal disruption of near Earth asteroids (NEAs) by the Earth's gravitational field to determine the frequency of binary NEA formation and the typical characteristics of the resulting binary asteroids. The results from this research will give insight into the internal structure of NEAs and may have implications for hazard mitigation strategies. In another effort, Richard Binzel at MIT will measure the near-infrared spectral properties of 40-60 NEOs per year. The observations will balance measurements that push the state-of-the-art limits of the technology for the smallest and faintest objects and measurements that provide sufficient detail for detailed mineralogical analysis. Research in this area also represents a substantial fraction of the use of the Arecibo planetary radar system, characterizing sizes, shapes, rotation rates, and configurations (single or binary, e.g.). The smallest system yet observed (a binary of 120m and ~40 m diameter components) was discovered in 2003. Measurements from a combination of Arecibo and NASA's Goldstone antenna from 1991 through 2003 demonstrated the existence of the Yarkovsky effect. This effect is an acceleration of the body related to the time delay between the absorption of solar radiation and the re-emission in the infrared. The observations clearly indicated that the acceleration must be included in orbit predictions. We have observed that the number of proposals to investigate NEOs has been increasing annually for the last few years. Of the proposals we receive on this topic, those that do best in our merit review competition are those proposing to characterize the physical properties of the objects. What are they made of? How were they formed and when? I believe NSF is currently playing the role for which it is best suited. It is funding individual investigators to further our understanding of the physical make-up of NEOs. The proposals for these investigations are subject to our normal merit review, thus insuring high quality basic research on these objects. In addition, it provides access to tools such as Arecibo that can enhance the discovery process.
Looking to the Future In recent years, there has been an increasing appreciation for the hazards posed by near-Earth objects, those asteroids and periodic comets (both active and inactive) whose motions can bring them into the Earth's neighborhood. In August of 2002, our colleagues at NASA chartered a Science Definition Team to study the feasibility of extending the search for near-Earth objects to smaller limiting diameters. The formation of the team was motivated by the good progress being made toward achieving the Spaceguard goal of discovering 90% of all NEOs with diameters greater than 1 km by the end of 2008. This raised the question of what, if anything, should be done with respect to the much more numerous smaller, but still potentially dangerous, objects. The team was tasked with providing recommendations to NASA as well as the answers to seven specific questions. We believe that the answers to these questions could form a solid basis for the direction of our research efforts and for more detailed studies of the best integrated strategy to carry on at the end of Spaceguard in 2008. What are the smallest objects for which the search should be optimized? The Team recommends that the search system be constructed to produce a catalog that is 90% complete for potentially hazardous objects (PHOs) larger than 140 meters. Should comets be included in any way in the survey? The Team's analysis indicates that the frequency with which long-period comets (of any size) closely approach the Earth is roughly one-hundredth the frequency with which asteroids closely approach the Earth and that the fraction of the total risk represented by comets is approximately 1%. The relatively small risk fraction, combined with the difficulty of generating a catalog of comets, leads the Team to the conclusion that, at least for the next generation of NEO surveys, the limited resources available for near-Earth object searches would be better spent on finding and cataloging Earth-threatening, near-Earth asteroids and short-period comets. A NEO search system would naturally provide an advance warning of at least months for most threatening long-period comets. What is technically possible? Current technology offers asteroid detection and cataloging capabilities several orders of magnitude better than the presently operating systems. This report outlines a variety of search system examples, spanning a factor of about 100 in search discovery rate, all of which are possible using current technology. Some of these systems, when operated over a period of 7-20 years, would generate a catalog that is 90% complete for NEOs larger than 140 meters. How would the expanded search be done? From a cost/benefit point-of-view, the report concludes that there are a number of attractive options for executing an expanded search that would vastly reduce the risk posed by potentially hazardous object impacts. The Team identified a series of specific ground-based, space-based and mixed ground- and space-based systems that could accomplish the next generation search. The choice of specific systems would depend on the time allowed for the search and the resources available. What would it cost? For a search period no longer than 20 years, the Team identified several systems that they felt would eliminate, at varying rates, 90% of the risk for sub-kilometer NEOs, with costs they estimate to range between $236 million and $397 million for both ground and space components. They conclude that all of these systems have risk reduction benefits which greatly exceed the costs of system acquisition and operation. How long would the search take? The Team concludes that a period of 7-20 years is sufficient to generate a catalog 90% complete to 140-meter diameter, which will eliminate 90% of the risk for sub-kilometer NEOs. The specific interval would depend on the choice of search technology and the investment allocated. Is there a transition size above which one catalogs all the objects, and below which the design is simply to provide warning? The Team concluded that, given sufficient time and resources, a search system could be constructed to completely catalog hazardous objects with sizes down to the limit where air blasts would be expected (about 50 meters in diameter). Below this limit, there is relatively little direct damage caused by the object. Over the 7-20 year interval (starting in 2008) during which the next generation search would be undertaken, the Team suggests that cataloging is the preferred approach down to approximately the 140-meter diameter level and that the search systems would naturally provide an impact warning of 60-90% for objects as small as those capable of producing significant air blasts. The path from where we are today to where we should be in 2014 is not defined in the conclusions of the study that NASA sponsored. Clear goals are defined; how one might reach them is wisely left to the scientific and technical community. At the national level, we must now examine these goals in detail, validate the conclusions, and determine how they might best be achieved. NSF Plans for the Future We are considering asking the AAAC to form a subcommittee to advise on the effort that would be appropriate beyond Spaceguard. Broadly based in the scientific and technical community, this subcommittee would consider the conclusions of recent studies, extract necessary research directions that would help us better understand the origin and nature of the objects known to date and help to chart the most productive course into the future. By the very nature of the charge to the AAAC, this would be an integrated look at the ground-based and space-based efforts that would make the most effective scientific advances in this area. Of particular interest to NSF would be the expansion of the individual investigator-driven basic research that we currently support, and a more detailed understanding of how such projects as the Large Synoptic Survey Telescope (LSST) and Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) might best contribute to the discovery and characterization effort in the future.
The LSST is a proposed single 8.4meter aperture, very wide field telescope capable of surveying the entire sky visible from one hemisphere every two weeks. It has a variety of science drivers including the characterization of dark matter and dark energy, the discovery of many classes of transient objects such as supernovae and gamma-ray burst counterparts, and NEOs. Pan-STARRS, an Air Force funded project under construction in Hawaii, will be composed of 4 individual telescopes of 1.8meter aperture observing the same region of sky simultaneously. In survey mode, i.e., searching for NEOs, Pan-STARRS will cover 6,000 square degrees per night. The whole available sky as seen from Hawaii will be observed 3 times during the dark time in each lunation. The LSST's ability to make fast, wide, and faint observations may make it uniquely suited to detecting small NEOs. A model LSST survey covering 9,000 square degrees of sky along the ecliptic, three or four times a month, to a limiting V magnitude of 24.0, achieved a ten-year completeness of about 90% for NEOs larger than 250 m, and about 80% for NEOs down to 140 m as called for by the NASA study. The requirements placed on the telescope, telescope operations, data system and detectors by the NEO detection challenge are considerable. By reaching objects 100 times fainter than those currently observed in the NEO surveys, Pan-STARRS is being designed to help complete the Congressional mandate to find and determine orbits for the 1-km (and larger) threatening NEOs. Further, it should push the detection limit for a complete (99%) sample down to objects as small as 300-meters in diameter. Design studies over the next several years will be needed to determine the strategy for attacking the NEO problem and whether it is best carried out with a single telescope like the LSST or whether an array of smaller telescopes such as Pan-STARRS is more appropriate for this particular problem. NSF's Division of Astronomical Sciences has begun planning for such studies and we have been actively joined by our colleagues at NASA, who will contribute their knowledge and experience in the handling of large data bases and archives. Conclusion In conclusion, Mr. Chairman, NSF is already pursuing a significant amount of basic research in this important area. We are guided, as always, by the scientific community through our merit review process. We are laying plans for new facilities and expanded research activity that speak to many basic questions about the nature and origin of these objects, and are confident that the body of knowledge so gained will have important application to any eventual risk-mitigation effort. Again I thank you for the opportunity to appear and would be happy to respond to any questions.
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