Hearing Statement: "The Threat of Near-Earth Asteroids", Joseph Burns, Solar System Exploration Survey Committee

Status Report From: National Research Council
Posted: Thursday, October 3, 2002

Statement of Joseph A. Burns, Ph.D., Member of the Solar System Exploration Survey Committee, National Research Council/The National Academies


Irving Porter Church Professor of Engineering and Professor of Astronomy, Cornell University before the Subcommittee on Space and Aeronautics, Committee on Science, U.S. House of Representatives 


Mr. Chairman, Ranking Minority Member, and members of the subcommittee: thank you for inviting me to testify on behalf of the National Academies' Solar System Exploration Survey. My name is Joseph Burns, and I am Irving Porter Church Professor of Engineering and Professor of Astronomy at Cornell University. I appear today in my capacity as a steering group member of the Solar System Exploration (SSE) Survey, and as a former chair of the National Research Council's Committee on Planetary and Lunar Exploration (COMPLEX). I was also a member of the Astronomy & Astrophysics Survey's panel on Ultraviolet and Infrared Astronomy from Space.

As you know, the Astronomy and Astrophysics community has a long history of creating, through the National Research Council (NRC), decadal surveys of their field. These surveys lay out the community's research goals for the next decade, identify key questions that need to be answered, and propose new facilities with which to conduct this fundamental research.

In April 2001, NASA Associate Administrator for Space Science Edward Weiler asked the NRC to conduct a similar survey for planetary exploration. Our report, New Frontiers in the Solar System, is the result of that activity. The Solar System Exploration Survey was conducted by an ad hoc committee of the Space Studies Board (SSB), overseen by COMPLEX. This committee was comprised of some 50 scientists, drawn from a diverse set of institutions, research areas, and backgrounds; it also received input from more than 300 colleagues. The SSE Survey had four subpanels which focused on issues pertaining to different types of solar system bodies (Inner Planets, Giant Planets, Large Satellites, and Primitive Bodies) and received direct input from COMPLEX on Mars issues and from the Committee on the Origins and Evolution of Life on issues pertaining to Astrobiology.

New Frontiers in the Solar System (the Executive Summary is appended to this statement) recommends a scientific and exploration strategy for NASA's Office of Space Science that will both enable dramatic new discoveries in this decade and position the agency to continue to make such discoveries well into the future. Your invitation indicated that I should focus on the conclusions that the SSE Survey reached in the area of Near-Earth Objects (NEOs).

Near-Earth Objects

The SSE Survey's charge from NASA included a request to summarize the extent of our current understanding of the solar system. This task was delegated to the subpanels, which in the particular case of NEOs was handled by the Primitive Bodies Panel.

Scientifically, the history of impacts on the Earth is vital for understanding how the planet evolved and how life arose. For example, it has been suggested that a majority of the water on this planet was delivered by comet impacts. A better known example of the role of impacts is the Cretaceous-Tertiary event that led to global mass extinctions, including that of the dinosaurs. Another case is the 20 megaton (MT) equivalent-energy explosion that devastated 2000 square-kilometers of pine forest in the Siberian tundra in 1908. The SSE Survey identifies the exploration of the terrestrial space environment with regards to potential hazards as a new goal for the nation's solar system exploration enterprise.

Current surveys have identified an estimated 50 percent of NEOs that have a diameter of 1 kilometer or greater and approximately 10-15 percent of objects between 0.5 and 1 km. The vast majority of these latter objects have yet to be discovered, but a statistical analysis indicates a 1% probability of impact by a 300-m body in the next century. Such an object would deliver 1000 MT of energy, cause regional devastation, and (assuming an average of 10 people per square-kilometer on Earth) result in 100,000 fatalities. The damage caused by an impact near a city or into a coastal ocean would be orders of magnitude higher. As of a year ago, 340 objects larger than a kilometer had been catalogued as Potentially Hazardous Asteroids. In addition, the number of undiscovered comets with impact potential is large and unknown.

The Primitive Bodies panel went on to state:

"Important scientific goals are associated with the NEO populations, including their origin, fragmentation and dynamical histories, and compositions and differentiation. These and other scientific issues are also vital to the mitigation of the impact hazard (emphasis added), as methods of deflection of objects potentially on course for an impact with Earth are explored. Information especially relevant to hazard mitigation includes knowledge of the internal structures of near-Earth asteroids and comets, their degree of fracture and the presence of large core pieces, the fractal dimensions of their structures, and their degree of cohesion or friction."

While almost all of the SSE Survey's recommendations involved NASA flight missions, the Primitive Bodies subpanel recommended that ground-based telescopes be used to do a majority of the study of NEOs, supplemented by airborne and orbital telescopes.

A survey for NEOs demands an exacting observational strategy. To locate NEOs as small as 300 m requires a survey down to 24th magnitude (16 million times fainter than the feeblest stars that are visible to the naked eye). If images are to be taken every 10 sec to allow the sky to be studied often, the necessary capability is almost 100 times better than that of existing survey telescopes. NEOs spend only a fraction of each orbit in Earth's neighborhood, where they are most easily seen. Repeated observations over a decade would be required to explore the full volume of space populated by these objects. Such a survey would identify several hundred NEOs per night and obtain astrometric (positional) measurements on the much larger (and growing) number of NEOs that it had already discovered. Precise astrometry is needed to determine the orbital parameters of the NEOs and to assign a hazard assessment to each object. Astrometry at monthly intervals would ensure against losing track of these fast-moving objects in the months and years after discovery.

Large-aperture Synoptic Survey Telescope

In its most recent decadal survey, the Astronomy and Astrophysics community selected the proposed Large-aperture Synoptic Survey Telescope (LSST) as their third major ground-based priority. In addition, our SSE Survey chose LSST to be its top-ranked ground-based facility. Telescopes like HST and Keck peer at selected, very localized regions of the sky or study individual sources with high sensitivity. However, another type of telescope is needed to survey the entire sky relatively quickly, so that periodic maps can be constructed that will reveal not only the positions of target sources, but their time variability as well. The Large-aperture Synoptic Survey Telescope is a 6.5-m-effective-diameter, very wide field (~3 deg) telescope that will produce a digital map of the visible sky every week. For this type of survey observation, the LSST will be a hundred times more powerful than the Keck telescopes, the world's largest at present. Not only will LSST carry out an optical survey of the sky far deeper than any previous survey, but also –just as importantly-- it will also add the new dimension of time and thereby open up a new realm of discovery. By surveying the sky each month for over a decade, LSST would revolutionize our understanding of various topics in astronomy concerning objects whose brightnesses vary on time scales of days to years. NEOs, which drift across a largely unchanging sky, are easily identified. The LSST could locate 90 percent of all-near-Earth objects down to 300 m in size, enable computations of their orbits, and permit assessment of their threat to Earth. In addition, this facility could be used to discover and track objects in the Kuiper Belt, a largely unexplored, primordial component of our solar system. It would discover and monitor a wide variety of variable objects, such as the optical afterglows of gamma-ray bursts. In addition, it would find approximately 100,000 supernovae per year, and be useful for many other cosmological observations.

The detectors of choice for the temporal monitoring tasks would be thinned charge-coupled devices (CCDs); the requisite extrapolation from existing systems should constitute only a small technological risk. An infrared capability of a comparably wide field would be considerably more challenging but could evolve as the second phase of the telescope's operation. Instrumentation for LSST would be an ideal way to involve independent observatories with this basically public facility.

NASA/NSF Cooperation

Historically, the National Science Foundation (NSF) has built and operated ground-based telescopes, whereas NASA has done the same for space-based observatories. Although the Astronomy and Astrophysics Survey was noncommittal on who should build the LSST, the SSE Survey included a recommendation that NASA share equally with NSF in the telescope's construction and operations costs.

Such an arrangement has precedent. The SSE Survey noted that

"NASA continues to play a major role in supporting the use of Earth-based optical telescopes for planetary studies. It funds the complete operations of the IRTF (InfraRed Telescope Facility), a 3-m diameter telescope located on Hawaii's Mauna Kea. In return for access to 50 percent of the observing time for non-solar-system observations, the NSF supports the development of IRTF's instrumentation. This telescope has provided vital data in support of flight missions and will continue to do so. As another example, NASA currently buys one-sixth of the observing time on the privately operated Keck 10-m telescopes. This time was purchased to test interferometric techniques in support of future spaceflight missions such as SIM (Space Interferometry Mission) and TPF (Terrestrial Planet Finder)."

The solar system exploration community is concerned that the NSF is often unwilling to fund solar system research. This is particularly unfortunate given NSF's charter to support the best science and its leadership role in other aspects of ground-based astronomy.

The shared responsibility between NASA and the NSF that we recommend is also endorsed by the more general findings last year of the NRC's Committee on the Organization and Management of Research in Astronomy and Astrophysics (COMRAA), chaired by Norman Augustine. COMRAA's report recommended that NASA continue to "support critical ground-based facilities and scientifically enabling precursor and follow-up observations that are essential to the success of space missions." COMRAA also noted that in 1980 the NSF provided most of the research grants in astronomy and astrophysics, but today NASA is the major supporter of such research.

The roles of the agencies also affect the ability of scientists to conduct a census of Near-Earth Objects. The SSE Survey commented that:

"interestingly enough, NASA has no systematic survey-capability to discover the population distribution of the solar-system bodies. To do this, NASA relies on research grants to individual observers who must gain access to their own facilities. The large NEOs are being efficiently discovered using small telescopes for which NASA provides instrumentation funding, but all the other solar system populations—e.g., comets, Centaurs, satellites of the outer planets, and Kuiper Belt Objects—are being characterized almost entirely using non-NASA facilities. This is a major deficiency…"

The construction of the LSST would provide a central, federally sponsored location for such research.

LSST Costs and Survey Below 300 Meters

The costs of the LSST are projected by the 2001 Astronomy and Astrophysics Survey as being $83 million for capital construction and $42 million for data processing and distribution for 5 years of operation, for a total cost of $125 million. Routine operating costs, including a technical and support staff of 20 people, are estimated at approximately $3 million per year. The LSST will be able to routinely discover and characterize NEOs down to 300 m in diameter. Increasing the sensitivity of the survey to 100 m would mean increasing the sensitivity of the telescope by a factor of ten. This may represent a "beyond the state-of-the-art" challenge to telescope builder, and certainly a much larger telescope - 3 times the LSST and probably 10 to 100 times the cost unless innovative designs are found. The number of discovered objects would correspondingly increase substantially; this large data set may challenge current capabilities.

Concluding Thoughts

By way of summary, let me place the LSST into the context of a robust scientific program. Systematically building an inventory of the Near-Earth Objects is crucial to an improved understanding of Earth's environment, especially to the prediction of future hazards posed to our species. It is also a necessary first step towards a rational program of NASA's exploration of these bodies with spacecraft: many of the most interesting targets may remain, as yet, undiscovered. The ability to create and play a "motion picture" of the night sky will also provide new insights in a wide variety of disciplines from cosmology to astrophysics to solar system exploration. A suitable analog might be the deepened knowledge that is obtained from dynamic movies of swirling clouds and weather patterns, as compared to an occasional static photo.

The immense volume of data from the LSST would provide a reservoir of information for numerous graduate students and researchers, as well as established scientists. Further, LSST will support flight missions – for example, identifying possible fly-by targets for a spacecraft mission to explore the Kuiper Belt. All in all, the SSE Survey committee believes that broad areas of planetary science, particularly NEO studies, would benefit very substantially from the construction of the LSST for a relatively small investment.

Thank you again, Mr. Chairman, for the opportunity to appear before the subcommittee today. I would be glad to answer any questions that you or your subcommittee members may have.


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