Subject: A summary of NASA report on sub-km NEO risk
From: Michael Strauss
Submitted: Thu, 4 Dec 2003 13:13:33 -0500
Message number: 180
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Hello all,
Subscribers to David Morrison's NEO newsletter will have already
seen this, but below please find a summary of the NASA report on the
risk from NEO's of size below a kilometer; it should be of interest to
many of you. Note the link below where you can download the full
report. Contact David directly (dmorrison@arc.nasa.gov) if you want
to subscribe to the newsletter.
Many thanks,
Michael Strauss
NEO News (12/01/03) Sub-km NEOs
This edition of NEO News is devoted to the report of the NASA Science
Definition Team (SDT), led by Grant Stokes, describing the situation
with respect to NEAs less than 1 km in diameter: both the impact
threat and proposals to deal with it. SDT members Don Yeomans and
Steve Chesley will be presenting many of these results to a
scientific audience at the American Geophysical Union Meeting in San
Francisco on December 12, in a session organized by David Morrison
and Alan Harris to mark ten years of progress on Spaceguard.
This NASA SDT was chartered to study what should be done to find
near-Earth objects less than 1 kilometer in size. While impacts by
these sub-km objects are not expected to cause global devastation,
impacts on land and the tsunamis resulting from ocean impacts could
still cause massive regional damage and pose a significant long-term
hazard. Following this introduction are the Executive Summary of the
SDT Report plus links to a website providing the entire 166-page
report with figures and references.
This report is a successor to two other influential NASA studies of
the NEO impact hazard. In 1992, the NASA Spaceguard Survey Report
(David Morrison, chair) first identified the magnitude of the impact
hazard and recommended a survey to find asteroids large enough to
threaten global disaster if they struck (approximately those larger
than 1 km in diameter). In 1995 a NASA follow-up report chaired by
Gene Shoemaker developed a more detailed survey strategy and
established the current Spaceguard goal to find 90% of the NEAs
larger than 1 km diameter within 10 years.
In 1998 NASA commenced its part of the Spaceguard effort, adopting
the goal of 90% of the NEAs >1 km by the end of 2008. Approximately
60% of the estimated 1,000 to 1,200 NEAs in this size range have
already been discovered.
To understand the next steps toward discovering the population of
potentially hazardous asteroids and comets whose orbits can bring
them into the Earth's neighborhood, NASA selected to this Science
Definition Team of 12 scientists. The SDT, chaired by Grant Stokes of
the MIT Lincoln Laboratory, was asked to study the feasibility of
extending the search effort to the far more numerous, perhaps
hundreds of thousands, of NEOs whose diameters are less than one
kilometer. (As an aside, just one scientist, Don Yeomans,
participated on all three of these NASA studies).
Among the new results in this SDT Report are (1) a conclusion that
comets account for less than 1% of the hazard from sub-km objects, so
that they can be neglected, (2) a focus on reducing or retiring the
risk as opposed to setting goals in terms of a percentage of NEAs to
be discovered; (3) an analysis of the tsunami hazard that suggests
the risk is less than had sometimes been asserted previously, but is
still significant, (4) a discussion of the roles of long-term
prediction through a survey vs. attempts to find asteroids shortly
before their impacts, and (5) a comparison of ground-based and
space-based survey strategies.
There are respects in which the SDT report differs from most previous
discussions, so that some of the conclusions might at first seem
contradictory. Most of the analysis in this report deals with
potentially hazardous objects (PHOs), which are a subset of the NEOs
or NEAs. Only about 20% of the NEAs are PHOs. Thus some numbers (on
population and discovery rate for example) appear to be only 1/5 of
those we usually see quoted, but they are in fact equivalent. The SDT
also chose to assume no improvements in the current survey system,
and they therefore conclude that the Spaceguard Survey will fall
short of meeting its goal of 90% of NEAs >1 km by the end of 2008. In
contrast, most past analyses have assumed continuing improvements.
Finally, the SDT made very conservative assumptions concerning the
operations of future ground-based surveys (by the proposed LSST, for
example), thus concluding that such telescopes will not be as capable
as others have suggested for finding the sub-km NEAs.
This detailed report with its careful quantitative hazard analysis
establishes the new standard for evaluating the impact hazard,
setting the stage for a broader policy discussion of the desirability
of carrying out a survey of sub-km NEAs.
David Morrison
=========================================
Science Definition Team members
Grant H. Stokes (Chair)
MIT Lincoln Laboratory
Donald K. Yeomans (Vice-Chair)
Jet Propulsion Laboratory/Caltech
William F. Bottke, Jr.
Southwest Research Institute
Steven R. Chesley
Jet Propulsion Laboratory/Caltech
Jenifer B. Evans
MIT Lincoln Laboratory
Robert E. Gold
Johns Hopkins University, Applied Physics Laboratory
Alan W. Harris
Space Science Institute
David Jewitt
University of Hawaii
Col. T.S. Kelso
USAF/AFSPC
Robert S. McMillan
Spacewatch, University of Arizona
B. Spahr
Smithsonian Astrophysical Observatory
Brig. Gen. S. Peter Worden
USAF/SMC
Thomas H. Morgan
NASA Headquarters
Lindley N. Johnson
NASA Headquarters
===================================================
STUDY TO DETERMINE THE FEASIBILITY OF EXTENDING THE SEARCH FOR
NEAR-EARTH OBJECTS TO SMALLER LIMITING DIAMETERS
Report of the Near-Earth Object Science Definition Team
August 22, 2003
Prepared at the Request of the NASA Office of Space Science,
Solar System Exploration Division
EXECUTIVE SUMMARY
In recent years, there has been an increasing appreciation for the
hazards posed by near-Earth objects (NEOs), those asteroids and
periodic comets (both active and inactive) whose motions can bring
them into the Earth's neighborhood. In August of 2002, 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 so-called Spaceguard goal of discovering 90% of
all near-Earth objects (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 the following 7
specific questions:
1. What are the smallest objects for which the search should be optimized?
2. Should comets be included in any way in the survey?
3. What is technically possible?
4. How would the expanded search be done?
5. What would it cost?
6. How long would the search take?
7. Is there a transition size above which one catalogs all the
objects, and below which the design is simply to provide warning?
Team Membership
The Science Definition Team membership was composed of experts in the
fields of asteroid and comet search, including the Principal
Investigators of two major asteroid search efforts, experts in
orbital dynamics, NEO population estimation, ground-based and
space-based astronomical optical systems and the manager of the NASA
NEO Program Office. In addition, the Department of Defense (DoD)
community provided members to explore potential synergy with military
technology or applications.
Analysis Process
The Team approached the task using a cost/benefit methodology whereby
the following analysis processes were completed:
Population estimation -- An estimate of the population of near-Earth
objects (NEOs), including their sizes, albedos and orbit
distributions, was generated using the best methods in the current
literature. We estimate a population of about 1100 near-Earth objects
larger than 1 km, leading to an impact frequency of about one in half
a million years. To the lower limit of an object's atmospheric
penetration (between 50 and 100 m diameter), we estimate about half a
million NEOs, with an impact frequency of about one in a thousand
years.
Collision hazard -- The damage and casualties resulting from a
collision with members of the hazardous population were estimated,
including direct damage from land impact, as well as the
amplification of damage caused by tsunami and global effects. The
capture cross-section of the Earth was then used to estimate a
collision rate and thus a yearly average hazard from NEO collisions
as a function of their diameter. We find that damage from smaller
land impacts below the threshold for global climatic effects is
peaked at sizes on the scale of the Tunguska air blast event of 1908
(50-100 m diameter). For the local damage due to ocean impacts (and
the associated tsunami), the damage reaches a maximum for impacts
from objects at about 200 m in diameter; smaller ones do not reach
the surface at cosmic speed and energy.
Search technology -- Broad ranges of technology and search systems
were evaluated to determine their effectiveness when used to search
large areas of the sky for hazardous objects. These systems include
ground-based and space-based optical and infrared systems across the
currently credible range of optics and detector sizes. Telescope
apertures of 1, 2, 4, and 8 meters were considered for ground-based
search systems along with space-based telescopes of 0.5, 1, and 2
meter apertures. Various geographic placements of ground-based
systems were studied as were space-based telescopes in low-Earth
orbit (LEO) and in solar obits at the Lagrange point beyond Earth and
at a point that trailed the planet Venus.
Search simulation -- A detailed simulation was conducted for each
candidate search system, and for combinations of search systems
working together, to determine the effectiveness of the various
approaches in cataloging members of the hazardous object population.
The simulations were accomplished by using a NEO survey simulator
derived from a heritage within the DoD, which takes into account a
broad range of "real-world" effects that affect the productivity of
search systems, such as weather, sky brightness, zodiacal background,
etc.
Search system cost -- The cost of building and operating the search
systems described herein was estimated by a cost team from SAIC. The
cost team employed existing and accepted NASA models to develop the
costs for space-based systems. They developed the ground-based system
cost estimates by analogy with existing systems.
Cost/benefit analysis -- The cost of constructing and operating
potential survey systems was compared with the benefit of reducing
the risk of an unanticipated object collision by generating a catalog
of potentially hazardous objects (PHOs). PHOs, a subset of the
near-Earth objects, closely approach Earth's orbit to within 0.05 AU
(7.5 million kilometers). PHO collisions capable of causing damage
occur infrequently, but the threat is large enough that, when
averaged over time, the anticipated yearly average of impact-produced
damage is significant. Thus, while developing a catalog of all the
potentially hazardous objects does not actually eliminate the hazard
of impact, it does provide a clear risk reduction benefit by
providing awareness of potential short- and long-term threats. The
nominal yearly average remaining, or residual, risk in 2008
associated with PHO impact is estimated by the Team to be
approximately 300 casualties worldwide, plus the attendant property
damage and destruction. About 17% of the risk is attributed to
regional damage from smaller land impacts, 53% to water impacts and
the ensuing tsunamis, and 30% to the risk of global climatic
disruption caused by large impacts, i.e. the risk that is expected to
remain after the completion of the current Spaceguard effort in 2008.
For land impacts and all impacts causing global effects, the
consequences are in terms of casualties, whereas for sub-kilometer
PHOs causing tsunamis, the "casualties" are a proxy for property
damage. According to the cost/benefit assessment done for this
report, the benefits associated with eliminating these risks justify
substantial investment in PHO search systems.
PHO Search Goals and Feasibility
The Team evaluated the capability and performance of a large number
of ground-based and space-based sensor systems in the context of the
cost/benefit analysis. Based on this analysis, the Team recommends
that the next generation search system be constructed to eliminate
90% of the risk posed by collisions with sub-kilometer diameter PHOs.
Such a system would also eliminate essentially all of the global risk
remaining after the Spaceguard efforts are complete in 2008. The
implementation of this recommendation will result in a substantial
reduction in risk to a total of less than 30 casualties per year plus
attendant property damage and destruction. A number of search system
approaches identified by the Team could be employed to reach this
recommended goal, all of which have highly favorable cost/benefit
characteristics. The final choice of sensors will depend on factors
such as the time allotted to accomplish the search and the available
investment (see Figures 9.3 and 9.4).
Answers to Questions Stated in Team Charter
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. NEO search performance
is generally not driven by technology, but rather resources. 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 (see Figure 9-4).
How would the expanded search be done? From a cost/benefit
point-of-view, 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 groundbased, space-based and mixed ground- and space-based
systems that could accomplish the next generation search. The choice
of specific systems will 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 would eliminate, at varying
rates, 90% of the risk for sub-kilometer NEOs, with costs ranging
between $236 million and $397 million. All of these systems have risk
reduction benefits which greatly exceed the costs of system
acquisition and operation.
How long would the search take? 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 depends 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.
Science Definition Team Recommendations
The Team makes three specific recommendations to NASA as a result of
the analysis effort:
Recommendation 1 -- Future goals related to searching for potential
Earth-impacting objects should be stated explicitly in terms of the
statistical risk eliminated (or characterized) and should be firmly
based on cost/benefit analyses.
This recommendation recognizes that searching for potential Earth
impacting objects is of interest primarily to eliminate the
statistical risk associated with the hazard of impacts. The "average"
rate of destruction due to impacts is large enough to be of great
concern; however, the event rate is low. Thus, a search to determine
if there are potentially hazardous objects (PHOs) likely to impact
the Earth within the next few hundred years is prudent. Such a search
should be executed in a way that eliminates the maximum amount of
statistical risk per dollar of investment.
Recommendation 2 -- Develop and operate a NEO search program with the
goal of discovering and cataloging the potentially hazardous
population sufficiently well to eliminate 90% of the risk due to
sub-kilometer objects.
The above goal is sufficient to reduce the average casualty rate from
about 300 per year to less than 30 per year. Any such search would
find essentially all of the larger objects remaining undiscovered
after 2008, thus eliminating the global risk from these larger
objects. Over a period of 7-20 years, there are a number of system
approaches that are capable of meeting this search metric with quite
good cost/benefit ratios.
Recommendation 3 -- Release a NASA Announcement of Opportunity (AO)
to allow system implementers to recommend a specific approach to
satisfy the goal stated in Recommendation 2. Based upon our
analysis, the Team is convinced that there are a number of credible,
current technology/system approaches that can satisfy the goal stated
in Recommendation 2. The various approaches will have different
characteristics with respect to the expense and time required to meet
the goal. The Team relied on engineering judgment and system
simulations to assess the expected capabilities of the various
systems and approaches considered. While the Team considers the
analysis results to be well-grounded by current operational
experience, and thus, a reasonable estimate of expected performance,
the Team did not conduct analysis at the detailed system design level
for any of the systems considered. The next natural step in the
process of considering a follow-on to the current Spaceguard program
would be to issue a NASA Announcement of Opportunity (AO) as a
vehicle for collecting search system estimates of cost, schedule and
the most effective approaches for satisfying the recommended goal.
The AO should be specific with respect to NASA's position on the
trade between cost and time to completion of the goal.
================================
Full 166-page report available here as a PDF document:
http://neo.jpl.nasa.gov/neo/neoreport030825.pdf
--
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NEO News (now in its tenth year of distribution) is an informal
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(NEOs) and their impacts. These opinions are the responsibility of
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