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.