Is there a transition size above which one catalogs all the objects, and below which the
design is simply to provide warning?
The Science Definition Team membership was composed of experts in the fields of asteroid
comet search, including the Principal Investigators of two major asteroid search efforts,
in orbital dynamics, NEO population estimation, ground-based and space-based
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
military technology or applications.
The Team approached the task using a cost/benefit methodology whereby the following
processes were completed:
Population estimation - An estimate of the population of near-Earth objects (NEOs),
their sizes, albedos and orbit distributions, was generated using the best methods in the
literature. We estimate a population of about 1100 near-Earth objects larger than 1 km,
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
hazardous population were estimated, including direct damage from land impact, as well as
amplification of damage caused by tsunami and global effects. The capture cross-section of
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
the threshold for global climatic effects is peaked at sizes on the scale of the Tunguska air
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
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
These systems include ground-based and space-based optical and infrared systems across
currently credible range of optics and detector sizes. Telescope apertures of 1, 2, 4, and 8
were considered for ground-based search systems along with space-based telescopes of 0.5,
and 2 meter apertures. Various geographic placements of ground-based systems were
were space-based telescopes in low-Earth orbit (LEO) and in solar obits at the Lagrange
beyond Earth and at a point that trailed the planet Venus.
Search simulation - A detailed simulation was conducted for each candidate search
for combinations of search systems working together, to determine the effectiveness of the
various approaches in cataloging members of the hazardous object population. The
were accomplished by using a NEO survey simulator derived from a heritage within the
which takes into account a broad range of "real-world" effects that affect the productivity
search systems, such as weather, sky brightness, zodiacal background, etc.
Search system cost - The cost of building and operating the search systems described
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
cost estimates by analogy with existing systems.
Cost/benefit analysis - The cost of constructing and operating potential survey systems
compared with the benefit of reducing the risk of an unanticipated object collision by
a catalog of potentially hazardous objects (PHOs). PHOs, a subset of the near-Earth
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
over time, the anticipated yearly average of impact-produced damage is significant. Thus,
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,
2008 associated with PHO impact is estimated by the Team to be approximately 300
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
tsunamis, and 30% to the risk of global climatic disruption caused by large impacts, i.e. the
that is expected to remain after the completion of the current Spaceguard effort in 2008. For
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
damage. According to the cost/benefit assessment done for this report, the benefits
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
the Team recommends that the next generation search system be constructed to eliminate
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
2008. The implementation of this recommendation will result in a substantial reduction in
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
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
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%
potentially hazardous objects (PHOs) larger than 140 meters.
Should comets be included in any way in the survey? The Team's analysis indicates that
frequency with which long-period comets (of any size) closely approach the Earth is
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
fraction, combined with the difficulty of generating a catalog of comets, leads the Team to
conclusion that, at least for the next generation of NEO surveys, the limited resources
for near-Earth object searches would be better spent on finding and cataloging Earth-
near-Earth asteroids and short-period comets. A NEO search system would naturally
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
performance is generally not driven by technology, but rather resources. This report outlines
variety of search system examples, spanning a factor of about 100 in search discovery rate,
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
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
posed by potentially hazardous object impacts. The Team identified a series of specific
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
and the resources available.
What would it cost? For a search period no longer than 20 years, the Team identified
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
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
complete to 140-meter diameter, which will eliminate 90% of the risk for sub-kilometer
The specific interval depends on the choice of search technology and the investment
Is there a transition size above which one catalogs all the objects, and below which the
is simply to provide warning? The Team concluded that, given sufficient time and
search system could be constructed to completely catalog hazardous objects with sizes
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
in 2008) during which the next generation search would be undertaken, the Team suggests
cataloging is the preferred approach down to approximately the 140-meter diameter level
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
should be stated explicitly in terms of the statistical risk eliminated (or characterized) and
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;
event rate is low. Thus, a search to determine if there are potentially hazardous objects
likely to impact the Earth within the next few hundred years is prudent. Such a search
executed in a way that eliminates the maximum amount of statistical risk per dollar of
Recommendation 2 - Develop and operate a NEO search program with the goal of
and cataloging the potentially hazardous population sufficiently well to eliminate 90% of the
due to sub-kilometer objects.
The above goal is sufficient to reduce the average casualty rate from about 300 per year to
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
search metric with quite good cost/benefit ratios.
Recommendation 3 - Release a NASA Announcement of Opportunity (AO) to allow
implementers to recommend a specific approach to satisfy the goal stated in
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
assess the expected capabilities of the various systems and approaches considered. While
Team considers the analysis results to be well-grounded by current operational experience,
thus, a reasonable estimate of expected performance, the Team did not conduct analysis at
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
Announcement of Opportunity (AO) as a vehicle for collecting search system estimates of
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.