[meteorite-list] NASA Releases Near-Earth Object Search Report

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:53 2004
Message-ID: <200309101539.IAA17704_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/neo/report.html

NASA Releases Near-Earth Object Search Report

NASA has released a technical report on potential future search efforts for
near-Earth objects after a year of analysis by scientists working on this
issue. This Science Definition Team was chartered to study what should be
done to find near-Earth objects less than 1 kilometer in size. While impacts
by these smaller objects would not be expected to cause global devastation,
impacts on land and the tsunamis resulting from ocean impacts could still
cause massive regional damage and still pose a significant long-term hazard.

In 1998 NASA commenced its part of the "Spaceguard" effort, with the goal of
discovering and tracking over 90% of the near-Earth objects larger than one
kilometer by the end of 2008. An Earth impact by one of these relatively
large objects would be expected to have global consequences and, over time
scales of a few million years, they present the greatest impact hazard to
Earth. Approximately 60% of the estimated 1,000 to 1,200 large near-Earth
objects have already been discovered, about 45% since NASA efforts started,
and each of the five NASA-supported search facilities continue to improve
their performance, so there has been good progress toward eliminating the
risk of any large, undetected impactor.

To understand the next steps to discovering the population of potentially
hazardous asteroids and comets whose orbits can bring them into the Earth's
neighborhood, NASA turned to this Science Definition Team of 12 scientists.
The Team, chaired by Dr. 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 near-Earth objects whose
diameters are less than one kilometer.

NASA considers the Science Definition Team's findings to be preliminary, and
a much more in-depth program definition, refining objectives and estimating
costs, would need to be conducted prior to any decision to continue
Spaceguard projects beyond the current effort to 2008.

This link will allow a download of the complete Science Definition Team
report (pdf format):

http://neo.jpl.nasa.gov/neo/report030825.pdf

and the Executive Summary of this report is below.

The Science Definition Team members include:

Dr. Grant H. Stokes (Chair) MIT Lincoln Laboratory
Dr. Donald K. Yeomans (Vice-Chair) Jet Propulsion Laboratory/Caltech
Dr. William F. Bottke, Jr. Southwest Research Institute
Dr. Steven R. Chesley Jet Propulsion Laboratory/Caltech
Jenifer B. Evans MIT Lincoln Laboratory
Dr. Robert E. Gold Johns Hopkins University, Applied
                                    Physics Laboratory
Dr. Alan W. Harris Space Science Institute
Dr. David Jewitt University of Hawaii
Col. T.S. Kelso USAF/AFSPC
Dr. Robert S. McMillan Spacewatch, University of Arizona
Dr. Timothy B. Spahr Smithsonian Astrophysical Observatory
Dr./Brig. Gen. S. Peter Worden USAF/SMC

Ex Officio Members
Dr. Thomas H. Morgan NASA Headquarters
Lt. Col. Lindley N. Johnson
(USAF, ret.) NASA Headquarters

Team Support
Don E. Avery NASA Langley Research Center
Sherry L. Pervan SAIC
Michael S. Copeland SAIC
Dr. Monica M. Doyle SAIC

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  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
               National Aeronautics and Space Administration
                          Office of Space Science
                     Solar System Exploration Division

           Full 166-page report available here as a PDF document:

              http://neo.jpl.nasa.gov/neo/neoreport030825.pdf

----------------------------------------------------------------------------

                              EXECUTIVE SUMMARY

 A Study to Determine the Feasibility of Extending the Search for Near-Earth
                   Objects to Smaller Limiting Diameters

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).

                   [Figure 9-3: Costs vs Risk Reduction]

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).

                 [Figure 9-4: Cost to Reduce Risk by 90%]

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.
Received on Wed 10 Sep 2003 11:39:59 AM PDT