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SDT-1 GHS 6/8/2017
Science Definition Team
Grant Stokes
Chairman
June 2017
Update to Determine the Feasibility of Enhancing the Search and Characterization of NEOs
SDT-2 GHS 6/8/2017
Outline – Charter provided questions and approach – Members – Study approach
Population and baseline Collision risk assessment Search systems – capabilities and costs Search system effectiveness Cost-benefit analysis
– Answers to specific questions – Findings
SDT-3 GHS 6/8/2017
Specific Study 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?
There continues to be increasing interest by the public and by the Congress, highlighted by the Chelyabinsk Event in February 2013, as to the viability of extending the current effort to objects smaller than 140 meters. The following study is being undertaken to determine:
SDT-4 GHS 6/8/2017
Charter Provided Approach
• Science definition team (SDT) composed of 10 to 12 scientists and engineers
– Currently leading NEO search teams – Model the NEO population – Technical expertise in the design and operation of ground-based and
space-based survey telescopes, including the areas of visible and IR large detector arrays
• Address the questions included and provide a non-advocate technical report to the director of the PDCO
• Applied Physics Laboratory provided logistical and technical support
• Interagency Working Group established by OSTP will be informed as to the progress and product of the SDT
• Duration of the study is expected to be 9 months
SDT-5 GHS 6/8/2017
Team Members Chair: Grant Stokes * MIT LL Members: Brent Barbee NASA Goddard William Bottke * SWRI Marc Buie SWRI Steve Chesley * NASA JPL Paul Chodas NASA JPL Jenifer Evans * MIT LL Robert Gold * JHU APL Tommy Grav PSI Alan Harris * SSI Robert Jedicke U of Hawaii Amy Mainzer NASA JPL Donovan Mathias NASA Ames Tim Spahr * NEO Sciences Lorien Wheeler CSRA/NASA Ames Donald Yeomans * NASA JPL (ret) Ex Officio: Lindley Johnson * NASA HQ Kelly Fast NASA HQ Michael Kelley NASA HQ
Study Support: Jane Daneu MIT LL Cheryl Reed JHU APL Dorothy Ryan MIT LL Erik Syrstad SDL Lawrence Wolfarth JHU APL * Served on 2003 SDT
SDT-6 GHS 6/8/2017
Study Process
Population Estimate
Object Risk
Search Technology
Search Strategy
Risk = f(type, size, orbit, ∆t to
potential impact)
Search approach and capability
= f(object type, size)
Cost-Benefit
Findings and Answers
Quantitative, cost-benefit based analysis: Elimination of statistical risk in return for investment
risk an unknown hazard Estimate
System Costs
SDT-7 GHS 6/8/2017
Progress Since 2003 SDT
• NEO population better understood – ~15 years of searching and cataloguing since 2003 report
• Albedos of NEOs have been measured – Provides a better understood connection between measured
brightness (H) and “diameter” • Sophistication of impact damage modeling greatly increased • Survey technology improved
– Especially to provide space-based IR capabilities • Survey simulation updated to include space-based IR systems • Benefits estimation process updated and now includes statistical
value of injury • Current NASA cost estimation process used
SDT-8 GHS 6/8/2017
Determining PHA Population
• NEO population model developed using results from NEO surveys
– The population has a “wavy” shape like its primary source population the main asteroid belt.
– We estimate that cumulative number N (D > 1 km) = N (H < 17.75) = 934.
• Potentially hazardous asteroids (PHAs)
– PHAs are a subset of NEOs of interest to study
Pass close enough to Earth’s orbit (< 0.05 AU) to present a potential near-term danger
– PHAs constitute ~20% of NEO population
– Can’t be differentiated until orbit is known
Estimate of the NEO Population
100
102
104
106
108
1010
910111213141516171819202122232425262728293031
10-1 102 105 108
100
102
104
106
108
0.01 0.1 1 10
Brown et al. 2002Constant power lawBolide flux 1994-2013Infrasound bolide fluxPopulation estimate, 2014Discovered to Jan 2016Population estimate, 2016Population 2016, re-bin
Chel
ya.
Chicx
ulu b
Tung
uska
Absolute Magnitude, H
Diameter, Km
N (< H
) o
r N
(>D )
Impa
c t In
t er v
al, y
ears
Impact Energy, MT
SDT-9 GHS 6/8/2017
Determining NEO Orbits and Sizes
• Primary source regions of NEOs identified, e.g. Main belt, High inc families, JFCs (Granvik et al. 2016)
• Compute the model distribution of NEOs: – Numerical integration codes used to
track dynamical evolution of NEOs From source regions to observed orbits.
– Created model of observed NEO orbits/sizes from Catalina Sky Survey
Model contains several free parameters.
– Solve for free parameters Use results from NEO surveys with well-
understood observational biases.
NEO Orbital Distribution
Earth-crossing region
SDT-10 GHS 6/8/2017
Asteroid Population Estimate
H = 17.75 D = 1 km 100
102
104
106
108
91113151719212325272931
0.01 0.1 1 10
H, bottom scaleD, top scale
HN(
<H) o
r N(>
D)
Diameter, km
• Albedo distribution of NEOs generated for our synthetic population – It was derived from 415 NEOs detected by NEOWISE (Wright et al. 2016) – We assume albedo distribution is constant across diameter space.
• The H and D distributions compare favorably to one another.
SDT-11 GHS 6/8/2017
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
141618202224262830H
Cumulative Completeness
2003 Population
2016 Population
2023 Model
Population Results Summary
• Best current data used to generate population/orbit estimate – Current and 2023 catalog created through search simulation process
• Focus on PHA population only – Different from previous community focus on all NEAs – PHAs represent the collision danger – the rest are chaff
H<=17.75 D =>1km
H<=22 D =>140m
SDT-12 GHS 6/8/2017
Probabilistic Asteroid Impact Risk Model
SDT-13 GHS 6/8/2017
Differential Casualty Estimates
• Average (total) casualty estimates plotted for each size bin for total PHA population
• Vertical bars represent one standard deviation uncertainty bounds – Bars that extend to bottom of plot indicate that zero casualty
results fall within one standard deviation of the mean
Small irons impact populated areas
Larger objects penetrate deeper into atmosphere and cause larger damage regions
Largest objects result in global effects. The fall off with size > 2km occurs because the impact frequency decreases with size while damage per strike remains constant
SDT-14 GHS 6/8/2017
Expected Casualties from Asteroid Impacts
Cumulative average annual casualties, by hazard, for the total PHA population.
Global effects from large impacts dominate the risk
Local effects drive the risk for impactors smaller than 500 m.
Tsunami effects can be important for specific scenarios, but on average contribute ~10x less risk than local effects.
SDT-15 GHS 6/8/2017
Primarily sub-global risk Primarily global risk
Cumulative Expected Casualties
• Cumulative expected casualties per year – Total PHA population – Assuming current survey discovery rate up to 2023 – At point where 90% of the sub-global risk uncertainty has been
reduced • In 2023, largest risk uncertainty
reduction associated with large objects
• At 90% completion – Additional large-object risk
uncertainty reduced – Largest uncertainty reduction
occurs in the “hundreds of meter” size range
SDT-16 GHS 6/8/2017
Risk Results Summary
• Total nominal risk from PHA impact ~ 2500 casualties/year – Dominated by global effects of large objects
• Risk associated with undiscovered PHAs (in 2023) ~ 180 casualties/year – 10 casualties/year for land impact – <1 casualties/year for water impact – 170 casualties/year for remaining global effects
• At 90% of 140 m survey completeness, undiscovered objects pose risk ~ 80 casualties/year
– Risk dominated by the small chance of still undiscovered objects in the 500m-2km size range
– Local and tsunami damage combine for ~ 2 casualties/year
• Long-period comet risk is small by comparison to remaining asteroid risk
– ~10 casualties/year – Short-period comets modeled in asteroid population – Analysis from 2003 STD used – few updates available
SDT-17 GHS 6/8/2017
Search Technology Approach
• Identify range of potential sensors – Spectral range
Visible for ground-based systems Visible and IR considered for space-based systems
– Aperture size – Field of view – Location - space/ground
• Develop capability estimates for each sensor – Modeled sensitivity = f(integration time) – Search rate – Estimate of “real-world” degradations not already in model
• Provide inputs to search strategy sub-team – Iterate and trim sensor suite
• Develop cost model inputs for refined set of sensors
SDT-18 GHS 6/8/2017
Search Rates for Ground-based and Space-based Systems
• System search adjusted by duty cycle and atmospheric effects – Accounts for seeing, atmospheric losses, sky brightness
10
100
1000
10000
18 20 22 24 26
Sear
ch R
ate
(squ
are
deg/
hr)
SNR5 Limiting Magnitude (V)
2m Visible Space1m Visible Space0.5m Visible Space8m GBO4m GBO2m GBO
Matched to seeing for 1 deg/day object Matched to pixel size for 1 deg/day object
10
100
1000
10000
020406080100120140160
SNR 5 Sensitivity (uJy)
0.5m1m
IR Systems Visible Systems
Operation point
Sear
ch R
ate
(squ
are
deg/
hr)
SDT-19 GHS 6/8/2017
Search Technology Results Summary
• Examples of “realizable”, as opposed to “optimized”, sensors chosen for analysis
– Ground-based telescopes in 2 - 8 meter range – Space-based telescopes in 0.5 - 2 meter range visible,
0.5m – 1m range IR
• Realistic performance and cost estimated for each sensor – Costs include 10-year operations rollup
SDT-20 GHS 6/8/2017
Outline – Charter provided questions and approach – Members – Study approach
Population and baseline Collision risk assessment Search systems – capabilities and costs Search system effectiveness Cost-benefit analysis
– Answers to specific questions – Findings
SDT-21 GHS 6/8/2017
Inputs
Simulation for Performance Assessment
• Sensors: Ground and space
network
Individual properties
Individual search strategies
• PHAs: Orbital elements
Size
Albedo
Outputs • PHA Detections:
Object number
Sensor
Location of PHA
Time
Simulation
Space Based Visible
Ground Based Optical
Space Based
Infrared
• Fast Resident Object Surveillance Simulation Tool (GBO and SBV) • Survey Simulation Tool (SBIR)
SDT-22 GHS 6/8/2017
Space-based Systems
• Space systems characteristics – ~100% duty cycle
More productive search time
– Access to ~ 3π steradians of sky More timely access for warning
– Unique viewing Quickly changing viewing geometry
LEO, GEO
Favorable viewing geometry L1, L2, Venus-trailing orbit
– Allows for IR and visible – More costly to build and operate
• Ground and space systems have different execution and risk profiles
Sensor in Venus-trailing orbit
SDT-23 GHS 6/8/2017
Cataloging as Performance Measure
Integral completeness, year-by-year, of 10-year search with a
1m LEO visible system.
90% complete to 796m in 2023 90% complete to 200m in 2033
Outputs: Detections, unique detections and new cataloged objects over 10-year search
Telescope Detections Unique Detections
Corr. Unique Detections
0.5m LEO 5,837,141 39,049 35,274
1m LEO 3,539,527 54,918 47,686
2m LEO 2,562,339 66,456 53,354
Cataloging 1 detection = 3 out of 4 frames.
3 detections in 25 days
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
10 13 16 20 25 32 40 50 63 80 100
126
159
200
252
317
399
502
632
796
1000
1259
1589
2000
2518
3170
3991
5018
6321
7955
1000
0
Diameter (m)
Progression of Completeness for LEO 1m
Jan-23Year 1Year 2Year 3Year 4Year 5Year 6Year 7Year 8Year 9Year 10
Input population: 148,071 unknown PHAs, 10m – 12.5km diameter
SDT-24 GHS 6/8/2017
Cataloging From GEO and Venus-trailing
Space-based systems with apertures as small as 50 cm provide more cataloging capability than 4 m ground-based system
Venus-trailing and L1 orbits provide particularly favorable vantage point for cataloging operations
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Diameter (m)
Cataloguing Completeness for GEO
4m GBO
Vis GEO 50cm
Vis GEO 1m
Vis GEO 2m
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Diameter (m)
Cataloguing Completeness for L1 and Venus
Vis L1 50cmVis Venus 50cmIR L1_50cmIR Venus 50cmGBO 4m
SDT-25 GHS 6/8/2017
140m
140m 140m 140m
System Assessment Cataloging Results
Completeness at 10 Years
SDT-26 GHS 6/8/2017
System Assessment Warning Results
D= 50m
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Diameter (m)
Warning Efficiency for
SDT-27 GHS 6/8/2017
Search Effectiveness Results Summary
• System effectiveness realistically estimated via simulation – Performance estimates based on operational experience and current
state-of-art SNR 5 detection, 4 frames processed
– Evaluation metric tuned to developing catalog Current state-of-art: 3 detections in 25 days required
– Operations concepts included and explored – Real-world effects included
• Sky coverage largely the key to system effectiveness • Catalog completeness achieved more quickly with more costly
space-based systems • Mixed-basing systems achieve most complete capability
– Venus-trailing orbit and L1 best base for catalog – Warning best done local to Earth or L1
SDT-28 GHS 6/8/2017
Cost-Benefit Analysis
• Cost to retire* impact risk estimated by simulation process – Is the cost worth the retired risk?
• Requires a way to value the risk – casualties and property damage
– Property damage scaled to casualties Value set using International Emergency Disasters Database (EM-DAT) From ~ 5000 natural disasters in decade from 2004 to 2013 Value of Statistical Property Damage = $1.77M in FY17
– Value of Statistical Life set as 20% of US value Global value set by comparison of many national estimates Compared developed and developing national values Value of Statistical Life = $2.28M in FY17
* Of course if an asteroid is discovered on collision course with Earth, the hazard is not retired – only the risk of an unknown hazard
SDT-29 GHS 6/8/2017
Cost-Benefit Process
• Search systems assumed built between 2017 and end of 2022 – FY17 value of investment calculated
• Search systems assumed to operate between 2023 and 2033 (run to 2043) – Operations costs accrued on FY17 value basis
• Benefits accrued – Catalog benefits accrued over 5, 10, and 20 years of search – Catalog benefits are everlasting
• Warning efficiency calculated for all systems – Warning for objects discovered 6 days to 1 year from impact – Warning defined as statistical value of deaths and injuries avoided
SDT-30 GHS 6/8/2017
Year by Year Benefit Results
0
100
200
300
400
500
600
700
800
2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033
Glo
bal N
EO Im
pact
Ris
k U
ncer
tain
ty E
limin
ated
($M
/yea
r)
Years of Operation
GBO4m + Vis Venus 1m
Vis Venus 1m
GBO4m + Vis Venus 50cm
Vis Venus 50cm
IR+Vis L1 50cm
Vis LEO 2m
Vis GEO 2m
GBO4m + IR Venus 50cm
Vis L1 1m
GBO4m + Vis GEO 1m
Vis GEO 1m
Vis L2 1m
Vis LEO 1m
GBO4m + IR L1 50cm
IR L1 1m
IR L1 50cm
GBO4m + Vis GEO 50cm
Vis L1 50cm
Vis GEO 50cm
Vis L2 50cm
Vis LEO 50cm
IR Venus 50cm
GBO4m + IR 20cm
GBO 8m
GBO 4m
IR GEO 20cm
IR Venus 1m
GBO 2m
Status Quo
Annual Risk Uncertainty Eliminated Cataloging Benefit
SDT-31 GHS 6/8/2017
Year by Year Warning Benefit
0
2
4
6
8
10
12
14
16
2023 2024 2025 2026 2027 2028 2029 2030 2031 2032
War
ning
Ben
efit
($M
/yea
r)
Years of Operation
GBO4m + Vis Venus 1m
Vis Venus 1m
Vis LEO 2m
GBO4m + Vis Venus 50cm
Vis GEO 2m
IR+Vis L1 50cm
Vis Venus 50cm
Vis L1 1m
GBO4m + Vis GEO 1m
Vis GEO 1m
Vis LEO 1m
Vis L2 1m
GBO4m + IR L1 50cm
GBO4m + IR Venus 50cm
IR L1 1m
IR L1 50cm
GBO4m + Vis GEO 50cm
Vis L1 50cm
Vis GEO 50cm
Vis L2 50cm
Vis LEO 50cm
IR Venus 50cm
GBO4m + IR 20cm
GBO 8m
GBO 4m
IR Venus 1m
IR GEO 20cm
GBO 2m
Status Quo
SDT-32 GHS 6/8/2017
Total Benefit
0
1000
2000
3000
4000
5000
6000
7000
2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033
Tota
l Ben
efit
for E
limin
atio
n of
Ris
k U
ncer
tain
ty ($
M)
Years of Operation
GBO4m + Vis Venus 1m
Vis Venus 1m
Vis LEO 2m
GBO4m + Vis Venus 50cm
Vis GEO 2m
IR+Vis L1 50cm
Vis Venus 50cm
Vis L1 1m
GBO4m + Vis GEO 1m
Vis GEO 1m
Vis LEO 1m
Vis L2 1m
GBO4m + IR L1 50cm
GBO4m + IR Venus 50cm
IR L1 1m
IR L1 50cm
GBO4m + Vis GEO 50cm
Vis L1 50cm
Vis GEO 50cm
Vis L2 50cm
Vis LEO 50cm
IR Venus 50cm
GBO4m + IR 20cm
GBO 8m
GBO 4m
IR Venus 1m
IR GEO 20cm
GBO 2m
Status Quo
Cumulative Benefit Cataloging + Warning
SDT-33 GHS 6/8/2017
Benefit Analysis Results Summary
• Risk-reduction divided between two components – Cataloging – permanent reduction in risk – Warning – proportional to rate of discovery of new objects
• Spaceguard program retires global risk via cataloging – Very compelling cost-benefit basis
Will retire 92% of total risk by 2023 Will retire 92% of global risk by 2023 Will retire 56% of sub-global risk by 2023
– Poor warning capability
• The residual risk left un-retired by Spaceguard justifies substantial additional investment
– Residual uncharacterized risk almost $8B over 10 years – Based on global values for infrastructure and casualties
SDT-34 GHS 6/8/2017
Outline – Charter provided questions and approach – Members – Study approach
Population and baseline Collision risk assessment Search systems – capabilities and costs Search system effectiveness Cost-benefit analysis
– Answers to specific questions – Findings
SDT-35 GHS 6/8/2017
Question 1
• Define objective relative to risk eliminated by search over specified interval
• Panel suggestion: address 90% of risk from sub-kilometer objects – Survey to 90% complete for objects larger than 140 m
What are the smallest objects for which the search should be optimized?
90% of sub-Global risk addressed
SDT-36 GHS 6/8/2017
Question 2
• No special design consideration should be given to comets – Comets represent ~ 1% of the total collision risk to the Earth – Comets will be naturally detected by a system designed to address asteroids
Months of warning provided in most cases Cataloging long-period comets beyond current technology
• After asteroids are addressed, comets will represent larger, but not dominant, fraction of remaining risk
– 10 Casualties per year on average vs. 80 per year remaining from NEOs after search program characterizes 90% of sub-Global risk
Should comets be included in any way in the survey?
SDT-37 GHS 6/8/2017
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
10 13 16 20 25 32 40 50 63 80 100
126
159
200
252
317
399
502
632
796
1000
1259
1589
2000
2518
3170
3991
5018
6321
7955
1000
0
Diameter (m)
Warning Efficiency
IR+Vis L1 50cm
Vis L1 50cm
Vis GEO 1m
GBO4m + IR L1 50m
GBO4m + IR GEO 20cm
IR GEO co-host 20cm
GBO4m + Vis Venus 50cm
GBO 4m
IR L1 50cm
IR Venus 50cm
Vis Venus 50cm80%
85%
90%
95%
100%
63 80 100 126 159 200 252 317 399 502 632 796
Diameter (m)
Cataloguing Completeness GBO4m + Vis Venus 1m
Vis Venus 1m
GBO4m + Vis Venus 50cm
Vis Venus 50cm
IR+Vis L1 50cm
Vis GEO 1m
GBO4m + IR Venus 50cm
IR L1 1m
GBO4m + IR L1 50m
IR L1 50cm
IR Venus 50cm
GBO4m + IR GEO 20cm
GBO 4m
Question 3
What is technically possible?
D= 50m D= 140m
• Current technology allows the design of an asteroid search system capable of substantially addressing asteroid threats to the atmospheric penetration limit for non-metallic bodies of ~50 meters (24 mag.)
– Over long cataloging interval – Cost-benefit decision
SDT-38 GHS 6/8/2017
Questions 4, 5, and 6
How would the expanded search be done? What would it cost?
How long would the search take?
• Substantial reduction in uncharacterized risks from asteroid impact may be achieved via additional search systems
– Space-based systems required to achieve progress quickly
0
500
1000
1500
2000
2500
0% 10% 20% 30% 40% 50%% Risk Remaining After 10 Years
Costs vs. Risk Reduction
Cos
t ($M
FY1
7)
SDT-39 GHS 6/8/2017
Cost of Retired Risk
* System replenishment not included
0
500
1000
1500
2000
2500
3000
3500
4000
0 5 10 15 20 25
Cost
($M
FY1
7)
Years to Reduce Risk by 90%
Costs to Reduce Risk by 90%
Vis + IR L1 50cm
IR L1 1m
GBO4m + Vis Venus 1m
Vis Venus 1m
GBO4m + IR L1 50cm
IR L1 50cm
GBO4m + IR Venus 50cm
Vis LEO 2m
GBO4m + Vis Venus 50cm
Vis GEO 2m
IR Venus 50cm
Vis GEO 1m
Vis LEO 1m
Vis L1 1m
Vis L2 1m
Vis Venus 50cm
GBO4m + Vis GEO 1m
SDT-40 GHS 6/8/2017
Question 7
Is there a transition size above which one catalogs all the objects, and below which the design is simply to provide warning?
• Cataloging preferred and cost effective approach to ~140 m asteroids – Warning provided across size range during search
System below ~50% effective at atmospheric penetration limit (50m)
– Space-based systems provide most capable warning capability
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
10 13 16 20 25 32 40 50 63 80 100
126
159
200
252
317
399
502
632
796
1000
1259
1589
2000
2518
3170
3991
5018
6321
7955
1000
0
Diameter (m)
Warning Efficiency
IR+Vis L1 50cm
Vis L1 50cm
Vis GEO 1m
GBO4m + IR L1 50m
GBO4m + IR GEO 20cm
IR GEO co-host 20cm
GBO4m + Vis Venus 50cm
GBO 4m
IR L1 50cm
IR Venus 50cm
Vis Venus 50cm
80%
85%
90%
95%
100%
63 80 100 126 159 200 252 317 399 502 632 796
Diameter (m)
Cataloguing Completeness GBO4m + Vis Venus 1m
Vis Venus 1m
GBO4m + Vis Venus 50cm
Vis Venus 50cm
IR+Vis L1 50cm
Vis GEO 1m
GBO4m + IR Venus 50cm
IR L1 1m
GBO4m + IR L1 50m
IR L1 50cm
IR Venus 50cm
GBO4m + IR GEO 20cm
GBO 4m
D= 50m D= 140m
SDT-41 GHS 6/8/2017
Findings
• 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 analysis
Finding 1
SDT-42 GHS 6/8/2017
Findings
• The study panel finds the goal as stated by the 2003 SDT to be appropriate:
– “Develop and operate an asteroid search program with the goal of discovering and cataloging the potentially hazardous population sufficiently to eliminate 90% of the risk uncertainty associated with sub-kilometer asteroid population”
• The goal translates to developing a metric of cataloging 90% of all objects with diameters greater than 140 m
• Results in reduction in average uncharacterized casualty rate from ~180/year in 2023 to < 80/year post search campaign
– Global risk well addressed by search effort meeting this goal
Finding 2
SDT-43 GHS 6/8/2017
Finding 3
Findings
• Satisfaction of the 140 meter cataloguing objective will require space-based search systems
• IR and/or visible sensors in the 0.5-1.0 meter diameter range are credible, cost benefit favorable, options using available technology
• Best cost/benefit and lowest risk space system options located at L1*
• Fastest completion of 140 m objective and best warning provided by large aperture IR or combined visible and IR systems located at L1
• Search systems located near-Earth have substantial warning benefit
• The addition of a single 4-meter ground-based search system aids completion timeline for any of the space-based options
* IR systems in GEO (ex 20cm) and LEO not assessed
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