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%

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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

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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