Estimated ARM Candidate Target Population and Projected Dis - PowerPoint Presentation

Slide1

Estimated ARM Candidate Target Population and Projected Discovery Rate of ARM CandidatesPaul Chodas (JPL/Caltech)with contributions from Bob Gershman, Rob Jedicke, Eva Schunova, and others…

Asteroid Redirect Robotic Mission (ARRM)

NASA Pre-Decisional - Sensitive But Unclassified (SBU)

1Slide2

ARRM is not currently proposed as a science mission, although science will certainly benefit from it. ARRM is a technology demonstration mission which not only creates a destination for human exploration but also advances high-power Solar Electric Propulsion (SEP) technology.ARRM meets the needs of the STMD SEP Technology Demonstration Mission.High-power SEP is an enabling technology for future missions, both human and robotic.

ARM would:Capture a 4- to 10-m near-Earth asteroid, with mass as much as 1000 metric tons,“Retrieve” the asteroid (ie, guide it towards an encounter with the Moon that captures it into the Earth-Moon system), and

Maneuver the asteroid into a stable Distant Retrograde Orbit (DRO) about the Moon, where it could be visited and explored by astronauts.

ARM: Asteroid Redirect Mission

2

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide3

ARM: Mission Overview10) Orion Rendezvous & Crew

Operations

Initial Earth Orbit

Moon’s Orbit

3)

Spiral Out

to

Moon

if

Atlas V

551

(1 to 1.5 years)

or, launch direct to Lunar Gravity Assist

if SLS or Falcon

Heavy (< 0.1 years)

Asteroid

Orbit

2) Separation & S/A Deployment

4) Lunar Gravity Assist (if needed)

5) SEP Low-thrust Cruise to Asteroid(2 to 3 years)

7) SEP Redirect to Lunar Orbit (2 to 5 years)

6) Asteroid Operations: Characterize, deploy bag, capture, and despin (60 days)

Launch: Atlas V 551, or SLS, or Falcon Heavy

Earth

8) Lunar

Gravity

Assist

9) SEP Transfer

to Safe DRO

(~1.5 yrs.)

Phase

Delta V

% Fuel

Duration

To Earth Escape

4,662 m/s

29%

1.4

yr

To Asteroid

3,868 m/s

21%

1.8

yr

Earth Return

152 m/s

36%

3.0

yr

To Moon Orbit

60 m/s

14%

1.4 yr

Example

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide4

Characteristics of ARRM Target Candidates4NASA Pre-Decisional - Sensitive But Unclassified (SBU)

CharacteristicReference ValueOrbit:

Vinfinity relative to Earth

< 2 km/s desired; upper bound ~2.6 km/s

Orbit: Natural return to Earth

Orbit-to-orbit distance (MOID) <

~0.03

au,

Natural

return to Earth in early 2020s (or 2020-2026)

(“Return” means close

approach within

~0.3

au)

Mass

<1,000 metric

tons

(Upper bound varies according to Vinfinity)Rotation StateSpin period > 0.5 minNon-Principal-Axis rotation is assumed to be likelySize and Aspect Ratio 4 m < mean diameter < 10 m (roughly, 27 < H < 31)

Upper limit on max dimension: ~14 mAspect ratio < 2:1Spectral Class

Known Type preferred, but not required(C-type with hydrated minerals desired)Slide5

Roughly, Vinfinity is the asteroid’s relative velocity when it encounters Earth, with the acceleration due to Earth’s gravity removed; it is closely related to the Tisserand parameter w.r.t. Earth, TE, which depends on a, e and i.

Vinf ≤ 2.6 km/s implies 2.99233 < TEDefine “Population 1” by this constraint + additional constraints on

a and e:

0.7 au < perihelion < 1.05 au and 0.95 au < aphelion < 1.45 au e > -1.40591 a + 1.33562 and e > +0.89132

a – 0.93588

Details on ARM

V

infinity

Constraint

5

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide6

ARM candidate orbit should be fairly Earthlike (a = ~1 au, low eccentricity, low inclination), since these have the lowest Vinfinities.Object should make a natural close approach to Earth (within ~0.3 au) in the right timeframe (“early 2020s”). Timeframe is dictated by the desired time for the Orion mission to visit the retrieved asteroid.Minimum Orbit Intersection Distance (MOID) < ~0.03 au.

Orbit knowledge should be fairly good: Orbit Cond. Code ≤ ~5; 3σ along-track position uncertainty at arrival should be < ~20,000 km.

Orbit will likely become well characterized (OCC ≤ 2) as a by-product of the physical characterization.There are no constraints on the angular orbital elements, although these will obviously feed into the mission design and timeline.

ARM Candidate Orbit Constraint Summary

6

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide7

7Numbers of Near-Earth AsteroidsCurrent number of known NEAs: 10,006 increasing at ~1000 per year.

NASA’s NEO Observation Program has been key to coordinating and funding the NEO discovery and characterization effort, and this arrangement should continue as the goal moves to smaller asteroids.Currently, most NEA discoveries are made by:

Catalina Sky Survey (64%), and

Pan-STARRS (25%)

Several new and improved surveys will come online in the next couple years. Some could be accelerated by additional funding.

10-m-class asteroids have been found:

Number currently known (27 ≤ H ≤ 30): ~370

Number that meet orbital criteria for ARM: ~14

Catalina Sky

Survey – Mt. Lemmon 60”

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide8

8

NEAs: Population vs. Absolute Magnitude & Size

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Numbers (powers of 10)

Number (<

H

)

ARM Size Range

7 m

Diameter (km), assuming

Albedo

= 0.14

Diagram courtesy of Al HarrisSlide9

Jedicke & Schunova (J&S) performed simulations of the ARM candidate discovery process, based on the Greenstreet NEO orbit distribution model. They included a detailed simulation of the upcoming ATLAS and PS2 surveys and used realistic sky coverage, cadence, and loss factors (see

Schunova’s talk in next session).The J&S simulation results had to be normalized to match known PS1 detection rates, revealing deficiencies in the

Bottke 2002/

Greenstreet orbit distribution

model.

Their normalized results suggest that on the order of 50,000 10-m class NEAs

in Pop1 (

ie

, that approach

Earth with a small enough

V

infinity

);

the number that

also

satisfy the

MOID and natural

return requirements would then be ~15,000.Only a tiny fraction of these will come close enough to the Earth (~0.03au) over the next few years to be discovered by current asteroid surveys.The J&S normalized simulations suggest the ARM candidate discovery rate will be ~5 per year

for PS2 and ~10 per year for ATLAS (see next session).9

ARM Candidate Discovery Rates from SimulationsNASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide10

10Current List of Potential ARM CandidatesNASA Pre-Decisional - Sensitive But Unclassified (SBU)

14 known asteroids satisfy the ARM orbit and absolute magnitude criteria (27 ≤ H ≤ 30), although most have not been adequately characterized.

These potential ARM candidates were discovered at a rate of

~2.5 per year.

While

this

discovery rate is admittedly a sparse base for statistics, there is no reason to expect this discovery rate to decrease.

4 candidates on this list have been, or will be, at least partially characterized:

2009 BD, 2011 MD, 2013 EC20 and 2008 HU4.

Name

First detected by

Apparent Magnitude at First Detection

Absolute Magnitude

H

V



(km/sec)

Approach Date

Distance at Approach (AU)

Good

retrieval trajectories

found

2007 UN12

CSS

17.7

28.7

1.2

9/15/2020

0.043

2008 EA9

ML

21.0

27.7

1.9

11/15/2020

0.073

2013 EC20

CSS

17.7

28.5

2.6

3/15/2021

0.067

2010 UE51

CSS

19.2

28.3

1.2

10/15/2022

0.023

2009 BD

ML

18.4

28.2

0.7

6/26/2023

0.199

2011 MD

LIN

19.2

28.1

0.9

8/10/2024

0.150

2008 HU4

CSS

17.9

28.2

0.5

3/27/2026

0.149

Good

retrieval trajectories

may be possible

2010 XU10

ML

20.0

27.4

2.5

10/22/2021

0.167

2012 WR10

CSS

19.0

28.6

2.6

12/6/2021

0.292

2011 BQ50

PS

22.8

28.3

2.6

11/4/2022

0.078

2011 PN1

PS

22.0

27.5

n/a

6/30/2023

0.300

2005 QP87

SW18.227.71.53/1/20240.4572010 AN61CSS19.427.02.66/10/20250.2512013 GH66PS20.328.02.04/15/2025TBDCSS = Catalina Sky Survey/Mt Bigelow, ML= CSS/Mt. Lemmon, SW = Spacewatch, LIN= LINEAR, PS = PanSTARRS

Current baseline

KISS baselineSlide11

11Projected Future Discovery Rate of ARM Candidates

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The ARM candidate discovery rate will almost certainly increase

due to enhancements to existing surveys and new surveys coming online.

Many enhancements are already in process and funded by the NEOO Program. Some could be accelerated with additional funding.

A conservative projection, based on improved coverage and cadence, is that the discovery rate will at least double within a year or

so

to at least

~5 per year

.

The final ARM target selection can occur as late as 6 months before launch.

With at least another 3-4 years to accumulate ARM candidate discoveries, at least

~15 more ARM candidates

discoveries are expected

;

favorable mission design trajectories should be available for at least half of these.

There should be opportunities to physically characterize future ARM candidates (

eg. with radar), making them stronger candidates than those in the current list.Slide12

Options for Increasing the ARM Candidate Discovery Rate12*Discoveries per year that meet ARM’s rough size and orbit criteria for retrieval. Vlim

= limiting magnitude N.B. Discoveries are not additive. There will be duplications of detections, particularly in the optimistic scenarios. Predictions for future discovery rates are based on extrapolated coverage and cadence.

Current

Future

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide13

Precise characterization of physical properties will be difficult without a characterization mission, but it should be possible to set reasonable upper bounds on these parameters.Radar will be essential for obtaining an accurate estimate of size, shape and rotation state.Ground-based and space-based IR measurements will be important for estimating

albedo and spectral class, and, indirectly, approximate density.Light curves

will be important to estimate shape and rotation state.Long-arc high-precision astrometry

will be important for determining the area-to-mass ratio. Use of Gaia catalog promises an order-of-magnitude improvement in area-to-mass estimation.

Mass will be estimated by combining an inferred or assumed density with the size and shape estimate, but

mass may

also be constrained by the area-to-mass ratio estimate.

13

Physical Characterization of

ARM Candidates

Assumed albedo

r

= 0.04

Assumed albedo

r

= 0.34

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide14

Size and Shape: 4 m < mean diameter <10 m; aspect ratio < 2:1. Dimensions should be known to within ~2 m. Upper bound on maximum dimension: ~

14 m.Mass: < ~1000 metric tons. Precise upper bound varies from case to case,

according to V

infinity, MOID and available time for thrusting.

Mass may only be known to within a factor of 3 or 4.

Rotation State

: Lower bound on primary rotation period: 0.5 min.

Non-principal-axis

rotation

is assumed to be likely.

Multiplicity

:

Solitary

body preferred for simplicity of capture process.

Final ARM target selection will probably be based largely on how the estimated upper bound on the

mass estimate for each candidate compares with the spacecraft’s return mass capability for that candidate's orbit.Biasing the target selection to smaller objects (eg. ~5-m size) may be necessary to increase the chances that retrieval will be successful.

14Summary of ARM Candidate Physical Constraints

NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide15

Rapid response after discovery is essential, since the asteroid will likely be near closest approach and will not likely be any closer for decades.Request interrupt radar observations at Goldstone and/or Arecibo. (NB: The Goldstone interrupt observation process needs to be streamlined.)Solicit

follow-up astrometry from the observing community, and frequently update the orbit solution on Horizons.Request interrupt observations from IRTF and other assets that can provide thermal IR data for faint objects. (This may require interagency agreements for target-of-opportunity observing time.)

Solicit high precision astrometry, photometry and light curve measurements from geographically dispersed observatories (e.g. Palomar, Keck, European Southern Observatory in Chile).

ARM Candidate Characterization Process

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NASA Pre-Decisional - Sensitive But Unclassified (SBU)Slide16

Discovered 7 March 2013 (during ARM study), by Catalina Sky SurveyInitial size estimate: ~6m, Close approach 8 March at 0.5 LDManually recognized as potential ARM target (a process now automated).Request follow-up astrometry => orbit update to enable IRTF observationIRTF Interrupt: Spectra and thermal IR [Moskovitz

& Binzel]:L- or Xe-type, inferred albedo range of 0.1-0.4, density range of 2.0-3.0 g/ccDiameter = 2.6 - 8.4 m, mass = 20 - 930 tSpin rate ~0.5

rpmArecibo radar @ ~3 LD [Borozovic]:

Diameter = 1.5 - 3 m => albedo > ~0.4

Constrains mass to < 50 tFaster spin rate: 0.5 – 2

rpm

Preliminary mission design

indicates

a feasible

retreival

trajectory

for

2021.

ARM Candidate Characterization Process Exercised for 2013 EC20

NASA Pre-Decisional - Sensitive But Unclassified (SBU)

16Slide17

Characteristics of Current ARM Potential Candidates17NASA Pre-Decisional - Sensitive But Unclassified (SBU)

CharacteristicReference Value2009 BD2011 MD

2013 EC202008 HU4

2007 UN122010 UE51

Orbit Confidence OCC < 4

Excellent

Good

Recoverable

Recoverable

Recoverable

Good

Orbit:

Vinfinity

(km/s)

< 2

(< 2.6 req.)

0.7

0.92.60.51.21.2Orbit: Natural return yearEarly 2020s(2020-26)20232024202020262020

2023Size (m)< 10 and > 4 < 8 [1]< 30 [4]

2-3 [6]< 28 [4]< 22 [4]< 27 [4]Mass (t)< 1000< 500 [2]< 50,000 [5]< 50

< 40,000 [5]< 20,000 [5]< 36,000 [5]Spin Rate (rpm)< 2

< 0.01 [3]0.1 [3]< 2 [6]UnknownUnknownUnknownSpectral ClassKnown

(C preferred)UnknownUnknownL or XeUnknownUnknownUnknown

Next Observation OpportunityA=AstrometricO=OpticalIR=InfraredR=Radar2013-Oct: IR2014: IR?2013-Aug: A?2016-Apr: A, O?, RNone2014: IR??

Notes: [1] NEOWISE stacked non-detection; [2] Upper bound density: 1.5 g/cc from Micheli et al.; [3] Magdalena Ridge lightcurve; [4] Lower bound on abs. mag. and lower bound albedo of 3%; [5] Upper bound density of 3.5 g/cc; [6] Arecibo radar.Slide18

There are ~80 spacecraft and rocket bodies in heliocentric orbits with low enough Vinfinities to be possibly mistaken as ARM candidate targets.Natural objects outnumber artificial objects by 1 or 2 orders of magnitude.Rocket Bodies and Spacecraft Masquerading as Asteroids18

NASA Pre-Decisional - Sensitive But Unclassified (SBU)

Artificial objects can be distinguished via 3 methods:

Best-fit orbit solution has a high area-to-mass ratio (

eg

. > 1 x 10

-3

m

2

/kg).

A backwards orbit propagation with high area-to-mass ratio puts the object near the orbit node at the time of a launch, and the Earth was near the node at the same time.

Reflectance spectra inconsistent with a natural body.

It will be important to characterize the orbit and physical properties of an ARM candidate well enough to eliminate

the possibility that it is artificial

.

Apollo 8 S-IVBSlide19

ARRM is primarily a technology demonstration mission, not a science mission.ARM candidates should reside in fairly Earthlike orbits, and must naturally return to Earth in the right timeframe.Simulations suggest there are thousands of suitable ARM candidates; the challenge is to find them.

ARM potential candidates are currently being discovered at the rate of ~2.5/year.With several survey enhancements in process and new surveys coming online within the next 2 years, the ARM potential candidate discovery rate should at least double to ~5 per year.

Rapid response after discovery is critical for physical characterization of ARM candidates. The

process was already successfully exercised for a small candidate.

Radar is a key characterization asset for ARM candidates.

The mass of ARM

candidates

may only be known to within a factor of 3 or 4.

Once an ARM

candidate

is characterized, it should be clear whether or not it is an old rocket body.

Summary

19

NASA Pre-Decisional - Sensitive But Unclassified (SBU)