Estimated ARM Candidate Target Population and Projected Dis PowerPoint Presentation, PPT - DocSlides

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)

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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), andManeuver 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

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ARM: Mission Overview

10) Orion Rendezvous & CrewOperations

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

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Characteristics of ARRM Target Candidates

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Characteristic

Reference

Value

Orbit:

V

infinity

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

V

infinity

)

Rotation State

Spin period > 0.5 min

Non-Principal-Axis

rotation

is assumed to be likely

Size and Aspect

Ratio

4 m < mean diameter < 10 m (roughly, 27 <

H

< 31)

Upper limit on max dimension:

~

14

m

Aspect

ratio

< 2:1

Spectral

Class

Known Type

preferred, but not required

(C-type with hydrated minerals

desired)

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 Vinfinity Constraint

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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

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Numbers of Near-Earth Asteroids

Current 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”

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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 Harris

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 Vinfinity); 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).

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ARM Candidate Discovery Rates from Simulations

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Current List of Potential ARM Candidates

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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.

NameFirst detected byApparent Magnitude at First DetectionAbsolute Magnitude HV (km/sec)Approach DateDistance at Approach (AU)Good retrieval trajectories found2007 UN12CSS17.728.71.29/15/20200.0432008 EA9ML21.027.71.911/15/20200.0732013 EC20CSS17.728.52.63/15/20210.0672010 UE51CSS19.228.31.210/15/20220.0232009 BDML18.428.20.76/26/20230.1992011 MDLIN19.228.10.98/10/20240.1502008 HU4CSS17.928.20.53/27/20260.149Good retrieval trajectories may be possible2010 XU10ML20.027.42.510/22/20210.1672012 WR10CSS19.028.62.612/6/20210.2922011 BQ50PS22.828.32.611/4/20220.0782011 PN1PS22.027.5n/a6/30/20230.3002005 QP87SW18.227.71.53/1/20240.4572010 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 baseline

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Projected 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.

Options for Increasing the ARM Candidate Discovery Rate

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*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

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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.

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Physical Characterization of ARM Candidates

Assumed albedo

r = 0.04

Assumed albedor = 0.34

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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 Vinfinity, 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.

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Summary of ARM Candidate Physical Constraints

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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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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 > ~0.4Constrains mass to < 50 tFaster spin rate: 0.5 – 2 rpmPreliminary mission design indicates a feasible retreival trajectory for 2021.

ARM Candidate Characterization Process Exercised for 2013 EC20

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Characteristics of Current ARM Potential Candidates

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CharacteristicReference Value2009 BD2011 MD2013 EC202008 HU42007 UN122010 UE51Orbit Confidence OCC < 4ExcellentGoodRecoverableRecoverableRecoverableGoodOrbit: Vinfinity(km/s)< 2(< 2.6 req.)0.70.92.60.51.21.2Orbit: Natural return yearEarly 2020s(2020-26)202320242020202620202023Size (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 XeUnknownUnknownUnknownNext 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.

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 Asteroids

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Artificial objects can be distinguished via 3 methods:Best-fit orbit solution has a high area-to-mass ratio (eg. > 1 x 10-3 m2/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-IVB

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

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