Asteroid Redirect Robotic Mission ( ARRM )

07.09.201307.09.2013

Estimated ARM Candidate Target Population and Projected Discovery Rate of ARM Candidates

Paul 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) 1

07.09.2013

• 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

2NASA Pre-Decisional - Sensitive But Unclassified (SBU)

07.09.2013

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)

1) 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)

07.09.2013

Characteristics of ARRM Target Candidates


Characteristic Reference Value

Orbit: Vinfinity relative to Earth < 2 km/s desired; upper bound ~2.6 km/s

Orbit: Natural return to EarthOrbit-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 State Spin period > 0.5 minNon-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 mAspect ratio < 2:1

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

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• 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 < TE

• Define “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


Num

bers

(pow

ers

of 1

0)

N

umbe

r (<

H)

ARM Size Range

7 m

Diameter (km), assuming Albedo = 0.14

Diagram courtesy of Al Harris

07.09.2013

• 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


• 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 by

Apparent Magnitude at

First DetectionAbsolute

Magnitude H V (km/sec) Approach DateDistance at

Approach (AU)Good retrieval trajectories found2007 UN12 CSS 17.7 28.7 1.2 9/15/2020 0.0432008 EA9 ML 21.0 27.7 1.9 11/15/2020 0.0732013 EC20 CSS 17.7 28.5 2.6 3/15/2021 0.0672010 UE51 CSS 19.2 28.3 1.2 10/15/2022 0.0232009 BD ML 18.4 28.2 0.7 6/26/2023 0.1992011 MD LIN 19.2 28.1 0.9 8/10/2024 0.1502008 HU4 CSS 17.9 28.2 0.5 3/27/2026 0.149Good retrieval trajectories may be possible2010 XU10 ML 20.0 27.4 2.5 10/22/2021 0.1672012 WR10 CSS 19.0 28.6 2.6 12/6/2021 0.2922011 BQ50 PS 22.8 28.3 2.6 11/4/2022 0.0782011 PN1 PS 22.0 27.5 n/a 6/30/2023 0.3002005 QP87 SW 18.2 27.7 1.5 3/1/2024 0.4572010 AN61 CSS 19.4 27.0 2.6 6/10/2025 0.2512013 GH66 PS 20.3 28.0 2.0 4/15/2025 TBDCSS = 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


• 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.

07.09.2013

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. V lim = 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.

Cur

rent

Futu

re


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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 albedor = 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


07.09.2013

• 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


07.09.2013

• Discovered 7 March 2013 (during ARM study), by Catalina Sky Survey– Initial size estimate: ~6m, Close approach 8 March at 0.5 LD

• Manually recognized as potential ARM target (a process now automated).

• Request follow-up astrometry => orbit update to enable IRTF observation

• IRTF 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/cc– Diameter = 2.6 - 8.4 m, mass = 20 - 930 t– Spin rate ~0.5 rpm

• Arecibo radar @ ~3 LD [Borozovic]:– Diameter = 1.5 - 3 m => albedo > ~0.4– Constrains mass to < 50 t– Faster 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) 16

07.09.2013

Characteristics of Current ARM Potential Candidates


Characteristic Reference Value

2009 BD 2011 MD 2013 EC20 2008 HU4 2007 UN12 2010 UE51

Orbit Confidence OCC < 4 Excellent Good Recoverable Recoverable Recoverable Good

Orbit: Vinfinity(km/s)

< 2(< 2.6 req.)

0.7 0.9 2.6 0.5 1.2 1.2

Orbit: Natural return year

Early 2020s(2020-26)

2023 2024 2020 2026 2020 2023

Size (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] Unknown Unknown Unknown

Spectral Class Known(C preferred)

Unknown Unknown L or Xe Unknown Unknown Unknown

Next Observation Opportunity

A=AstrometricO=OpticalIR=InfraredR=Radar

2013-Oct: IR

2014: IR?

2013-Aug: A?

2016-Apr: A, O?, R

None 2014: 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.

07.09.2013

• 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


• 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

07.09.2013

• 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


Asteroid Redirect Robotic Mission ( ARRM )

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