Asteroid Redirect Robotic Mission (ARRM) 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 07.09.2013 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 1 ARM: Asteroid Redirect Mission 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 2 ARM: Mission Overview 6) Asteroid Operations: Characterize, deploy bag, capture, and despin (60 days)
Asteroid Orbit 7) SEP Redirect to Lunar Orbit (2 to 5 years) 5) SEP Low-thrust Cruise to Asteroid (2 to 3 years) 4) Lunar Gravity Assist (if needed) 2) Separation & S/A Deployment Moons 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) Initial Earth Orbit 1) Launch: Atlas V 551, or SLS, or Falcon Heavy Earth 07.09.2013 8) Lunar Gravity Assist 9) SEP Transfer to Safe DRO (~1.5 yrs.) 10) Orion Rendezvous & Crew Operations NASA Pre-Decisional - Sensitive But Unclassified (SBU)
Example 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
Characteristics of ARRM Target Candidates Characteristic 07.09.2013 Reference Value Orbit: 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 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) NASA Pre-Decisional - Sensitive But Unclassified (SBU)
4 Details on ARM Vinfinity Constraint Roughly, Vinfinity is the asteroids relative velocity when it encounters Earth, with the acceleration due to Earths 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 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 5 ARM Candidate Orbit Constraint Summary 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; 3 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 6
Numbers of Near-Earth Asteroids Current number of known NEAs: 10,006 increasing at ~1000 per year. NASAs 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 07.09.2013 Catalina Sky Survey Mt. Lemmon 60 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 7 NEAs: Population vs. Absolute Magnitude & Size Diameter (km), assuming Albedo = 0.14 Number (< H) Numbers (powers of 10) 7m ARM Size Range
Diagram courtesy of Al Harris 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 8 ARM Candidate Discovery Rates from Simulations 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 Schunovas 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). 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 9 Current List of Potential ARM Candidates Apparent First Magnitude at Absolute Distance at V (km/sec)km/sec) Name detected by First Detection Magnitude H Approach Date Approach (km/sec)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 Current baseline 2011 MD LIN 19.2
28.1 0.9 8/10/2024 0.150 CSS 17.9 28.2 0.5 3/27/2026 0.149 KISS baseline 2008 HU4 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
SW 18.2 27.7 1.5 3/1/2024 0.457 2010 AN61 CSS 19.4 27.0 2.6 6/10/2025 0.251 2013 GH66 PS 20.3 28.0 2.0 4/15/2025 TBD CSS = Catalina Sky Survey/Mt Bigelow, ML= CSS/Mt. Lemmon, SW = Spacewatch, LIN= LINEAR, PS = PanSTARRS 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 10 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 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 11 Future Current Options for Increasing the ARM Candidate Discovery Rate *Discoveries per year that meet ARMs 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 12 Physical Characterization of ARM Candidates 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-tomass 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) Assumed albedo r = 0.04 Assumed albedo r = 0.34 13 Summary of ARM Candidate Physical Constraints 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 spacecrafts 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU)
14 ARM Candidate Characterization Process 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). 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 15 ARM Candidate Characterization Process Exercised for 2013 EC20 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 16 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
< 2  Unknown Unknown Unknown Spectral Class Known (C preferred) Unknown Unknown L or Xe Unknown Unknown Unknown Next Observation Opportunity A=Astrometric O=Optical IR=Infrared R=Radar 2013-Oct: IR 2014: IR? 2013-Aug: A?
2016-Apr: A, O?, R None 2014: IR?? Notes:  NEOWISE stacked non-detection;  Upper bound density: 1.5 g/cc from Micheli et al.;  Magdalena Ridge lightcurve;  Lower bound on abs. mag. and lower bound albedo of 3%;  Upper bound density of 3.5 g/cc;  Arecibo radar. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 17 Rocket Bodies and Spacecraft Masquerading as Asteroids 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. Apollo 8 S-IVB 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 18
Summary 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. 07.09.2013 NASA Pre-Decisional - Sensitive But Unclassified (SBU) 19
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