ARM Architectural Framework

Report
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)
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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.
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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
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)
Initial Earth Orbit
1) Launch: Atlas V 551,
or SLS, or Falcon Heavy
Earth
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8) Lunar
Gravity
Assist
9) SEP Transfer
to Safe DRO
(~1.5 yrs.)
10) Orion
Rendezvous &
Crew
Operations
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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
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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)
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Details on ARM Vinfinity Constraint
• 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
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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;
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.
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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.
Catalina Sky Survey – Mt. Lemmon 60”
• 10-m-class asteroids have been found:
Number currently known (27 ≤ H ≤ 30): ~370
Number that meet orbital criteria for ARM: ~14
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NEAs: Population vs. Absolute
Magnitude & Size
Diameter (km), assuming Albedo = 0.14
of 10)
Numbers
(< H)
Number(powers
7m
ARM Size Range
Diagram courtesy of Al Harris
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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 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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Current List of Potential ARM Candidates
Apparent
First
Magnitude at
Absolute
Distance at
Name
detected by
First Detection Magnitude H
V (km/sec)
Approach Date 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
Current baseline 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
KISS baseline 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
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.
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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.
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Future
Current
Options for Increasing the
ARM Candidate Discovery Rate
*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.
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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.
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Assumed albedo
r = 0.04
Assumed albedo
r = 0.34
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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
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.
07.09.2013
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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
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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
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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=Astrometric
O=Optical
IR=Infrared
R=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.
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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.
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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
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