Asteroid Redirect Mission: Candidate Targets Paul Chodas, NEO Program Office, JPL Small Bodies Assessment Group Meeting #12, January 7, 2015
NEA Discovery Rates Are Increasing Overall discovery rate of Near- Earth Asteroids (NEAs) has increased 45% since last year Total known NEAs: >12000 Pan-STARRS survey is now dedicated to NEO search; its discovery rate is up 70% Catalina Sky Survey discovery rate is same as last year; its camera will be upgraded in 2015 NEOWISE is reactivated and finding several NEAs every month Discovery rate for accessible targets (per NHATS criteria) is up 65% Discovery rate for small (< ~10m) NEAs is up 80% BUT, only 1 potential candidate for ARM Option A was found in 2014, and it could not be fully characterized due to radar being down 2
Summary of NEO Radar Observations in 2014 88 NEAs and 2 comets were observed by radar in 2014, up 13% from 2013, and more than in any previous year This is remarkable considering Arecibo was down for ~120 days due to equipment problems mainly due to damage from an earthquake, and Goldstone was down for ~35 days In just the last 2 weeks, Arecibo characterized two ~10m NEAs that were almost suitable candidates for ARM Option A 14 NHATS targets were observed by radar this year Boulders were detected on two 100m-class NEAs this year, but neither of them was in suitable orbit to be candidate for Option B 2014 BR57 2014 HQ124 3
2014 BR57: Radar Evidence for Boulders 2014 February 18, resolution = 3.75 m x 0.06 Hz Monthly report for February 2014 Goldstone images of 2014 BR57. The images reveal that 2014 BR57 is a rounded object and suggest a diameter of ~80 m. Radar-bright spots at two locations are visible and persist as the asteroid rotates. The figure above shows one of them with arrows. The spots resemble similar features seen in radar images on numerous larger NEAs that are candidates for boulders. 2014 BR57 is the smallest NEA by about a factor of four with these features imaged to date by radar. This hints that smaller NEAs than previously realized can have boulders. 4 4 4
2014 HQ124: Radar Evidence for Boulders 2014 June 8, resolution = 3.75 m x 0.00625 Hz Monthly report for June 2014 Bistatic Goldstone-Arecibo radar images of 2014 HQ124. This was the first test of new data taking equipment installed at Arecibo in May. 2014 HQ124 is an elongated, bifurcated object with a rotation period of ~20 h and a long axis of about 380 m. Numerous radar-bright spots (some highlighted with green circles) are visible that are candidate boulders. 5
Asteroid Redirect Mission: Two Robotic Capture Options Option A for Asteroid Rendezvous with a <10 m asteroid Demonstrate some basic planetary defense techniques Match its spin, capture it and despin Guide the asteroid back to lunar orbit Max return mass depends on orbit, but is likely on the order of 100 tons Option B for Boulder Rendezvous with a 100+ m asteroid Characterize its surface, select boulder Land on the asteroid and collect the boulder, probably ~2-3 m in size Demonstrate gravity tractor deflection Bring the boulder back to lunar orbit Max return mass depends on orbit, but is likely on the order of 10 tons 66
Earth-like orbit Favorable Close Approach Not too big, Not rapidly spinning Rotation rate less than 0.5 rpm 7 7
Trajectory of 2009 BD in a Rotating Frame NASA Asteroid Initiative Opportunities Forum #AskNASA 8
Why So Few ARM Option A Candidates in 2014? In 2014, 10 NEAs were discovered in Earth-like orbits, similar to the number found in 2013 But unlike in 2013, more than half of the objects were in orbits that return to Earth too late (2030s or later), and all but one of the remaining NEAs were too large (~15 35 m) The only new potential candidate, 2014 BA3, was too far away for Goldstone, and Arecibo was down due to earthquake damage Thus, no new valid candidates for Option A in 2014: the bottom line remains the same: 9 potential candidates, 3 valid candidates 2014 was unlucky: historical trends indicate 2-3 new potential candidates per year even without enhancements to the observation campaign, and 1-2 of these per year should become valid candidates Another potential candidate, 2008 HU4, has radar opportunity in 2016 As the ARM launch pushes later, candidates drop off the list, but if the return date/arcm accessibility date also pushes later, other potential candidates get added to the list 9
Summary of Potential Candidates for Option A KISS baseline Current baseline Estimated Was Radar Size Return Comment Name Possible? (m) V (km/s) Date 2008 HU4 Y 4 22 0.5 4/2026 Characterizable in 2016 2009 BD Y 2.6 6 1.2 6/2023 Valid Candidate 2010 UE51 Y 4 22 1.2 12/2022 2011 MD Y 3 15 1.0 8/2024 Valid Candidate 2013 EC20 Y 2 3 2.6 9/2024 Valid Candidate 2013 GH66 Y 4 22 2.0 4/2025 2013 LE7? 6 33 2.5 5/2023 2013 PZ6 N 5 23 n/a 8/2023 2014 BA3 Y 4 21 1.9 1/2024 Arecibo Down 9 Potential Candidates: Pre-2013 discovery rate: ~1/year; in 2013: 4/year; in 2014: 1/year Expected discovery rate without enhancements: 2-3 per year Expected discovery rate with enhancements: ~5 per year 3 Valid Candidates (yellow): 2009 BD, 2011 MD and 2013 EC20 Additional valid candidates still expected at ~1-2 per year 10
11 Option A Candidates 2009 BD and 2011 MD Both are well observed (hundreds of observations, dozens of observatories) Neither observed by radar or ground IR, so sizes initially not well constrained But, both were targeted by the Spitzer Space Telescope (space-based IR) 2009 BD: Not detected, but this still constrains the size (Mommert et al., 2013): Probable diameter: ~4 m; likely range is 2.6 m - 7 m Upper bound on mass: 150 tons, which is within the return mass capability if ARM launches before the end of 2020 2011 MD: Detected. (Mommert et al., 2014): Probable diameter: ~6 m; likely range is 3 m - 15 m Upper bound on mass: 350 t, which is within the return mass capability if ARM launches before the end of 2020 4.0 m 2009 BD and 2011 MD are Valid Candidates for Option A 2011 MD observed by Spitzer
Characterization of Option A Candidates 2009 BD and 2011 MD: Both objects very well observed from the ground, and orbits well determined Spitzer detection/non-detection puts bounds on the sizes (Mommert et al.) Non-gravitation parameters can be modeled, yielding distributions on size and mass with good constraints on uncertainties 2013 EC20: Less well observed from the ground Detection by Arecibo radar puts bounds on the size Non-grav parameters were not modeled: mass was estimated from size and assumed range of densities 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 1 2 3 4 5 6 7 8 Diameter [m] 1.2 x 10 5 1 0.8 0.6 0.4 0.2 2009 BD Size Distribution 2009 BD Mass Distribution 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Mass [kg] x 10 5 12
Summary of Option A Candidate Characteristics Candidate Target Mass (t) Size (m) 2009 BD 95% upper limit Median 95% lower limit 157 70 26 6.0 4.4 2.6 Spin Period (minutes) > 180 2011 MD 95% upper limit Median 95% lower limit 670 110 27 13.0 6.0 3.1 12 2013 EC20 95% upper limit Median 95% lower limit 43 20 8 3.0 2.5 2.0 ~2 13
Medium-sized NEA Accessible orbit Observational evidence of 14 boulders 14
Characterizing Option B Candidates In order to be a Valid Candidate for Option B, the surface of the host asteroid must be characterized, and the existence of boulders of the size which can be returned must at least be inferred There are two means of adequately characterizing the surface: 1) In situ imaging from a precursor mission 2) Ground-based radar with high enough SNR Candidates must approach within ~8 lunar distances to have high enough SNR. Amount of mass which can be returned from Option B candidates which can be characterized is <~50 t, corresponding to boulder sizes of <~4 m 15
In Situ Characterization for Option B Candidates One Option B candidate has been visited by a spacecraft and characterized well enough to detect boulders of a size which could be returned: Itokawa, visited by Hayabusa in 2005. Two other Option B candidates are planned to be similarly characterized by spacecraft in 2018: Bennu (OSIRIS-Rex), and 1999 JU3 (Hayabusa 2). 535 m Itokawa is a Valid Candidate for Option B, and Bennu and 1999 JU3 are expected to become Valid Candidates Itokawa 16
Radar Characterization for Option B Candidates Radar cannot detect individual <4m boulders, but if the SNR is high enough, ~10m-scale features can be seen, and the presence of <4m boulders can be inferred Radar observed ~10m-scale features interpreted as boulders on 2014 HQ124 and 2005 YU55, but neither is in an accessible orbit ~15m-scale boulders are observed on 2 candidates: Bennu & 2008 EV5 (both C- types); we infer the presence of <4m boulders on these two asteroids 2014 HQ124 2005 YU55 Bennu and 2008 EV5 are considered Valid Candidates for Option B because of the inferred presence of boulders 2008 EV5 17
Candidates for Option B: Mid-2019 Launch & 2023-24 Return Target Type Asteroid V (km/s) Earth Launch or Escape Earth return Max Return mass (t) Boulder max diam (m) c CharacterizaHon Itokawa a S 5.7 3/2019 10/2023 6 1.6-1.8 Visited by Hayabusa in 2005 Bennu b C 6.4 5/2019 11/2023 12 2.0-2.3 Radar, OSIRIS- REx, mid- 2018 1999 JU3 C 5.1 6/2019 7/2023 13 2.0-2.3 Hayabusa 2, mid- 2018 2008 EV5 a C 4.4 1/2020 6/2024 32 2.7 3.1 Radar in Dec. 2008, SNR= 240,000 2011 UW158? 5.3 7/2018 7/2024 10 1.8-2.1 Radar in Jul. 2015, SNR = 280,000 2009 DL46 a? 5.7 11/2019 8/2024 11 1.9-2.2 Radar in May 2016, SNR= 48,000 Valid Valid Expected Valid Possible Possible a Earth gravity assist ~1yr prior to capture c Assuming densities in the range 2.0 to 3.0 g/cm 3 NB: Max Return masses and Boulder max diameters vary significantly with launch date and return date Assumes Falcon Heavy launch vehicle, Earth departure in mid 2019, 400-day stay and return in 2023 Green: In situ characterization from precursor mission Grey: Characterization by radar and inference of appropriate-sized boulders Lots of Potential Candidates (characterizable candidates tabulated above) 3 Valid Candidates: Itokawa, Bennu, 2008 EV5 (radar only) Additional candidates validated by radar at ~1 per year.
Summary of Candidate Asteroids for Option B Itokawa: Precursor: Hayabusa in 2005 S-type, 535 x 200 m, 12 hr spin Bennu: Precursor: OSIRIS-REx in 2018 B/C-type, 500 m size, 4 hr spin 2008 EV5: No precursor, but radar detected boulders in 2008 C-type, 400 m size, 4 hr spin 1999 JU3: Precursor: Hayabusa 2 in 2018/19 C-type, 870 m size, 8 hr spin Possible Future Candidates: No precursors, but radar could detect boulders on 2011 UW158 in 2015, 2009 DL46 in 2016 Itokawa 2008 EV5 19
End-to-End ARRM Mission Examples OpHon A OpHon B Crew Accessible Date (LDRO) Aug 2024 Aug 2024 Dec 2024 Sep 2025 Launch Date Oct 2019 Oct 2020 Dec 2019 Oct 2020 Target 2009 BD 2009 BD 2008 EV5 2008 EV5 Launch Vehicle D4H SLS SLS D4H Solar Array Power, BOL 50 kw 50 kw 50 kw 50 kw Total Flight Time 4.8 yrs 3.8 yrs 5.0 yrs 4.6 yrs Return Mass Capability 170 t 157 t 10 t 22 t DuraSon of Planetary Defense Demo < 20 min. < 20 min. 55 days 32 days Planetary Defense DV 1.0 mm/s 1.0 mm/s 0.016 mm/s 0.016 mm/s Stay Time at Asteroid 90 days 90 days 430 days 458 days Total Xenon with margin 3.2 t 6.3 t 7.9 t 4.7 t 20