1. Physical Properties of Decameter-scale Asteroids Andrew Rivkin (JHU/APL) Target NEO 2 Workshop Washington DC

2. Getting us on the same page  Compositions/albedos/den sities/etc. go together, often assume other pieces when only one thing known S/Q complex, albedo ~0.2-0.3, 3 g cm -3, OC meteorites C complex, albedo ~0.04-0.08, 2 g cm -3, CC meteorites, water/organics X complex, albedo ~0.04-0.5, 2-8 g cm -3, iron meteorites or CC meteorites or aubrites or…? (Lunar rocks, albedo ~0.1-0.3, 3 g cm -3 look like S/D asteroids?) Higher albedos associated with anhydrous bodies “Key” of 0.5-2.5 µm spectral shapes from DeMeo et al. (2009)

3. Getting us on the same page  Rarely have measured sizes, just brightnesses Use absolute magnitude H as proxy for size 8 m ~ H=28 for average albedo, 27.2 for S-like albedo, 29-29.5 for C-like albedo  Data scarce for these sizes, extrapolate from larger sizes and assuming similar mix. May be off! “Key” of 0.5-2.5 µm spectral shapes from DeMeo et al. (2009)

4. Getting us on the same page  Rarely have measured sizes, just brightnesses Use absolute magnitude H as proxy for size 8 m ~ H=28 for average albedo, 27.2 for S-like albedo, 29-29.5 for C-like albedo  Data scarce for these sizes, extrapolate from larger sizes and assuming similar mix. May be off!

5.  Meteorite falls give direct info about 0.1-10 m scale objects in NEO population  80% ordinary chondrite (OC), 4% carbonaceous chondrites (CC)  8% achondrites (5% HED), 6% iron, 1% stony-iron  Notable meteorite parent bodies  Tagish Lake, Carancas, Peekskill, TC3 ~3-4 m  Chelyabinsk ~15 m  Gold Basin L4 6-8 m diameter  These may not match what’s out in space!  Weaker material screened out at various stages  Meteorites needn’t be on low delta-v orbits! What do we know about 10-m class asteroids compositionally?

6.  Visible-wavelength surveys biased against low-albedo objects (q.v. Mainzer in re IR surveys)  Debiased surveys suggest NEO “dark to bright ratio” of 1.6 (Stuart and Binzel 2004)  C:S ratio 1:2.2  X complex ~33% of NEOs, spans large albedo range: wildcard.  Based on ~1 km+ targets, caveat scientificus.  Few spectra at H>26: 2 C-ish, one S (Polishook et al., Hicks and Rhoades) What do we know about 10-m class asteroids compositionally?

7.  Visible-wavelength surveys biased against low-albedo objects (q.v. Mainzer in re IR surveys)  Debiased surveys suggest NEO “dark to bright ratio” of 1.6 (Stuart and Binzel 2004)  C:S ratio 1:2.2  X complex ~33% of NEOs, spans large albedo range: wildcard.  Based on ~1 km+ targets, caveat scientificus.  Few spectra at H>26: 2 C-ish, one S (Polishook et al., Hicks and Rhoades)  Getting albedos will be critical!  H = 27.2 for S/Q = 8 m, but for C = 24 m  H= 29 for C = 8 m, but for S/Q = 3 m What do we know about 10-m class asteroids compositionally?

8.  Constraints from visible observations  C-complex is 10% (debiased) of NEOs  Ch ~ 0.5% of all NEOs? (Binzel et al. 2004)  Few NEOs with measurements in 3-µm region  Difficult measurements due to atmosphere, high NEO temperatures  Hydrated CM/CI ~50-60% of CC meteorites  If CCs are 10% of NEOs, hydrated CCs ~5% of NEOs?  Other classes?  “Missing” low-albedo X material from meteorites?  “Expect” ~1:1 carbonaceous:non-carbonaceous (depending on orbit distributions)  WISE studies show low-albedo objects ~ high albedo objects in inner belt  Have 1:20-25 in meteorite collection Hydrated minerals in the NEOs

9. 8-m scale comets?  Theoretically possible: estimates of ~50 5-10 m Kreutz sungrazing comets  At 260 K even buried ice lasts ~1000 yr  NEO lifetimes ~1-10 My  Very unlikely to find an icy object!  Also very likely to have them in Earth-like orbits SOHO view of sungrazing comet

10.  ~2-4 meter body discovered 20 hours before impact near Sudan/Egypt border  Rotation period 1.6 minutes  F-class (C-complex) spectrum reported  Recovered as Almahata Sitta meteorite  Mostly ureilites (achondrite)  20-30% other meteorite types! (Jenniskens et al. 2011)  Clearly a rubble pile at top of atmosphere (next talk)!  Macroporosity estimated at 20-50% (Kohout et al. 2011)  Centimeter-size fragments recovered (but doesn’t rule out larger sizes being present pre-impact)  Despite “dark” spectral type, poor in water/OH (but organics present) 2008 TC3

11. Spectra of Almahata Sitta (2008 TC3)  Spectrally, meteorite is all over the place Chips are low albedo, relatively flat spectra Automated classification of #44 and #36 chips as Ch and Cb Powders are a bit higher albedo, but show more S-like spectra. Automated classification of #44 and #36 powders as Q!  Demonstrates pitfalls of blindly using associations mentioned above! From Hiroi et al. 2010

12. NameDiameterHPeriod (m) 2001 WJ40.01 27.454.2 2003 WT1530.007 287.02 2005 UW50.009 27.514.44 2006 DD10.015 26.52.74 2006 MV10.013 26.85.71 2006 RH1200.003 29.92.750 2008 JL240.004 29.63.23117 2008 TC30.004 30.91.6165 2009 FH0.01 26.66.438 2009 KW20.014 26.63.412 2009 UD 20090.01 27.21.3948 2009 WV510.011 27.14.60 2010 AL300.011 27.28.796 2010 JL880.01326.80.4098 2010 TD540.00528.71.376 2010 WA 20100.003300.5148 2011 MD 20110.0072811.62 2012 BX340.00927.6108.50 2012 KP240.0226.612.500 2012 KT420.00628.793.634 2012 TC40.01426.712.23 Asteroids with H>26.5, good quality lightcurves Assuming albedo 0.17 Average size 9 m Median size 11 m Average period 12 min (dominated by 1 object) Median period 3.6 min Mean amplitude 0.69 Axial ratio ~ 1.38:1 ~6.4x9x9 m to ~7.2x7.2x10 m Rotation of small NEOs

13. Asteroids with H>26.5, good quality lightcurves Assuming albedo 0.17 Average size 9 m Median size 11 m Average period 12 min (dominated by 1 object) Median period 3.6 min Mean amplitude 0.69 Axial ratio ~ 1.38:1 ~6.4x9x9 m to ~7.2x7.2x10 m Rotation of small NEOs

14. What intact, 8-m single objects do we know of?  Hoba iron meteorite: 2.7x2.7x0.9 m (largest known intact piece)  ‘Decameter-scale’ boulders on 2005 YU55  Boulders on Eros, Itokawa

15. In context: 8-m sized objects

16.  For a given brightness, likeliest albedos lead to factor of ~3 uncertainty in size → factor of ~25-30 in mass A combination of preparatory observations and S/C abilities will be needed to reduce or accommodate this factor  Range of likely-seeming porosities from zero (if monolith) to 50% (if like larger asteroids, high end of TC3 estimate) → Densities from ~1 g/cm 3 - ~3 g/cm 3 → another factor of 3 in mass I suspect this factor of 3 will be irreducible prior to S/C visit Putting together some uncertainties

17.  Expectations for decameter-scale asteroids extrapolated from larger bodies, informed by meteorite studies  Expect ~5% of NEOs to have hydrated minerals as lower limit  Fraction of X-complex asteroids that are low albedo leads to great uncertainty in understanding in overall population composition  Mission requirements plus wide range of target albedos makes albedo/size measurements imperative!  Rotation rates will be fast, likely < 5 minutes Summary