Physical Properties of Decameter-scale Asteroids

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/densities/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 shapesfrom DeMeoet 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 shapesfrom DeMeoet 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. What do we know about 10-m class asteroids compositionally? • 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 • TagishLake, 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!

6. What do we know about 10-m class asteroids compositionally? • 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)

7. Hydrated minerals in the NEOs • Constraints from visible observations • C-complex is 10% (debiased) of NEOs • Ch ~ 0.5% of all NEOs? (Binzelet 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

8. 8-m scale comets? • Theoretically possible: estimates of ~50 5-10 m Kreutzsungrazing 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

9. 2008 TC3 • ~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 AlmahataSitta 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)

10. Spectra of AlmahataSitta(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

11. Rotation of small NEOs Asteroids with H>26.5, good quality lightcurves Assuming albedo 0.17 Average size 9 mMedian 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

12. Rotation of small NEOs Asteroids with H>26.5, good quality lightcurves Assuming albedo 0.17 Average size 9 mMedian 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

13. In context: 8-m sized objects

14. Putting together some uncertainties • 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 (ifmonolith) to 50% (if like larger asteroids, high end of TC3 estimate) → Densities from ~1 g/cm3 - ~3 g/cm3→ another factor of 3 in mass I suspect this factor of 3 will be irreducible prior to S/C visit

15. Summary • 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