Physical Properties of Decameter-scale Asteroids

Andrew Rivkin (JHU/APL)Target NEO 2 WorkshopWashington DC

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 DeMeo et al. (2009)

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 DeMeo et al. (2009)

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!

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?

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?

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

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 orbitsSOHO view of sungrazing comet

~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

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

Name Diameter H Period (m)2001 WJ4 0.01 27.4 54.22003 WT153 0.007 28 7.022005 UW5 0.009 27.5 14.442006 DD1 0.015 26.5 2.742006 MV1 0.013 26.8 5.712006 RH120 0.003 29.9 2.7502008 JL24 0.004 29.6 3.231172008 TC3 0.004 30.9 1.61652009 FH 0.01 26.6 6.4382009 KW2 0.014 26.6 3.4122009 UD 2009 0.01 27.2 1.39482009 WV51 0.011 27.1 4.602010 AL30 0.011 27.2 8.7962010 JL88 0.013 26.8 0.40982010 TD54 0.005 28.7 1.3762010 WA 2010 0.003 30 0.51482011 MD 2011 0.007 28 11.622012 BX34 0.009 27.6 108.502012 KP24 0.02 26.61 2.5002012 KT42 0.006 28.79 3.6342012 TC4 0.014 26.7 12.23

Asteroids with H>26.5, good quality lightcurves

Assuming albedo 0.17Average 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

Rotation of small NEOs

Asteroids with H>26.5, good quality lightcurves

Assuming albedo 0.17Average 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

Rotation of small NEOs

In context: 8-m sized objects

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/cm3 - ~3 g/cm3 → another factor of 3 in mass

I suspect this factor of 3 will be irreducible prior to S/C visit

Putting together some uncertainties

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