Decameterscale Asteroids Andrew Rivkin JHUAPL Target NEO 2 Workshop Washington DC Getting us on the same page Compositionsalbedosdensitiesetc go together often assume other pieces when only one thing known ID: 229738
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Slide1
Physical Properties of Decameter-scale Asteroids
Andrew Rivkin (JHU/APL)
Target NEO 2 Workshop
Washington DCSlide2
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 meteoritesC complex, albedo ~0.04-0.08, 2 g cm-3, CC meteorites, water/organicsX 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)Slide3
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)Slide4
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!Slide5
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-ironNotable meteorite parent bodies Tagish Lake, Carancas, Peekskill, TC3 ~3-4 mChelyabinsk ~15 mGold Basin L4 6-8 m diameterThese may not match what’s out in space!Weaker material screened out at various stagesMeteorites needn’t be on low delta-v orbits! What do we know about 10-m class asteroids compositionally?Slide6
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 1:2.2X 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?Slide7
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 1:2.2X 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 mH= 29 for C = 8 m, but for S/Q = 3 mWhat do we know about 10-m class asteroids compositionally?Slide8
Constraints from visible observations
C-complex is 10% (
debiased
) of NEOsCh ~ 0.5% of all NEOs? (Binzel et al. 2004)Few NEOs with measurements in 3-µm regionDifficult measurements due to atmosphere, high NEO temperaturesHydrated CM/CI ~50-60% of CC meteoritesIf 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 beltHave 1:20-25 in meteorite collectionHydrated minerals in the NEOsSlide9
8-m scale comets?
Theoretically possible: estimates of ~50 5-10 m
Kreutz
sungrazing cometsAt 260 K even buried ice lasts ~1000 yrNEO lifetimes ~1-10 MyVery unlikely to find an icy object! Also very likely to have them in Earth-like orbitsSOHO view of sungrazing comet Slide10
~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 meteoriteMostly 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 TC3Slide11
Spectra of Almahata
Sitta
(2008 TC3)Spectrally, meteorite is all over the placeChips are low albedo, relatively flat spectraAutomated classification of #44 and #36 chips as Ch and CbPowders 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. 2010Slide12
Name
Diameter
H
Period (m)2001 WJ40.01
27.454.2
2003 WT153
0.007
28
7.02
2005 UW50.00927.514.442006 DD1
0.01526.52.74
2006 MV1
0.013
26.8
5.71
2006 RH120
0.003
29.9
2.750
2008 JL24
0.004
29.6
3.23117
2008 TC3
0.004
30.9
1.6165
2009
FH
0.01
26.6
6.438
2009 KW2
0.014
26.6
3.412
2009 UD 2009
0.01
27.2
1.3948
2009 WV51
0.01127.14.602010 AL300.01127.28.7962010 JL880.01326.80.40982010 TD540.00528.71.3762010 WA 20100.003300.51482011 MD 20110.0072811.622012 BX340.00927.6108.502012 KP240.0226.612.5002012 KT420.00628.793.6342012 TC40.01426.712.23
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 NEOsSlide13
Asteroids with H>26.5,
good quality
lightcurves
Assuming albedo 0.17Average size 9 mMedian size 11 mAverage 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 mRotation of small NEOsSlide14
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 YU55Boulders on Eros, ItokawaSlide15
In context:
8-m sized objectsSlide16
For a given brightness, likeliest albedos lead to factor of ~3 uncertainty
in size
→
factor of ~25-30 in massA combination of preparatory observations and S/C abilities will be needed to reduce or accommodate this factorRange 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 massI suspect this factor of 3 will be irreducible prior to S/C visitPutting together some uncertaintiesSlide17
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