Invited Review:
Physical Properties of Small Bodies
from Atens to TNOs
Clark R. Chapman
Southwest Research Inst.
Boulder, Colorado, USA
Asteroids Comets Meteors 2005
Buzios, Rio de Janeiro, Brazil,
9 a.m., Monday, 8 August 2005
Gary Emerson
Classes of “Small Bodies”
By Orbital Class
 Inner-Earth Objects (IEOs or Apoheles)
 NEAs (Atens, Apollos, Amors)
 Main-Belt Asteroids (incl. Hungarias,
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Cybeles, Hildas, etc.)
Trojans (of Mars, Jupiter, Neptune…)
Centaurs, Scattered-Disk Objects
KBOs (Plutinos, Cubewanos)
Oort Cloud (inner)
Comets (JFCs, longer period comets)
Planetary satellites (irregular, regular)
By Size
 IDPs, Meteoroids, Meteorites
 “Small bodies” ~10 m to 1000 km diam.
 Pluto, 2003 UB313, other large TNOs
Kinds of Physical Properties:
Observables and “How Well?”
What is Learned  Types of Observations
composition, regolith
spin, shape, volatiles
mass (density)
structure, geology
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spectral reflectance & emission (UV – radio)
temporal variations (lightcurves, outbursts)
satellite orbits, perturbations on other bodies
imaging
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cosmochemistry,
geophysics
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Earth-based (optical/IR AO, radar)
Fly-by/orbital/lander spacecraft
in situ measurements/sample return [future]
 Degrees of Knowledge of Properties
Minimal info/most objects  rough size (no albedo), vis./IR colors
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Maximum info/few bodies 
spin period, albedo, spectral type, oblong/sph.
detailed shape, major minerals/ices, spots
detailed lab data on samples; parent unknown
large-scale geology, spatial compos., mass
Detailed obs./measurement by orbiter/lander
Colors of Centaurs, KBOs, SDOs
Hainaut & Delsanti database
 Bi-modal colors
 especially Centaurs
 esp. not Cubewanos
 Weak correlations
with orbital elements,
dynamical groups
 Comets do not match
colors of sources
(implies processing)
B-R
Delsanti et al 2004
Doressoundiram et al 2005
i
e
aq
a
Hainaut & Delsanti database
Main-Belt Asteroid Colors:
Then…and Now
Chapman (1971)
 Asteroid data 35 years ago
Hapke (1971)
Lessons Learned
like TNO data today
Data from Gehrels (1970)
Burbine et al (2001)
Ivezic et al (2002)
Disputed clusters partly OK
Trends with a,e,i convincing
only after debiasing (~1975)
 Matching colors/reflectance
spectra to mineralogy only
fair (space weathering, etc.)
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 Today: abundant statistics,
hi-res spectra, good compos.
Colors for tens of thousands
Reflectance spectra: 1000’s
Good correspondence of
taxonomy with meteorites
 Relationship of NEAs to
main-belt asteroids clear
 Families as catastrophic
collision products of (usually)
homogeneous parent bodies
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NEA Colors
(Binzel et al. 2004)
 S/Q type colors
Space-weathered (like M.B.) >5 km
 Range from ord. chond. – M.B. <2 km
 Spread of fresh to matured surfaces
 Implies there may be small M.B. Q’s
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 NEA colors vs. M.B.
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Q’s are NEAs only
More extremes
D-types (upper-rt)
10-18% of NEOs
could be extinct
comets
Diversity like M.B.
Outer M.B. underrepresented a bit
(beyond low
albedo bias)
Size Distributions
 NEAs less “wavy” than large Main Belt ast.
 TNOs have shallow slope at <20 km diam.
 Comets “truncated” 0.6-4 km
(Meech et al. 2004)
 Separate SDs for different families/groups
TNOs
Bernstein et al. 2004
NEAs
NASA SDT 2003
Main Belt
Tedesco et al. 2005
Detailed Earth-based Studies
of Individual Objects (examples)
5145 Pholus
4 Vesta
4179 Toutatis
Cruikshank et al. 1998
The period of rotation,
shape, density, and
homogeneous surface
color of the Centaur
5145 Pholus
Vernazza et al. 2005
Kryszczynska et al. 1999
S.C. Tegler et al. (2005)
HST
Polarization
Bogard & Garrison 2003
Mukai et al. 1997
Hudson et al. 2003
Brown et al. 2000
Shapes of Comet
Nuclei & Asteroids
Kleopatra
Tempel 1
Mathilde
Wild 2
Gaspra
Geophysical Properties
 Spins, shapes, satellites, masses, densities, strengths, interior structures
 Most remote-sensing of surfaces reveals little about interior properties
 Rapid spins = monolithic structure; do slow spins imply rubble piles?
 Impact experiments, numerical modelling, scaling analysis
 NEAR laser altimetry probes interior of Eros
NEAR Laser
Altimeter:
Eros
Neumann & Barnouin-Jha 2005
Holsapple 2005
Korycansky & Asphaug 2005
Spacecraft: Orbiters, Landers,
and (soon) Sample Returns
 Many fly-bys of small bodies
 Significant reconnaissance
 Surprises: no 2 bodies same
 NEAR Shoemaker orbital
NEAR XRS data suggest Eros
composition ~ ordinary chondrites
mission to Eros (& landed!)
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Detailed remote-sensing
Composition: ord. chondrite
 Impact, landers, sample ret.
 Deep Impact experiment
 Contact with Itokawa soon
 Awaiting sample returns by
Stardust & Hayabusa
 Must extrapolate physical
properties measured for few
visited small bodies to vast,
heterogeneous population
Lim et al. 2005
Unexpected SmallScale Geology of Eros
 Flat ponds and “beaches”
 Small craters absent; dominant boulders
Surface Geology of Tempel 1
Preliminary answers at 11 am today!
 Flat, smooth areas; craters; ridges;
bright spots…
 What processes are at work? Over
what duration of time?
Dynamics: Relationships to
Physical Properties
 Dynamical processes cause physical properties
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Spins and axis orientations due to Yarkovsky Effect
Tidal interactions with planets/sun cause distortions and
disruptions/disintegrations
Collisions and catastrophic disruptions create families,
rubble pile structures, satellites (initial spins, sizes)
 Physical properties elucidate dynamics
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Colors help identify dynamical families
Yarkovsky/YORP effects depend on albedo, shape,
thermal inertia, spin, density, etc.
 Dynamical analysis can determine physical properties
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Mass (hence density)
Spins (very rapid spins indicate monolith, not rubble pile)
Non-gravitational forces imply features of comet nucleus
 Dynamical analysis helps us study physical processes
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Specific ages for families specify rates for processes like
space-weathering
How perihelia evolve and facilitate volatilization
NEO Impact Hazard:
99942 Apophis (2004 MN4)
 In astronomy, only solar
flares and impacts have
major practical effects
 1:8000 chance that 320m
asteroid impacts 4/13/36
(~ South Asia tsunami)
 Physical properties affect:
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In the extremely unlikely event
that it will hit, ground-zero will be
somewhere on the red line
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Whether it hits “keyhole”
How Yarkovsky affects it
How we could attach to it,
couple energy to divert it
How it responds to forces
How it responds to tidal
forces during 2029 fly-by
Consequences of impact
Themes and Issues
 How much are we astronomers fooled by the
space-weathered, impacted optical surfaces?
 Can we really comprehend how processes work
at near-zero gravity?
 Really what are the densities, porosities,
granular structures, strengths?
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Are these splitting/vanishing comets “dust bunnies”?
Are M-types metallic cores? (many evidently aren’t)
Regolith-free bare rocks vs. “talcum powder”
Biased view from what penetrates our atmosphere
 What are we missing?
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2003 UB313: we weren’t looking for high-inclinations
Hypotheticals: “vulcanoids”, Lou A. Frank “LAFOs”
Interstellar small bodies?
 Asteroid belts/Oort clouds around other stars
Asteroids/ Comets:
Evolving Perspectives…
Traditional View
ASTEROIDS
COMETS
Rocky, metallic, no active
geology, cratered,
collisional fragments, some
differentiated by heating
Icy, under-dense, no active
geology, pristine…until they
come close to the Sun, become
very active, disintegrate
Emerging Continuum
ASTEROIDS
COMETS
Under-dense, rubble piles,
many volatile-rich (except
at surfaces), some nonimpact geology, many
satellites; NEAs tidally
evolved
Active, fluffy, evolved
bodies with complex
geology (impact & nonimpact), easily split;
precursor KBOs have
satellites, interior “oceans”