ASTEROIDS/MINOR PLANETS

What is interesting about asteroids (minor planets)?
Possible remnants of early solar system, though collisionally evolved
As fragments, means to study interiors of differentiated bodies
Sources for meteorites
compositionally, they may help suggest original comp. of SS

What would you *expect* based on solar system formation?
composition gradient
planetesimal-sized, but mostly less than 500 km
heavily cratered at this point
In planetesimal stages, larger bodies may have differentiated
nothing large could form there due to Jupiter's gravity
collisions create smaller & smaller fragments
some may essentially be pure metallic (differentiated cores)
composition gradient from inner to outer belt
recent times: asteroids both ejected to and from the belt

When there is a periodic relation between orbits
regularity of perturbation
Keplerian orbits are 2-body phenomena
when 3 bodies matter, you get deviation from ellipse
Child on swing analogy for resonance
gaps in the asteroid belt?
Same reason there is a gap in the rings of Saturn!
objects in 1:1 (L4 & L5) resonance with Jupiter
objects in 3:2 resonance with Neptune -- Plutinos
Resonance both stable and unstable -- DELIVERY of meteorites!

Distribution

MAIN ASTEROID BELT, approx 2.1-3.3 AU mean distances (PLOT distribution)
most eccentricities 0.1 to 0.3, so wider range
inclinations <= 30 degrees, compare: 3 for planets
Discovered in pursuit of the "missing planet"
Jupiter's gravity causes both gaps & concentrations
Hilda asteroids -- 2:3 resonance, a=3.97, stable zone
Kirkwood gaps, integer relations to Jupiter's orbit
1:2 a=3.28 or 1:3 a=2.50, big gaps
resonances change orbits either gradual or rapid
Trojans lie 60 degrees before/behind a planet's orbit
Jupiter trojans vary up to 20 degrees LOTS of objects
more elongated than typical belt asteroid
Near-Earth populations: Amor, Apollo, Aten
short lived, ~10 million years, so need some "supply"
some could be extinct comets
Apollo asteroids 100 "Earth-crossers", est. 700 +- 300
100m object, 1 km crater, every 3000 years
70% fall to ocean, of course
population must be replenished, or long ago gone
Amor asteroids are "Mars-crossers", down to 1 AU
Aten asteroids are inside Earth at semi-major axis
asteroid "families" -- dynamically similar
Each distinct group/zone has different evolutionary history
"Families" form from a single breakup event
sleuthed by orbital elements: a, i, e
diagram is misleading about density
volume is something like 10 ²⁵ km³
even if 60 million asteroids, only one per Eart-Moon sized sphere

Comparative size figure (see fig. 7 in Ch. 25 of book)
largest is Ceres just under 1000 km
15 others > 240 km in diameter
Common method: IR modelling, larger bodies emit more IR radiation
larger bodies emit more IR radiation
unresolved from Earth
large/dark appears same size as small/bright
Best method: stellar occultation

Largely irregular, too small to self-gravitate
Larger ones are spheroidal or triaxial ellipsoids
Collisions play an important role in shaping asteroids
light curves suggest extreme shapes
Also radar: size, shape, spin, surface characteristics
mostly Earth-crossers
largest of main belt

Multi-filter Spectra from reflected sunlight, visible & IR
Early classes were broad, suggestive of meteorite types C & S
Tholen taxonomy from ECAS (Eight-Color Asteroid Survey)
Classes correlate with heliocentric distance
may indicate thermal processing from most (inner) to least (outer)
radioactive 26Al heat, or electrical induction heating
Bell "superclasses" -- Igneous, Metamorphic, Primitive
IR spectroscopy -- note olivine and pyroxine bands
types:
C low albedo, peak outer belt, most common (75%)
S moderate albedo, inner edge of belt, about 1/6
M types (maybe metallic) peak mid-belt
correlation with distance
thermal observations, blackbody peak 20micron or so

Increased radar observations of NEOs and inner belt
Hubble Space Telescope -- Vesta Geology & history
amazing Galileo photos (both S) -- first satellite discovered
Kuiper belt objects!!!
NEAR spacecraft -- Mathilde (first C) & Eros

Links between Vesta and meteorites
Need to understand how asteroids have been processed since formation
heating, melting, aqueous alteration, impact processing
What source for ordinary chondrites?
"space weathering" of S-types
a selection effect based on position relative to resonance
What effects are size-dependent?