Interplanetary bodies: asteroids
asteroid-- rocky object in orbit around the sun
includes:
Main Belt asteroid
Hilda and Thule asteroid
near-Earth asteroid (NEA)
Trojan asteroid
origin: rocky material that never accreted into a larger
object; survivors of the planetary sweep-up process
Asteroid orbits
object
approx. a (AU)
approx. e
Main Belt
asteroid
2.2 - 3.2
0 - 0.4
Hilda, Thule
asteroid
3.9, 4.3
0 - 0.2
Trojan
5.2 (= Jupiter)
0 - 0.2
NEA
0.7 - 3
0.2 - 0.8
NEAs derived from Main Belt by perturbations
Asteroid
locations
2.2 AU
Selected
NEA orbits
Main
3.2 AU
Asteroid semi-major axes
Hungariagroup
Main Belt
Hildagroup
Trojans
Thulegroup
Kirkwood
gaps
Kirkwood gaps
• represent areas in asteroid belt relatively
free of material
• form at locations (resonances) that experience
repeated perturbations from Jupiter
• strong resonance locations include 1:3 & 2:5
• material in gaps moved elsewhere, such as into
planet-crossing orbits (e.g., NEAs)
Asteroids:
spectra & albedos
Examples
of asteroid
spectral
types
Asteroid spectral types
type
albedo
(%)
main
location (a)
S
10-22
M
mineralogy
analogue
1.8-3.5 AU
pyroxene
+ - metal
+ - olivine
O-chondrites?
achondrites?
stony irons?
10-20
2.3-3.2 AU
metal
iron meteorites
C
3-7
2-4 AU
hydrated
silicates
+ carbon
C-chondrites
many meteors?
comets?
D+
P
2-6
3-5 AU
carbon /
organic-rich
silicates
none?
comets?
Asteroids: different types in
different locations
Asteroids: examples
Gaspra
Ida
Mathilde
Vesta
Eros
Main Belt,
S-type
Main Belt, S-type
member of family
Main Belt,
C-type
Many
large
craters !
NEA,
possible
chondrite
parent body
Eros: up close & personal
The ultimate “meteorite”
collecting site!
Relative
crater sizefrequency
diagram
(“R plot”)
Gaspra-younger
(recent
collision?)
Mathilde-surplus of
larger
craters (how?)
Asteroids:
Eros geology
Eros:
the Saddle
(Himeros)
Eros:
grooves & troughs
Eros:
ridges & grooves
Eros:
mass wasting
Eros:
ponds-asteroidal
sedimentary
deposit
Asteroids:
Landing the NEAR spacecraft
on Eros
(or how to make an asteroid landing, without
designing for it)
Asteroids:
Space weathering
Space weathering-• We have clear evidence for the importance
of space weathering on all asteroids seen
up close.
• On such asteroids, space weathering makes
objects redder, darker, and less crystalline.
Color-exagerrated
image
Eros spectral reflectance: mineralogy
0.9 -1 micron band:
Fe-bearing olivine, pyroxene
2 micron band:
Fe-bearing pyroxene
Eros: pyroxene signature
Himeros
Himeros
Psyche
Space weathering-A common process on asteroids
• Color variations on Gaspra & Ida: “redder” color =
more weathered
• Albedo variations on Eros: darker = more
weathered
• Spectral variations on Eros: weaker pyroxene
signature = more weathered
Space weathering-• Has important implications for making
links between asteroids and meteorites.
• Probably involved in “pond” formation on
Eros.
• Implies we can make sedimentary deposits on
asteroids or small-gravity objects by
space weathering. (Not seen on Moon.)
Asteroids:
NEAs
NEAs: the more we look, the more we find them
NEA
asteroid
1999 JM8
diameter ~ 3.5 km
a = 2.71 AU
e = 0.65
i = 13.8o
q = 0.96 AU
radar images
NEA asteroid 4179 Toutatis:
“contact binary” or elongated asteroid?
radar images
diameter = 4.6 x 2.4 x 1.9 km
a = 2.51 AU
e = 0.63
i = 0.5o
q = 0.92 AU
NEA asteroid 1999 KW4: binary asteroid
radar
images
diameter of primary object ~ 1.2 km
diameter of secondary object ~0.4 km
orbital period ~ 16 hrs
a = 0.64 AU
e = 0.69
i = 39o
q = 0.20 AU
Target: Earth?
NEA asteroid
1950 D4
diameter ~1.1 km
rot. period ~2.1 hrs
a = 1.70 AU
e = 0.51
i = 12.2o
q = 0.84 AU