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Asteroids
Asteroid Belt
• Between Mars and Jupiter there are a number of objects.
– Much smaller than a planet
• The small amount of light led them to be named asteroids.
 Jupiter’s
gravity keeps
asteroids from
growing too
large.
Mars’ orbit
Main belt
Jupiter’s orbit
Minor Planets
• 26 asteroids have diameters greater than 200 km.
– Largest: Ceres, 933 km
• Large asteroids show some differentiation into layers like a
planet, including lava flows.
Types of Asteroids
C-type
 75% of
asteroids
 extremely dark
(albedo 0.03)
 oldest material,
carbon and
other minerals.
S-type
 17% of
asteroids
 bright (albedo
0.10 to 0.22)
 iron mixed with
silicate rock.
M-type
 8% of asteroids
 bright (albedo
0.10 to 0.18)
 pure iron with
nickel and other
metals.
Mathilde
• Mathilde is a main-belt asteroid (66 km long).
– Dark type C
– Craters with little splash
NEAR/Mathilde
Trojan Asteroids
• The law of gravity permits an orbit around the sun exactly
60º ahead of and behind Jupiter, called Lagrange points.
– Asteroids collect there
– Several hundred Trojan asteroids locked to Jupiter
L4 Trojans
Jupiter
L5 Trojans
Near Earth Asteroids
• Asteroids nudged by
Jupiter’s gravity head into
the inner solar system.
• Many of these now have
orbits that cross the orbit
of Earth.
Eros
• Eros is a near-earth asteroid (30 km long).
– Bright type S
– Impact boulders on its surface
• The NEAR spacecraft also visited here, and safely touched
down on the surface.
NEAR/Eros