Astronomy 241: Foundations of Astrophysics I

18. Asteroids

100 AU

5 AU

S

20,000 AU

Overview: Structure

Inner system: terrestrial planets, asteroids.

Outer system: giant planets and moons, “KBOs”.

Oort Cloud: comets.

Asteroids and comets to scale

Comet Nuclei

Samples From the Asteroid Belt

Primitive meteorites are relics of the solar system’s formation, 4.6 Gyr old.

Processed meteorites come from differentiated asteroids, later broken up by collisions.

Element Abundances

The Solar System

Eros  

a  =  1.5AU,  e  =  0.22  34  x  11  x  11  km    S-­‐type    Density  =  2700  kg  m-­‐3  

Prot  =  5  hr    Disc.  1898  (Carl  WiI)  V  ~  8.1  mag  (binoculars)    Visited  by  NEAR-­‐Shoemaker  probe  

The Last Global Rotation Movie

Mission to Eros

Eros: The Final Approach

Asteroid Light Curves

An Asteroid Rotation Factory

Diameter vs Rotation Rate

Some asteroids are rubble piles: loose collections of fragmented rock held together by self-gravity.

Spin Limits of Solar System Bodies

Diameter vs Rotation Rate

Some small asteroids rotate so fast that they must be monoliths held together by material strength.

Spin Limits of Solar System Bodies

PIA15678: Full View of Vesta

Giant Asteroids

Giant Asteroids

Vesta

Largest Asteroid May Be 'Mini Planet'

Ceres975 km

Large asteroids are complex objects which appear to have differentiated.

530 km

PIA14321: Vesta Full Rotation Movie PIA15510: Vesta's Internal Structure

Basaltic crust

Mantle

Iron-Nickel core

Wikipedia: Asteroid belt

Jupiter

Hildas

Trojans

Trojans

Mars

Belt Structure 14 August 2006

Jupiter

Hildas

Mars

Trojan Asteroids

L4

L5

Wikipedia: Asteroid belt

Orbits starting near L4

ma/mb = 49

Orbits starting near L1

ma/mb = 49

Wikipedia: Hilda family

Hilda Asteroids

2:3 MMR; stable since they avoid Jupiter (e ~ 0.3).

Small inclinations (i ≲ 20°).Wikipedia: Hilda family

Asteroid Belt Populations

Asteroid Families

Many asteroids are members of families; they have similar orbits and compositions (indicated by colors).

Inner belt asteroids (left) and families (right).

Suspected Asteroid Collision Leaves Odd X-Pattern of Trailing Debris

A Suspected Asteroid Collision