27. asteroids - institute for astronomybarnes/ast241_f15/ac/ac1.pdfperiod determines semi-major axis...

Astronomy 241: Foundations of Astrophysics I

27. Asteroids

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 ago.

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

Asteroid Light Curves

Wikipedia: Light Curve

(Days)

Period

Diameter vs Rotation Rate

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

Planetary Sciences, Fig. 9.20

PIA15678: Full View of Vesta

Giant Asteroids

Giant Asteroids

Vesta

Largest Asteroid May Be 'Mini Planet'

Ceres

Giant 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

975 km

PIA19547: Ceres Rotation

Wikipedia: Asteroid belt

Jupiter

Hildas

Trojans

Trojans

Mars

Belt Structure 14 August 2006

Trojans

Mars

Jupiter

Trojans

Hildas

Belt Structure

Inner Belt: a < 2.5 AU

Mid Belt: 2.5 AU < a < 2.8 AU

Outer Belt: a > 2.8 AU

Wikipedia: Asteroid belt

Kirkwood Gaps

Inne

r

Mid

dle

Out

er

Troj

ans

Hild

asResonances with Jupiter group asteroids by orbit period; period determines semi-major axis (Kepler III: P2 = a3).

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).

Wikipedia: Asteroid belt

Trojans

Mars

Jupiter

Trojans

Hildas

Origin of Families

Fragments are scattered on similar orbits.

Large Asteroid Breakup — Don Davis

WHAM!

Suspected Asteroid Collision Leaves Odd X-Pattern of Trailing Debris

A Suspected Asteroid Collision

Jupiter

Hildas

Mars

Trojan Asteroids

L4

L5

Wikipedia: Asteroid belt

Orbits starting near L4

ma/mb = 49

Orbits starting near L1

ma/mb = 49

SMBC 3127 (revised)

Points 4 and 5 are usually stable.

They are unstable in:(a) Pluto-Charon system

(MC / MP > 0.04)(b) Earth-Moon system (other planets perturb)

Wikipedia: Hilda family

Hilda Asteroids

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

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