Astronomy 241: Foundations of Astrophysics I

18. Asteroids & Comets

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.

Wikipedia: Kuiper Belt

1. TRANS-NEPTUNIAN OBJECTS

Pluto and Charon

Hubble Maps Pluto

Double planet with 4 small moons; probably formed by giant impact (similar to Earth-Moon system).

• orbit ⇒ mass: MPluto = 0.002 ME

• density: ~ 2000 kg m-3

• composition: 1/3rd rock, 2/3rd ice

• thin atmosphere: N2, CH4, CO

Pluto has probably differentiated;Charon is too small to melt itself.

Hubble Discovers a Fifth Moon

Wikipedia: Comet Hale-Bopp

2. COMETS

Comets

Comets are icy objects which fall into the inner solar system.

Warmed by the Sun, they develop tails of gas & dust.

At other times, a comet is an inert lump of ice & dust.

To S

un

Comet’s Motion

Comet Halley

Asteroids and comets to scale

Comet Nuclei

Deep Impact: the Nucleus of Tempel 1

Evidence of Cometary Ice

Tempel Alive With Light

Older comets have crusts of inert material left by sublimation.

Asteroids and comets to scale

Comet Nuclei

3. ASTEROIDS

The Last Global Rotation Movie

Mission to Eros

Eros: The Final Approach

Asteroid Light Curves

An Asteroid Rotation Factory

Diameter vs Rotation Rate

Many asteroids may be rubble piles: loose collections of fragmented rock held together by self-gravity.

Spin Limits of Solar System Bodies

Diameter vs Rotation Rate

Many asteroids may be rubble piles: loose collections of fragmented rock held together by self-gravity.

Spin Limits of Solar System Bodies

Giant Asteroids

Vesta

Hubble Reveals Huge Crater on the Surface of the Asteroid Vesta

Computer Model

530 km

Largest Asteroid May Be 'Mini Planet'

Ceres975 km

Large asteroids are complex objects which appear to have differentiated.

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

POW!

Fragments are scattered on similar orbits.

Large Asteroid Breakup — Don Davis

Suspected Asteroid Collision Leaves Odd X-Pattern of Trailing Debris

A Suspected Asteroid Collision

Wikipedia: Asteroid belt

Mars

Origin of Near-Earth Objects (NEOs)

Some fragments wind up on orbits which are resonant with Jupiter.

Their orbits grow more elliptical, finally entering the inner solar system.

Fireball Meteor Over Groningen

A meteor enters Earth’s atmosphere; a meteorite survives the fall.

4. METEORS & METEORITES

Samples From the Asteroid Belt

Primitive meteorites are old (4.6 Gyr); they are relics of the solar system’s formation.

Processed meteorites come from differentiated asteroids which were fragmented by collisions.

Element Abundances

The Solar System

Hubble Maps the Ancient Surface of Vesta

A fragment of Vesta

Surface Lavas

Crust

Magnesium-Sliicate Mantle

(Olivine)

Stony Irons?

Iron-Nickle Core

As smaller bodies in the early Solar System fall together, the asteroid agglomerates.

Heavier elements sink to the center as the asteroid heats. This forms a separate core,

mantle, and outer crust. Lava from the interior flows to the surface.

Occasional impacts with other bodies break off pieces of the crust, exposing

the mantle.

Key Stages in the Evolution of the Asteroid Vesta

Family Members

PR95-20 • ST ScI OPO • April 19, 1995 • B. Zellner (GA Southern Univ.), NASA

Origin of Processed Meteorites

Hubble Maps the Ancient Surface of Vesta

Large Asteroid Breakup — Don Davis

Chicxulub Impactor: A Possible Timeline

1. Baptistina parent body (170 km diameter) smashed ~160 Myr ago.

2. Fragment hits Moon, forming Tycho crater (110 Myr ago).

3. Fragment hits Earth, forming Chixulub (65 Myr ago).

Comets and Meteor Showers

Comets shed dust and debris which slowly spread out as they move along the comet’s orbit.

If the Earth encounters one of these trails, we get a meteor shower.

Meteor Stream

Breakup of a Comet

Raining Perseids

Perseid Meteor Shower

Forty Thousand Meteor Origins Across the Sky

Major Meteor Showers