Chapter 8 Vagabonds of the Solar System

What do you think?

• Were the asteroids a planet that was somehow destroyed?

• How far apart are the asteroids on average?

• Why do comets have tails?

• In which direction does a comet tail point?

• What is a shooting star?

Most asteroids orbit the Sun between Mars and Jupiter

In general, asteroids are small

Asteroids are found by looking for moving objects (streaks) in long exposure photographs

Asteroid Ida and its tiny moon, Dactyl

A Short History of Asteroids• 1000 km Ceres was discovered in 1801

• 600 km Pallas was discovered in 1802

• Juno and Vesta were discovered in the 19th Century -- all the rest in the 20th Century

• Officially, there are about 7000 known asteroids, most tiny and less than 100 km across, but as many as 100,000 might be out there

• Even 100,000 spread out over an 18,000,000 mile orbit means that they are rather rare

Was the asteroid belt once a planet that has since been destroyed?

• If all the asteroids were assembled into a planet, it would have a tiny diameter of only 1500 km, or about 12% Earth’s diameter.

• The combination of the pull of the Sun’s gravity and Jupiter’s gravity keeps the asteroids “stirred up” enough to keep anything from coalescing.

Jupiter’s gravity creates wide

gaps in the

asteroid belt

Asteroids exist outside the asteroid

belt

Asteroids exist outside the asteroid belt

• Trojan asteroids in front and behind Jupiter

• Apollo asteroids which cross Earth’s orbit about the Sun

• Kuiper asteroids (Kuiperoids) exist beyond the orbit of Neptune– these Kuiperoids might not be rocky asteroids

at all, but rather, icy comets

Comets seem to come from two possible places

• Oort Cloud– Reservoir of long period comets that might only

come through the solar system once in billions of years and can come from any direction

• Kuiper Belt– Reservoir of short period comets that come

through the solar system regularly and only come in along the plane of the ecliptic

The Kuiper Belt of comets spreads from Neptune out 500 AU from the Sun

Kuiper Belt Object 1993SC - these images were taken 4.6 hours apart

Comet Kohoutek and Comet West

Comets lack tails until they enter the inner solar system

Comets often have two tails:

a thin ION tail and a curving DUST tail

Anatomy of a comet

15 km long by 8 km

wideComet Halley nucleus

Comets don’t last forever

Fragmentation of Comet West shortly after passing near the Sun in 1976

(sequence of photos is from March 8 to March 24)

Comet orbits are altered by

gravitational interactions with

planets

Small rocky debris peppers the solar system

• meteors

• falling stars

• shooting stars

• bolides

• fireballseach are caused by small

rocks colliding with Earth’s atmosphere and heating up due to friction with the air

Meteorites are space debris that land intact

Meteorite Types

• Stony meteorites – look much like ordinary rocks

• Iron meteorites – heavy and composed on iron and nickel minerals

• Stony-iron meteorites– contain roughly equal amounts of rock and iron– rare ones are carbonaceous chondrites which have

never melted and contain amino acids - one of the building blocks of life

Impact craters and meteor showers mark remnants of space

debris on Earth

Arizona crater is some 50,000 years old

The 1908 Siberean Tunguska mystery provides evidence of catastrophic collisions

A large asteroid’s impact with Earth may well have killed off the dinosaurs

some 65 million years ago

What did you think?• Were the asteroids a planet that was somehow destroyed?

No, the gravitational pull from Jupiter prevented a planet from ever forming there.

• How far apart are the asteroids on average?The distance between asteroids averages ten million kilometers.

• Why do comets have tails?Gas and dust that evaporate from the comet nucleus are pushed away from the Sun by sunlight

and the solar wind.

• In which direction does a comet tail point?Comet gas tails point directly away from the Sun; comet dust tails make arcs pointing away

from the Sun.

• What is a shooting star? A shooting star is a piece of space debris plunging through the Earth’s atmosphere -- a meteor.

Self-Check1: Sketch the location of the asteroid belt in the solar system and

explain the nature and origin of the Kirkwood gaps.

2: Describe the relationships among meteoroids, meteorites, meteors, and meteor showers.

3: List the principal classes of meteorites; compare and contrast their compositions and frequencies in space and on Earth.

4: Compare and contrast asteroids, meteoroids, and comets in terms of orbital characteristics, chemical composition, size, and structure.