The Asteroid Belt. Composition and Classification. Where Different Asteroids are Found

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ISP 205 - Astronomy Gary D. Westfall Lecture 14

Asteroids

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The orbits of most of the asteroids lie between those of Mars and Jupiter

Q Asteroid belt

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More than 10,000 asteroids have well-determined orbits

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Asteroids 2410 and 4859 are named for the two of the

authors of our book, Morrison and Fraknoi

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There are about a million asteroids with a diameter greater than 1 km

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The largest asteroid is Ceres and was the first to be discovered in 1801

Q Diameter just under 1000 km

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The total mass of the asteroids sums to about the mass of the Moon

Q Many are probably missing from the original distribution

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Jupiter Mars

The Asteroid Belt

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The asteroids all orbit the Sun in the same direction as the planets

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The asteroid belt contains orbits with semimajor axes between 2.2 and 3.3 AU

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The asteroids are not particularly close together

Q Typical spacing is millions

of km

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The asteroids seem to group into families that have similar physical characteristics and probably resulted from

collisions between asteroid

Trojans

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Composition and Classification

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Asteroids are not all alike

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Some are very dark like a lump of coal

Q Reflectivity = 3% Q C class, most numerous

X Carbonaceous or carbon-rich

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Some reflect like the Moon

Q Reflectivity = 20%

Q S class, second most populous X Stony composition

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Some are very bright

Q Reflectivity = 60%

Q M class, much less numerous X Metal C 250 3.15 1854 Euphrosyne C 260 3.07 1899 Patientia M 265 2.92 1852 Psyche S 265 3.67 1804 Juno C 275 3.48 1866 Sylvia C 280 3.1 1868 Europa C 280 3.43 1861 Cybele C 310 3.18 1903 Davida C 310 3.06 1910 Interamnia C 410 3.14 1849 Hygeia * 510 2.36 1807 Vesta C 540 2.77 1802 Pallaa C 940 2.77 1801 Ceres Class Dia. (km) D (AU) Year Name

The Largest Asteroids

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Where Different Asteroids are Found

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The different classes of asteroids are grouped

together at different distances from the Sun

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Apparently the asteroids are still located near their birthplaces

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Vesta

, A Volcanic Asteroid

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Vesta is a very unusual asteroid

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Much brighter than other main belt objects

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Surface is covered with basalt

Q Indicates volcanism in spite of its small size

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Some meteorites have been found with compositions similar to Vesta

Q 4.4 to 4.5 billion years

old

Q Whatever process

created Vesta was early and short lived

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Hubble found a crater on Vesta deep enough to expose the mantle

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Asteroids Up Close

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To get to Jupiter, Galileo had to traverse the asteroid belt

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Galileo has close encounters with two main-belt

asteroids, Gaspra and Ida

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Gaspra is and S-type asteroid 16 km long

Q Cratering suggests it is 200 million years old

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Ida is a larger S-type asteroid 56 km in length

Q Cratering shows it is 1 billion years old

Q Ida has a satellite, Dactyl, whose orbit was used to

calculate the mass and hence the density of Ida

X 2.5 g/cm3, similar to primitive rocks

Portraits of Asteroids

Gaspra Ida

Galileo images of the small main-belt asteroid, Gaspra. The dimensions of

Gaspra are 16 x 11 x 10 km.

Asteroid Ida from Galileo images. Ida is 56 km in length.

As Close as it Gets

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One February 12, 2001 the NEAR (Near Earth

Asteroid Rendezvous) Shoemaker spacecraft landed on the surface of the asteroid Eros

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The Moons of Mars

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The moons of Mars, Deimos and Phobos, are

thought to be captured asteroids

Deimos Phobos

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The Trojans

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The Trojan asteroids are located far beyond the

main belt at about the same distance as Jupiter

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The Trojan asteroids are dark and sizable

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There is one group ahead and one group behind

Jupiter

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Asteroid in the Outer Solar System

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Asteroids exist with orbits that carry them far outside the orbit of Jupiter

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Chiron is one such asteroid

Q Diameter of 200 km

Q During closest approach to the Sun, brightened by a

factor of 2

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Pholus is another such asteroid

Q Ventures out past the orbit of Neptune Q Is the reddest object ever observed

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Earth Approaching Asteroids

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In 1989, a 200 m object passed with 800,000 km

of Earth and in 1994 a 10 m object passed with 105,000 km of Earth

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About 500 NEOs (near earth objects) are known

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The orbits of NEO are unstable

Q Will collide with terrestrial planet Q Will be ejected from the solar system

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We are naturally interested in NEOs since an

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Comets

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A comet is a relatively small chuck of icy material

that develops an atmosphere as it approaches the Sun

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Comets can develop tails

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Comets move with respect to the background stars

but are much more unpredictable than planets

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Comets are the best preserved, most primitive

material available in the solar system

Q May provide unique access to the material that formed

the planets 4.5 billion years ago

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Comets spend most of their lives very far away

from the Sun where it is very cold

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The Orbits of Comets

• Newton recognized that the orbits of comets were highly eccentric

• Edmund Halley published calculations in 1705 for the orbits of 24 comets and predicted that a particular comet would return in 1758

Q It did and was named Halley’s Comet

• Halley’s Comet last appeared in 1986 and was studied by several spacecraft

The Comet

’

s Structure

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When we see a comet, we see its temporary

atmosphere of gas and dust

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This material comes from the nucleus of the

comet

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The comet has

Q Nucleus (1-10 km) Q Coma (105 km)

Q Hydrogen envelope (107 km)

Q Ion tail (directly away from the Sun) Q Dust tail (away from comet’s motion)

Location and Origin of Comets

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Most comets exist in the Oort cloud

Q Huge spherical cloud surrounding the solar system Q Extends out to 50,000 AU

X The gravitational sphere of influence of the Sun Q Orbits are stable

Q Occasionally a comet will be perturbed and enter the

solar system

X Only then is a comet visble

Q About 1013 comets may exist, 1000 times the mass of

the Earth

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Comets also are found in the Kuiper cloud

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The Fate of Comets

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Comets spend nearly their entire life in the Oort

cloud at a temperature near absolute zero

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It a comet, enters the inner solar system then

several things can happen

Q May survive passing the Sun and return to the Oort

cloud

Q May hit the Sun or come so close that it is vaporized Q May interact with a planet

X Impact the planet

X Get speeded up and ejected from the solar system X Perturbed into an orbit with a shorter period

+ Comet will rather quickly end its life

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Comet Shoemaker-Levy 9

Comet Shoemaker-Levy 9 broken up into 21 pieces photographed by Hubble

Hubble photo showing the impact of fragment G

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Meteors

• Meteors are the result of solid particles entering the Earth’s atmosphere from space

• These particles vaporize in the atmosphere at heights of 80 to 130 km

• The typical bright meteor is produced by a particle with a mass less than 1 gram

Q No larger than a pea

• If a particle the size of a golf hits the atmpshere, a much brighter trail is created

Q Fireball

• If a bowling ball size object hits the atmosphere, it has a good chance of reaching the ground

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Meteor Showers

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Most of the meteors that strike the Earth can be

associated with specific comets Q Some visible some not visible

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A meteor shower consists of passing through the

debris of a comet

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These meteor showers seem to come from one

spot in the sky Q Radiant

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Meteor showers are often

designated by the constellation they seem to come from

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Major Annual Meteor Showers

1.4 Phaethon December 13 Geminid 33 Tempel-Tutlle November 16 Leonid 3 Encke October 31 Taurid 76 Halley October 20 Orionid 7 Giacobini-Zinner October 9 Draconid 105 Swift-Tuttle August 22 Perseid --Unknown July 30 Delta Aquarid 76 Halley May 4 Eta Aquarid 415 Thatcher April 21 Lyrid --Unknown January 3 Quandrantid Comet’s Period (Years) Associated Comet Date of Maximum Shower Name 22

Nature of Meteor Showers

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No shower meteor has ever reached the surface

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From the flight paths, one can deduce that the

particles are very light or porous

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Comet dust is apparently fluffy, inconsequential

stuff

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The most reliable meteor shower is the Perseid

shower (comes from the Perseus constellation on August 11)

Q One can estimate that total mass of of the particles in

the Perseid swarm is nearly a billion tons from the Swift-Tuttle comet

X Comet Swift-Tuttle was last seen in 1992 and is predicted to

return in 2126 and will have a close pass with Earth

Meteorites

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A meteor that survives its fall through the atmosphere is called a meteorite

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Hundreds fall on the Earth every year

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Meteorites do not come from comets

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First documented case in modern times was

recorded in 1803

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Meteorites are discovered in two ways

Q Observed meteorite falls Q Meteorite finds

X About 25 per year are found

X Antarctica is a fertile ground for finding meteorites

Ice cap collects over a large area and preserves the meteorites

Meteorite Classification

• Traditionally meteorites have been placed into three broad classes

Q Irons

X Nearly pure nickel-iron Q Stones

X Silicate or rocky Q Stony-irons

X Mixture of stone and metallic iron

1% 5% 1% Stony-irons 2% 42% 3% Irons 12% 1% 8% Differentiated stones 85% 51% 88% Primitive stones Antarctic Finds Falls Class

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Ages and Compositions of Meteorites

• Meteorites include the oldest and most primitive materials available for direct study

• Using radioactive dating, the average age of meteorites is between 4.54 ± 0.1 billion years

Q Usually taken as the age of the solar system (4.5 billions years)

• Meteorites almost certainly originate from asteroids

• Two famous meteorites (both fell in 1969)

Q Murchison (Australia)

X Carbonaceous. Contained complex organic molecules, amino acids Q Allende (Mexico)

X Contained material older than the solar system