isp 205 - astronomy gary d. westfall1lecture 14 asteroids the orbits of most of the asteroids lie...

1ISP 205 - Astronomy Gary D. Westfall Lecture 14

AsteroidsAsteroids• The orbits of most of the asteroids lie between those of

Mars and Jupiter Asteroid belt

• More than 10,000 asteroids have well-determined orbits• Asteroids 2410 and 4859 are named for the two of the

authors of our book, Morrison and Fraknoi• There are about a million asteroids with a diameter

greater than 1 km• The largest asteroid is Ceres and was the first to be

discovered in 1801 Diameter just under 1000 km

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

Many are probably missing from the original distribution


Jupiter

Mars

The Asteroid BeltThe Asteroid Belt• The asteroids all orbit the

Sun in the same direction as the planets

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

• The asteroids are not particularly close together

Typical spacing is millions of km

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

Trojans


Composition and ClassificationComposition and Classification• Asteroids are not all alike

• Some are very dark like a lump of coal Reflectivity = 3% C class, most numerous

Carbonaceous or carbon-rich

• Some reflect like the Moon Reflectivity = 20% S class, second most populous

Stony composition

• Some are very bright Reflectivity = 60% M class, much less numerous

Metal

Name Year D (AU) Dia. (km) Class

Ceres 1801 2.77 940 C

Pallaa 1802 2.77 540 C

Vesta 1807 2.36 510 *

Hygeia 1849 3.14 410 C

Interamnia 1910 3.06 310 C

Davida 1903 3.18 310 C

Cybele 1861 3.43 280 C

Europa 1868 3.1 280 C

Sylvia 1866 3.48 275 C

Juno 1804 3.67 265 S

Psyche 1852 2.92 265 M

Patientia 1899 3.07 260 C

Euphrosyne 1854 3.15 250 C

The Largest Asteroids


Where Different Asteroids are FoundWhere Different Asteroids are Found• The different classes of asteroids are grouped

together at different distances from the Sun

• Apparently the asteroids are still located near their birthplaces

Orbit of Mars Orbit of Jupiter


Vesta, A Volcanic AsteroidVesta, A Volcanic Asteroid• Vesta is a very unusual asteroid• Much brighter than other main belt objects• Surface is covered with basalt

Indicates volcanism in spite of its small size

• Some meteorites have been found with compositions similar to Vesta 4.4 to 4.5 billion years old Whatever process created

Vesta was early and short lived

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


Asteroids Up CloseAsteroids Up Close• To get to Jupiter, Galileo had to traverse the

asteroid belt

• Galileo has close encounters with two main-belt asteroids, Gaspra and Ida

• Gaspra is and S-type asteroid 16 km long Cratering suggests it is 200 million years old

• Ida is a larger S-type asteroid 56 km in length Cratering shows it is 1 billion years old Ida has a satellite, Dactyl, whose orbit was used to

calculate the mass and hence the density of Ida 2.5 g/cm3, similar to primitive rocks


Portraits of AsteroidsPortraits of Asteroids

QuickTime™ and aSorenson Video decompressorare needed to see this picture.


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 GetsAs Close as it Gets• One February 12, 2001 the NEAR (Near Earth

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

Picture taken by NEAR Shoemaker at 120 m. The vertical lines at the bottom resulted when the spacecraft

struck the surface while transmitting the picture

Picture taken by NEAR Shoemaker at 170 m.


The Moons of MarsThe Moons of Mars• The moons of Mars, Deimos and Phobos, are

thought to be captured asteroids


Deimos Phobos



The TrojansThe Trojans• The Trojan asteroids are located far beyond the

main belt at about the same distance as Jupiter

• The Trojan asteroids are dark and sizable

• There is one group ahead and one group behind Jupiter


Asteroid in the Outer Solar SystemAsteroid in the Outer Solar System• Asteroids exist with orbits that carry them far

outside the orbit of Jupiter

• Chiron is one such asteroid Diameter of 200 km During closest approach to the Sun, brightened by a

factor of 2

• Pholus is another such asteroid Ventures out past the orbit of Neptune Is the reddest object ever observed


Earth Approaching AsteroidsEarth Approaching Asteroids• 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

• About 500 NEOs (near earth objects) are known

• The orbits of NEO are unstable Will collide with terrestrial planet Will be ejected from the solar system

• We are naturally interested in NEOs since an encounter with one would be unpleasant


CometsComets• A comet is a relatively small chuck of icy material

that develops an atmosphere as it approaches the Sun

• Comets can develop tails• Comets move with respect to the background stars

but are much more unpredictable than planets• Comets are the best preserved, most primitive

material available in the solar system May provide unique access to the material that formed

the planets 4.5 billion years ago

• Comets spend most of their lives very far away from the Sun where it is very cold


The Orbits of CometsThe 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

It did and was named Halley’s Comet

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


The Comet’s StructureThe Comet’s Structure• When we see a comet, we see its temporary

atmosphere of gas and dust

• This material comes from the nucleus of the comet

• The comet has Nucleus (1-10 km) Coma (105 km) Hydrogen envelope (107 km) Ion tail (directly away from the Sun) Dust tail (away from comet’s motion)


Location and Origin of CometsLocation and Origin of Comets• Most comets exist in the Oort cloud

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

The gravitational sphere of influence of the Sun Orbits are stable Occasionally a comet will be perturbed and enter the

solar system Only then is a comet visble

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

• Comets also are found in the Kuiper cloud Flattened disk just outside the orbit of Pluto


The Fate of CometsThe Fate of Comets• Comets spend nearly their entire life in the Oort

cloud at a temperature near absolute zero

• It a comet, enters the inner solar system then several things can happen May survive passing the Sun and return to the Oort

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

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

Comet will rather quickly end its life


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

Hubble photo showing the impact of fragment G


MeteorsMeteors• 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

No larger than a pea

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

Fireball

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


Meteor ShowersMeteor Showers• Most of the meteors that strike the Earth can be

associated with specific comets Some visible some not visible

• A meteor shower consists of passing through the debris of a comet

• These meteor showers seem to come from one spot in the sky Radiant

• Meteor showers are often designated by the constellation they seem to come from


Major Annual Meteor ShowersMajor Annual Meteor ShowersShower Name Date of Maximum Associated Comet

Comet’s Period (Years)

Quandrantid January 3 Unknown --

Lyrid April 21 Thatcher 415

Eta Aquarid May 4 Halley 76

Delta Aquarid July 30 Unknown --

Perseid August 22 Swift-Tuttle 105

Draconid October 9 Giacobini-Zinner 7

Orionid October 20 Halley 76

Taurid October 31 Encke 3

Leonid November 16 Tempel-Tutlle 33

Geminid December 13 Phaethon 1.4


Nature of Meteor ShowersNature of Meteor Showers• No shower meteor has ever reached the surface• From the flight paths, one can deduce that the

particles are very light or porous• Comet dust is apparently fluffy, inconsequential

stuff• The most reliable meteor shower is the Perseid

shower (comes from the Perseus constellation on August 11) One can estimate that total mass of of the particles in

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

Comet Swift-Tuttle was last seen in 1992 and is predicted to return in 2126 and will have a close pass with Earth


MeteoritesMeteorites• A meteor that survives its fall through the

atmosphere is called a meteorite• Hundreds fall on the Earth every year• Meteorites do not come from comets• First documented case in modern times was

recorded in 1803• Meteorites are discovered in two ways

Observed meteorite falls Meteorite finds

About 25 per year are found Antarctica is a fertile ground for finding meteorites

Ice cap collects over a large area and preserves the meteorites


Meteorite ClassificationMeteorite Classification• Traditionally meteorites have been placed into three broad classes

Irons Nearly pure nickel-iron

Stones Silicate or rocky

Stony-irons Mixture of stone and metallic iron

Class Falls Finds Antarctic

Primitive stones 88% 51% 85%

Differentiated stones

8% 1% 12%

Irons 3% 42% 2%

Stony-irons 1% 5% 1%


Ages and Compositions of Ages and Compositions of MeteoritesMeteorites

• 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

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) Murchison (Australia)

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

Contained material older than the solar system Material formed by previous generations of stars

isp 205 - astronomy gary d. westfall1lecture 14 asteroids the orbits of most of the asteroids lie...

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