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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
2ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
3ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
4ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
5ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
6ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
7ISP 205 - Astronomy Gary D. Westfall Lecture 14
Portraits of AsteroidsPortraits of Asteroids
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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.
8ISP 205 - Astronomy Gary D. Westfall Lecture 14
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.
9ISP 205 - Astronomy Gary D. Westfall Lecture 14
The Moons of MarsThe Moons of Mars• The moons of Mars, Deimos and Phobos, are
thought to be captured asteroids
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
Deimos Phobos
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
10ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
11ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
12ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
13ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
14ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
15ISP 205 - Astronomy Gary D. Westfall Lecture 14
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)
16ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
17ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
18ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
19ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
20ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
21ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
22ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
23ISP 205 - Astronomy Gary D. Westfall Lecture 14
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
24ISP 205 - Astronomy Gary D. Westfall Lecture 14
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%
25ISP 205 - Astronomy Gary D. Westfall Lecture 14
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