Chapter 14: Solar System Debris Asteroids, Comets, and Meteorites

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Chapter 14 Solar System DebrisAsteroids,
Comets, and Meteorites
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Solar System Debris

• Debris
• (1) scattered remains of something
  broken or destroyed
• (2) accumulation of fragments of rock.
• Solar system
• 1 large object
• Sun
• several medium-sized objects
• planets and moons
• lots of debris
• asteroids, meteoroids, comets, dust

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What is this debris?

• The classification of is a matter of minor
  controversy.
• Traditionally, the solar system has been divided
  into
• planets (the big bodies orbiting the Sun),
• their satellites (variously sized objects
  orbiting planets),
• asteroids (small dense objects orbiting the Sun)
  and
• comets (small icy objects with highly eccentric
  orbits).
• Unfortunately, the solar system has been found to
  be more complicated than this would suggest
• there are several small moons that are probably
  captured asteroids
• comets sometimes fizzle out and become
  indistinguishable from asteroids
• the Kuiper Belt objects and others like Chiron
  don't fit this scheme well
• Pluto/Charon system sometimes considered "double
  planet and like Kupier Belt objects.

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How much debris is out there and where is it?

• Most asteroids have been found orbiting the Sun
  between Mars and Jupiter, but they also share
  orbits with planets and cross into the inner
  solar system.
• Mass of all known asteroids
• Most comets have been found to orbit the Sun in
  two regions beyond the orbit of Neptune.
• Kupier Belt, 30-100 AU, never inside orbits of
  jovian planets
• Oort cloud, up to 50,000 AU from Sun
• Mass of all comets probably comparable to mass of
  terrestrial planets.

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Why Debris is Important?

• Early history of life on Earth pieced together
  from fossil record.
• To study early history of solar system, need
  cosmic fossils - materials that have remained
  relatively unchanged since the solar system was
  very young.
• planets melted, battered by impacts,
  tectonically active, and altered
• Look to smaller objects, asteroids,
  and comets for clues.

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Asteroids

• Asteroids are believed to be left over from the
  beginning of the solar system 4.6 billion years
  ago.

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Asteroids

• Rocky and metallic objects too small to be
  considered planets.
• Range in size from Ceres (diameter of 1000 km),
  down
  to objects a few centimeters or less across.
• Name asteroids, meaning "star-like", derives from
  the fact that they are more star-like in
  appearance than comets.

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Asteroids Discovery

• Too small be seen without a telescope.
• Ceres, largest of asteroids, orbiting Sun at 2.8
  AU was discovered first by Giovanni Piazzi in
  1801.
• He was searching for the missing planet predicted
  to be between Mars and Jupiter by Titius-Bode
  law.
• In next 6 years, three more objects found in
  region.
• Currently, more that 10,000 asteroids have
  well-determined orbits.
• Each given a number for order of discovery a
  name.
• Most orbits lie between those of Mars and Jupiter

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Asteroids Formation

• Believed to represent material left over from
  formation
  of solar system.
• Although sometimes suggested that asteroids are
  remains of a planet that was destroyed in a
  massive collision, it is more likely that they
  represent material that never coalesced into a
  planet.

• Highest concentration of asteroids in asteroid
  belt, the region lying between
  orbits of Mars and Jupiter.
• Likely that origin of the asteroid belt is linked
  to gravitational perturbation by Jupiter,
  which kept these planetisimals from coalescing
  into larger bodies.

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Asteroids Classification

• Asteroids are classified into types according to
  their
• spectra
• (and hence their chemical composition)
• and
• albedo.

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Asteroids Classes Compared

• S-type
• Appear bright (reflectivity 15-20)
• Predominately silicate materials
• C-type (e.g., Ceres and Pallas)
• Appear very dark (reflectivity 3-4)
• Carbonrich silicate materials
• composition thought to be similar to the Sun,
  
  depleted in hydrogen, helium, and
  other volatiles.
• M-type (e.g., Psyche)
• Relatively bright (reflectivity 10-18)
• Metals like iron and nickel
• Rare

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Vesta An Unusual Asteroid

• HST resolves features as small as 50 miles
  across, allowing astronomers to map Vesta's
  geologically diverse terrain.
• The surface is a complex record of Vesta's four
  billion-year history.
• Features include ancient lava flows, and a
  gigantic impact basin that is so deep, it exposes
  the asteroid's subsurface, or mantle.

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Asteroids Abundance Location

• Not all classes of asteroids are equally abundant
  and different classes of
  asteroids found at different distances from Sun.
• 17 S-type dominate inner belt region
• 75 C-type dominate outer belt region
• 8 other types found in middle belt region

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Known Asteroids
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Asteroids Orbits about the Sun

• Asteroids orbit the Sun in many regions at
  different distances.
• They are often grouped by characteristics of
  their orbits.
• Asteroid Belt between Mars and Jupiter
• Trojan asteroids co-orbital with Jupiter
• Apollo and Aten asteroids Earth-crossing
  asteroids
• Amor asteroids Mars-crossing asteroids
• Other asteroids

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The Asteroid Belt

• All orbit Sun in west-to-east direction (same
  as planets).
• Most orbits lie near plane of ecliptic.
• The asteroid belt defined as region containing
  asteroids with semi-major axes in the range of
  2.2-3.3 AU.
• Asteroids in belt take 3.3-6 years to orbit Sun.
• Contains 75 of known asteroids.
• Spacing of asteroids in belt several million
  km.
• Many classified into families - groups with
  similar orbital and physical characteristics .

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Asteroids Size and Location

• 100,000 asteroids lie in the asteroid belt.
• Asteroids differ from planets in both their
  orbits and their size.
• generally move on quite eccentric trajectories,
• few are 300 km in diameter, and most are far
  smaller (as small as 1/10 km across).
• Taken together, mass of known asteroids amounts
  to

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Asteroids View from Space

• Galileo flew by main-belt asteroids.
• Gaspra
• S-type
• 7 hour rotation period
• 16 x 11 x 10 km, irregular shape
• sparse crater count implies 200 million years old
• Ida
• larger S-type
• more heavily cratered, 1 billion years
  old
• satellite, Dactyl, 1.5 km diameter
• period 24 hours
• orbital distance 100 km
• Idas density 2.5 g/cm3

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Phobos

• Mars Global Surveyor images of Phobos

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Asteroids Orbital Characteristics

• The Asteroid Belt
• Most asteroids orbit in a zone between the orbits
  of Mars and Jupiter called the Asteroid Belt
  
• Ranges 2 - 3.5 A.U. from the Sun.
• About 5,000 orbits have been calculated,
  but 100,000 asteroids may exist.
• Orbits are elliptical but nearly circular and
  near the plane of the ecliptic.
• Their orbits are very similar to the planets'
  orbits.
• The Kirkwood gaps are found in the orbits of belt
  asteroids and are formed by Jupiter's strong
  gravitational influence.

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Orbits of other Asteroids
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Asteroids Orbital Characteristics

• Trojan Asteroids
• Found in the same orbit with Jupiter,
  but are 60o ahead
  and behind the planet.
• They are stable positions in Jupiter's orbit
  where the gravity of the Sun and Jupiter cancel.
  
• Such positions called Lagrange points.
• Apollo Asteroids
• Orbits cross Earth's orbit.
• About 50 known Apollo asteroids,
  but may be
  as many as 1,000.
• All potential "Earth-colliders".
• Eros is an example. It is about 30 kilometers
  across.

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Asteroids Orbital Characteristics

• Other asteroid groups
• A few stray asteroids have been found that lie
  completely outside of the asteroid belt.
• Chiron is the most famous example.
• Its orbit carries it between Saturn and Uranus.
• Chiron may actually be a dormant comet that has
  lost most of its volatiles.
• When it is closest to the Sun, a very diffuse
  atmosphere forms around it.
• If Chiron is a comet, it is the largest one known
  with a diameter of about 180 kilometers.

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Chiron

• Chiron is unusual
• has a detectable coma, indicating that it is a
  cometary body,
• over 50,000 times the characteristic volume of a
  comet, a
  size more commensurate with a large asteroid
• its curious orbit is unstable on time scales of a
  million years, indicating that it hasn't been in
  its present orbit long.

Mt. Wilson Observatory - California02 April
1995Charles Morris, TIE telescope
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Centaurs

• Chiron was the first of four bodies discovered so
  far with similar orbits and properties.
• These bodies have been designated Centaurs,
  after the race of
  half-man/half-horse beings from Greek mythology,
  in recognition of their dual comet/asteroid
  nature.
• It is believed that the Centaurs may be objects
  which have escaped from the Kuiper belt.

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Kirkwood Gaps

• Distribution of asteroid semi-major axes shows
  some prominent gaps caused by resonances with
  Jupiter's orbital motion. These are known as the
  Kirkwood Gaps .
• Asteroid in resonance with Jupiter receives a
  strong gravitational tug from planet each time
  they are close together. If asteroid's period is
  in a well-defined ratio with that of Jupiters,
  the effects reinforce each other.

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Near-Earth and Trojan Asteroids
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Trojan Asteroids

• Although most asteroids orbit in the main belt,
  2-3 A.U. from the Sun, an
  additional class of asteroids orbit at the
  distance of Jupiter and are called the Trojan
  asteroids.
• Trojan asteroids are locked into a 11 orbital
  resonance with Jupiter.
• Several hundred such asteroids are now known
  it is estimated that there may be a thousand or
  more altogether.

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Trojan Asteriods Lagrange Points

• There are exactly five places in the solar system
  where a small body can orbit the Sun in synchrony
  with Jupiter. These places are known as the
  Lagrange points of Jupiter's orbit.
• All five Lagrange points revolve around the Sun
  at the same rate as Jupiter.

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Near Earth Asteroids (NEAs)

• NEAs are asteroids that closely approach the
  Earth.
• Most are small, diameter

• Three groups
• Atens semi-major axes distance 0.983 AU
• Apollossemi-major axes 1.0 AU perihelion
  distance
• Amors perihelion distances between 1.017 and 1.3
  AU and only cross Mars orbit

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Earth-Crossing Asteroids

• Certain groups of asteroids have elliptical
  orbits that cross the orbit of Earth and other
  inner planets.
• About 300 asteroids are known to cross Earth's
  orbit.
• However, about 1500 unknown NEOs are estimated
  to exist.
• The good news is that none of the known asteroids
  will strike the Earth.
• The bad news is that we have discovered only a
  fraction of the total number of Earth-crossing
  asteroids, so there are many for which we do not
  know the orbit.

Animation of Aten asteriod
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Near Earth Asteroid Rendezvous

• NEAR spacecraft launched in 1996.
• Goals
• fly by Mathilde, C-type in main belt
• gravity measurements yield density of 1.6
  g/cm3
• match orbit with Eros, a C-type asteroid and
  largest of Earth-approaching asteroids
• map surface
• determine surface composition and density

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Close to EarthBinary Asteroids

• Binary asteroids -- two rocky
  objects orbiting about one another
  appear to be common in
  Earth-crossing orbits.  If one is ever found
  headed our way, it could be double trouble. 
• http//www.jpl.nasa.gov/releases/2002/release_2002
  _83.html
• Another near miss.
• http//www.jpl.nasa.gov/releases/2002/release_2002
  _79.html

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Falling Stars
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Meteoroids, Meteors, Meteorites

• Meteoroids are simply smaller versions of
  asteroids.
• may be chunks that have been broken off asteroids
  by impacts.
• Meteors are streaks of light across the sky
  caused by a meteoroid entering the Earth's upper
  atmosphere and burning up in the process.
• Sometimes called "shooting" or "falling stars".
• Typically, 5 or 6 meteors are visible per hour
  across the sky (sporadic meteors).
• Sometimes a portion of a large meteoroid may
  survive its passage through the atmosphere and
  reach the Earth's surface. This rock is called a
  meteorite.
• Meteorites provided astronomers with the first
  good estimate of the age of the Solar System.
  Radiometric dating of meteorites gives them an
  age of about 4.5 billion years.

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Meteorites

• Meteorites are bits of the solar system that have
  fallen to the Earth.
• most come from asteroids, including few are
  believed to have come specifically from Vesta
• a few probably come from comets
• a small number of meteorites have been shown to
  be of lunar (23 finds) or Martian origin (22).

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Types of Meteorites

• Iron
• Primarily iron and nickel similar to type M
  asteroids
• Stony Iron
• Mixtures of iron and stony material like type S
  asteroids
• Chondrite
• by far the largest number of meteorites fall into
  this class
• similar in composition to the mantles and crusts
  of the terrestrial planets
• Carbonaceous Chondrite
• similar to type C asteroids
• Achondrite
• similar to terrestrial basalts the meteorites
  believed to have originated on the Moon and Mars
  are achondrites

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Martian Meteor
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Meteor Showers

• At certain times of the year, many more meteors
  are observed to be radiating from a particular
  point in space.
• These so-called meteor showers are now known to
  be associated with comet orbits.
• When the Earth crosses the "dusty" trail of a
  comet, many more meteors per hour can be
  observed.
• There are several major meteor showers each year.

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Comet Debris
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Meteor Shower Radiant
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• Leonids, 1998

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Literature and Astronomy

• Meteor showers can be very impressive. Samuel
  Taylor Coleridge's famous lines from The Rime of
  the Ancient Mariner may have been inspired by the
  Leonid meteor shower that he witnessed in 1797.
• The upper air burst into life! And a
  hundred fire-flags sheen, To and fro they
  were hurried about! And to and fro, and in and
  out, The wan stars danced between.
• And the coming wind did roar more loud, And
  the sails did sigh like sedge And the rain
  poured down from one black cloud The Moon was at
  its edge .

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Meteorite Hunting
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Impact Sites
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Antarctica
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Meteor Craters
Barringer Crater
Chicxulub Crater
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The End?
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Predicted Annual Number of Meteorite Falls
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Educated guesses about consequences of impacts of
various sizesfrom 'The Impact Hazard', by
Morrison, Chapman and Slovic,

published in Hazards due to Comets and Asteroids
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Impact Energies

• Kinetic Energy 1/2 mv2
• m ?V
• 1 megaton 4 x 1016 joules

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Comets

• Comets small bodies made out of dust and ices
  ("dirty snowballs").
• The term "comet" derives from the Greek aster
  kometes, which means "long-haired star"---a
  reference to the tail.
• Since the observations of Tycho Brahe, comets are
  known to be members of the Solar System well
  beyond Earth's atmosphere.
• Most are on long elliptical orbits (perhaps
  parabolic in some cases) that take them from the
  outer reaches of the Solar System to the vicinity
  of the Sun.
• When they come near the Sun they are heated and
  emit gases and dust that are swept by the Solar
  Wind into the characteristic tail that always
  points away from the Sun.

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Comets

• Dirty snowballs
• Long term comets
• most in Oort cloud (up to 50,000 AU from Sun)
• normally orbit far from the Sun, very few enter
  planetary region of solar system
• highly elongated orbits
• not confined to ecliptic, all orbital
  inclinations
• prograde and retrograde orbits
• roughly uniform distribution
• Short term comets (periods
• most originate in region beyond Neptune called
  Kupier belt
• approximately circular, prograde orbits 30-100
  AU
• normally orbit outside jovian planets,
  occasionally kicked into inner solar system

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Parts of a Comet

• Comets have several distinct parts when near the
  Sun and active
• nucleus
• relatively solid and stable, mostly ice and gas
  with a small amount of dust and other solids
• coma
• dense cloud of water, carbon dioxide and other
  neutral gases sublimed from the nucleus
• hydrogen cloud
• huge (millions of km in diameter) but very sparse
  envelope of neutral hydrogen
• dust tail
• up to 10 million km long composed of smoke-sized
  dust particles driven off the nucleus by escaping
  gases
• most prominent part of a comet to the unaided eye
• ion tail
• as much as several hundred million km long
• composed of plasma laced with rays and streamers
  caused by interactions with the solar wind.

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Comet Tails

• Tails of bright comets can be 150 million
  kilometers (1 AU) in length, making them the
  "largest" objects in the Solar System.
• Many comets have two tails
• gas tail (or ion tail) composed of ions blown
  out of the comet away from the Sun by the solar
  wind, and
• dust tail composed of dust particles liberated
  from the nucleus as the ices are vaporized.

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Comet Orbits

• Comets interact gravitationally with the Sun and
  other objects.
• Their motion is also influenced to some degree by
  gases jetting out of them, so their orbits are
  not completely determined by gravity.
• Most comets orbits appear to be elliptical, or
  in some cases parabolic.
• The most common comets are called short-period
  comets that have only mildly elliptical orbits
  that carry them out to a region lying from
  Jupiter to beyond the orbit of Neptune. These
  are normally seen only with telescopes.
• Comets visible to the naked eye are rare and are
  thought to come from a great spherical cloud of
  cometary material surrounding the Solar System
  called the Oort Cloud.

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Comet Halley

• English astronomer Edmund Halley used Newton's
  new theory of gravitation to determine the orbits
  of comets from their recorded positions in the
  sky as a function of time.
• He found that the bright comets of 1531, 1607,
  and 1682 had the same orbits, and concluded that
  these were different appearances of the same
  comet.
• He used his calculations to predict the return of
  this comet in 1758.
• If one traces back in the historical records for
  recordings of bright comets and their positions
  in the sky, it can be concluded that Comet Halley
  has been observed periodically as far back as 240
  B.C.

Halley in 1910
Halley in 1986
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Comet Shoemaker-Levy 9

• In July of 1994, fragments of Comet
  Shoemaker-Levy 9 impacted the planet Jupiter. The
  points of impact could be observed by the Galileo
  spacecraft.

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Unique?

• Crater chain on surface of Callisto

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CONTOUR

• CONTOUR - Comet Nucleus Tour
• NASA Discovery mission.
• Planned launch July 1, 2002.
• Mission to visit two comets.
• Encke (3.2 year period inside orbit of Jupiter)
• Schwassmann-Wachmann 3
• Will intersect orbits when comets are near Sun
  and most active.

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Comets, Asteroids, and Meteors