Comets_ Asteroids_ and Meteors

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					Chapter 14: Solar System Debris
  Asteroids, Comets, and Meteorites
       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
                  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
   – 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.
     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 < 1/10 mass of Moon.
• 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.
 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.
Asteroids are believed to be left over from the beginning
         of the solar system 4.6 billion years ago.
 • 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.
            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
               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.
  Asteroids: Classification

Asteroids are classified into types according to their
      (and hence their chemical composition)
     Asteroids: Classes Compared
                                                     Gaspra: S-type asteroid
 – Appear bright (reflectivity 15-20%)
 – Predominately silicate materials
•C-type (e.g., Ceres and Pallas)
 – Appear very dark (reflectivity 3-4%)
 – Carbon–rich 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                                             Mathilde: C-type asteroid
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.
  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
Known Asteroids
  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
  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 .
        Asteroids: Size and Location
• > 100,000 asteroids lie in the
  asteroid belt.
• Asteroids differ from planets
  in both their orbits and their
  – generally move on quite eccentric
  – 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
     < 1/10 mass of Moon.
        Asteroids: View from Space
•Galileo flew by main-belt asteroids.
   • S-type
   • 7 hour rotation period
   • 16 x 11 x 10 km, irregular shape
   • sparse crater count implies 200 million years old
   • larger S-type
   • more heavily cratered,
     ~1 billion years old
   • satellite, Dactyl, 1.5 km diameter
      – period = 24 hours;
      – orbital distance = 100 km
      – Ida’s density ~ 2.5 g/cm3

Mars Global Surveyor images of Phobos
 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.
Orbits of other Asteroids
 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.
 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.
• 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 - California
                               02 April 1995
                      Charles Morris, TIE telescope

• 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.
• Distribution of
  asteroid semi-major
  axes shows some           Kirkwood Gaps
  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 Jupiter’s, the
  effects reinforce each
Near-Earth and Trojan Asteroids
                   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
• Trojan asteroids are
  locked into a 1:1 orbital
  resonance with Jupiter.
• Several hundred such
  asteroids are now known;
  it is estimated that there
  may be a thousand or
  more altogether.
  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.
      Near Earth Asteroids (NEAs)
• NEAs are
  asteroids that
  closely approach
  the Earth.
• Most are small,
  diameter < 1 km

•Three groups:
  • Atens: semi-major axes <1.0 AU; aphelion distance >0.983 AU
  •Apollos:semi-major axes >1.0 AU; perihelion distance <1.017 AU
  •Amors: perihelion distances between 1.017 and 1.3 AU and
           only cross Mars’ orbit
               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
  NEO’s are estimated to exist.
• The good news is that none of the
  known asteroids will strike the
• 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
   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-          Mathilde and Rhode Island
  approaching asteroids
   • map surface
   • determine surface composition
      and density

   Close to Earth:Binary 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.

Another near miss.
‘Falling Stars’
Meteoroids, Meteors, & Meteorites
• Meteoroids are simply smaller versions of asteroids.
   – may be chunks that have been broken off asteroids by
• 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.

• 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).
• Iron
                   Types of Meteorites
   – 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

      Chondrite                        Iron                     C. Chondrite
Martian Meteor
                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
• There are several major
  meteor showers each year.
Comet Debris
Meteor Shower Radiant
Leonids, 1998
        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 .
Meteorite Hunting
Impact Sites
         Meteor Craters

Barringer Crater
                     Chicxulub Crater
The End?
  Predicted Annual Number of
         Meteorite Falls
 AREA       0.1 kg   1.0 kg   10 kg

 106 km2     39        8       2

             920      190      38
Land area
            5,800    1,200    240
 of Earth
            19,000   4,100    830
 Educated guesses about consequences of impacts of various sizes:
                from 'The Impact Hazard', by Morrison, Chapman and Slovic,
                      published in Hazards due to Comets and Asteroids
Impactor Diameter        Yield           Interval
    (meters)           (megatons)         (years)
      < 50                < 10             <1       meteors in upper atmosphere;
                                                    most don't reach surface

       75                                           irons make craters like Meteor Crater;
                         10 - 100         1000      stones produce airbursts like Tunguska;
                                                    land impacts destroy area size of city
                                                    irons,stones hit ground;
                                                    comets produce airbursts;
      160               100 - 1000        5000
                                                    land impacts destroy area size of large    urban
                                                    area (New York, Tokyo)
                                                    land impacts destroy area size of small state;
      350             1000 - 10,000      15,000
                                                    ocean impact produces mild tsunamis
                                                    land impacts destroy area size of moderate
      700            10,000 - 100,000    63,000     state (Virginia);
                                                    ocean impact makes big tsunamis

      1700          100,000 -1,000,000   250,000    land impact raises dust with global implication;
                                                    destroys area size of large state
                                                    (California, France)
Impact Energies
 • Kinetic Energy : 1/2 mv2
         • m = V
• 1 megaton = 4 x 1016 joules
• 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.
• 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 < 200 years)
   – 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
                    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
• 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.
                       Comet Tails
• Tails of bright comets can be
  150 million kilometers (1 AU)
  in length, making them the
  "largest" objects in the Solar
• 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
                          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.
                          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
         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.

Crater chain on surface of   Crater chain on surface of
          Callisto                Earth’s Moon

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.
Comets, Asteroids, and Meteors

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