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Chapter 12: Comets and Asteroids: Debris of the Solar System


Chapter 12: Comets and Asteroids: Debris of the Solar System Discovery of Asteroids Most asteroid orbits lie in the asteroid belt between Mars and Jupiter. – PowerPoint PPT presentation

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Title: Chapter 12: Comets and Asteroids: Debris of the Solar System

Chapter 12 Comets and Asteroids Debris of the
Solar System
Discovery of Asteroids
  • Most asteroid orbits lie in the asteroid belt
    between Mars and Jupiter.
  • Too small to be visible without a telescope.
  • First discovered when astronomers were hunting
    for a planet between Mars and Jupiter
  • 1st discovered in the 1801
  • Name Ceres
  • Distance from the Sun 2.8 AU
  • Discoverer Giovanni Piazzi
  • Followed in subsequent years by the discovery of
    other small planets in similar orbits
  • By 1890, more than 300 objects had been
  • More than 10000 asteroids now have well
    determined orbits.

Asteroid Nomenclature
  • Asteroids are given a number and a name
  • Names originally chosen from Greek/Roman
    goddesses other female names all names go!
  • Asteroids 2410, and 4859 named after Morrison and

Asteroid Census
  • Total number of asteroids in the solar system
    very large.
  • Must be estimated on the basis of systematic
    sampling of the sky.
  • Studies indicate there are 106 asteroids with
    diameters greater than 1 km!
  • Largest Ceres - Diameter 1000 km
  • Pallas, and Vesta Diameter 500 km
  • 15 more larger than 250 km.
  • 100 times more objects of 10 km size than 100 km.
  • Total mass of asteroids is less than the mass of
    the Moon

Asteroid Orbits
  • Revolve around the sun in west-to-east.
  • Most lie in or near the ecliptic.
  • Asteroid belt defined as region that contains all
    asteroids with semi-major axes 2.2 to 3.3 au.
  • Periods 3.3 to 6 years.
  • 75 of known asteroids in the main belt.
  • Not closely spaced typically gtmillion km
    between them.
  • Japanese astronomer K. Hirayama found in 1917
    that asteroids fall into families.

Asteroid Families
  • Groups with similar characteristics
  • Each family may result from explosion of larger
    body (most likely by a collision)
  • In a family, asteroids have similar velocities
  • Several dozen families are found.
  • Physical similarities between largest asteroids
    of given families.

Asteroid Physical Appearance
  • Majority very dark
  • Do not reflect much light.
  • Reflectivity 3-4.
  • Some
  • Sizable group
  • Typical reflectivity 15-20 (similar to Moon)
  • Few
  • Reflectivity 60
  • Understanding of the reasons for the above
    difference provided by spectral analysis.

Asteroid Classification - 1
  • Primitive bodies
  • Dark asteroids
  • Chemically unchanged since beginning of Solar
  • Composed of silicates with dark organic carbon
  • Ceres, Pallas, and most object in outer third of
    the belt.
  • Most primitive asteroids part of Class C
  • Where C stands for carbonaceous carbon-rich

Asteroid Classification - 2
  • Class S
  • S stands for Stony composition.
  • No dark carbons.
  • Higher reflectivity.
  • Most asteroids of this type believed to be also

Asteroid Classification - 3
  • Class M
  • M stands for metal
  • Identification difficult
  • Done by radar for the largest asteroids such as
  • Much less numerous
  • Suspected to originate from collision of a parent
    body that had previously differentiated.
  • Enough metal in 1-km M-type asteroid to supply
    the world with iron for a long period of time.
  • Mines in Sudbury, ON, Canada originate from
    collision with class-M asteroid.

Trojan Asteroids
  • Located far beyond main belt
  • 5.2 AU, nearly same distance as Jupiter
  • Unstable orbits because of Jupiter.
  • Two points on the orbit where asteroids can stay
  • 2 points make equilateral triangle with Jupiter
    and the Sun
  • Collectively called trojans (Homer Illiad)
  • Discovered 1906-
  • Several hundreds found.
  • Dark, primitive, appear faint, but are
    nonetheless sizeable.

Outer Solar System Asteroids
  • Many asteroids with orbits beyond Jupiter
  • Example
  • Chiron, just inside the orbit of Saturn, to
    almost the distance of Uranus
  • Pholus (1992) 33 AU, red surface, of unknown
  • Named after Centaurs (half horse, half human)
  • so named because these objects have some
    attributes of comets, and asteroids.
  • 1988, on closest approach to the Sun, Chirons
    brightness doubled, much like the comets, which
    contain abundant volatile materials such as water
    ice, or carbon monoxide ice.
  • Chiron is however much bigger than comets.

Earth-Approaching Asteroids
  • 1989 a 200-m object passed within 800000 km of
    the Earth.
  • 1994 a 10-m object passed 105000 km away.
  • Some of these objects have collided with the
    Earth in the past, some are likely to do so again
    in the future.
  • Referred to as Near-Earth Objects (NEOs)

Near Earth Objects (NEOs)
  • 640 NEOs larger than 1km located by the end of
  • Actual population more likely to be gt millions.
  • Unstable orbits
  • Fate
  • Collide with our planet and be destroyed
  • Be ejected from the Solar System
  • Probability of impact once every 100 million
  • None of the known NEOs will end up crashing into
    the Earth in the foreseeable future
  • Larger impacts likely to generate environmental
  • A good argument towards further investigation of

NEO observation
  • 5-km NEO Toutatis,
  • approached the Earth at 3 million km in 1992
  • less than 3 times the distance to the Moon
  • Radar images show it is a double object (two
    irregular lumps) 3 and 2 km objects squashed

Appearance of Comets
  • Observed since antiquity
  • Typical comets appear as rather faint, diffuse
    spot of light smaller than the Moon, and many
    times less brilliant.
  • Small chunk of icy material that develop an
    atmosphere as they get closer to the Sun.
  • As they get very close they may develop a
    faint, nebulous tail extending far from the main
    body of the comet.
  • Appearance seemingly unpredictable
  • Typically remain visible for periods from a few
    days to a few months.

Comet Orbits
  • Scientific study of comets dates back to Newton
    who first recognized their orbits are elongated
  • Edmund Halley (a contemporary of Newton)
    calculated/published 24 cometary orbits (1705).
  • Noted that the orbits of bright comets seen in
    1531, 1607, 1682 were quite similar and could
    be the same comet returning to the perihelion
    every 76 years. Predicted a return in 1758.
  • When the comet did appear in 1758, it was given
    the name Comet Halley.

Comet Halley
  • Observed/Recorded on every passage at intervals
    from 74 to 79 years since 239 B.C.
  • Period variations caused by Jovian planets
  • 1910, Earth was brushed by the comet tail.
    causing much public concern
  • Last appearance in our skies 1986.
  • Met by several spacecrafts
  • Return in 2061.
  • Nucleus approximately 16x8x8 kilometers.

Comet Census
  • Records exist for 1000 comets
  • Comets are discovered at an average rate of 5- 10
    per year.
  • Most visible only on photos made with large
  • Every few years, a comet appears that is bright
    enough to be seen with the naked eye.
  • Recent flybys
  • Comet Hyakutake, long tail, visible for about a
    month, March (1996)
  • Hale-Bopp (1997)

Comet Structure
  • nucleus relatively solid and stable, mostly ice
    and gas with a small amount of dust and other
  • coma dense cloud of water, carbon dioxide and
    other neutral gases sublimed off of 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 this is the most prominent
    part of a comet to the unaided eye
  • ion tail as much as several hundred million km
    long composed of plasma and laced with rays and
    streamers caused by interactions with the solar

Comet Structure
dust tail
Nucleus and Coma
  • Nucleus ancient ice, dust and gaseous core
  • nucleus has low gravity cannot keep dust and
    gas from escaping
  • Coma the bright head of the comet seen from
    the Earth.
  • The coma is a temporary atmosphere of gas and
    dust around the nucleus.
  • The coma is 100,000's of kilometers across

halley's nucleus
halley's coma
Ion Tail
  • Sun spews out charged particles, called the solar
    wind. The solar wind travels along solar magnetic
    field lines extending radially outward from the
  • UV sunlight ionizes gases in the coma. These ions
    (charged particles) are pushed by solar wind
    particles along magnetic field lines to form the
    ion tail millions of kilometers long.
  • The blue ion tail acts like a "solar" wind sock.
    The ion tail always points directly away from the
    Sun, because the ions move at very high speed.
  • When the comet is moving away from the Sun, its
    ion tail will be almost in front of it!
  • The blue color is mostly from the light emitted
    by carbon monoxide ions but other types of ions
    also contribute to the light. Since the gas is so
    diffuse, the observed spectrum is an
    emission-line spectrum.

Dust Tail and Hydrogen Cloud
  • The dust tail forms when solar photons collide
    with the dust in the coma. Ejected dust particles
    form a long, curved tail that lies slightly
    farther our from the Sun than the nucleus' orbit.
  • The dust tail has a yellow-white color from
    reflected sunlight. Both of the tails will
    stretch for millions of kilometers.
  • The dust tail curves gently away from comets
    head, because dust particles are more massive
    than individual ions. They are accelerated more
    gently by the solar wind and do not reach the
    same high speeds as ions.
  • The hydrogen cloud forms when water vapor in the
    jets from the nucleus is dissociated by solar UV
    into oxygen and hydrogen.
  • The hydrogen cloud can be tens of millions of
    kilometers across the largest things in the
    solar system!
  • All of this is coming from a dirty snowball the
    size of a city!

Stardust Mission
Origin and Evolution of Comets
  • Originate from very great distances
  • Aphelia of new comets 50000 AU
  • Clustering of aphelia first noted by Dutch
    astronomer Jan Oort (1950).
  • Oorts Comet Origin Model
  • Stars sphere of influence extends a little
    beyond 50000 AU or 1 LY
  • Objects in orbit about the Sun at this distance
    can be easily perturbed by passing Stars.
  • Some perturbed object take on orbits that bring
    them much closer to the Sun.
  • Reservoir of ancient icy objects from which
    comets are derived is called Oort Comet Cloud.

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Oort Cloud
  • Estimated 1012 comets in the Oort cloud.
  • 10 times this number of comets could be orbiting
    the Sun between the planets and the Oort cloud.
  • Such objects undiscovered because to small, to
    reflect sufficient light to be detectable at
    large distances, and because their stable orbit
    do not bring them closer to the Sun.
  • Total number of comets in the sphere of influence
    of our Sun could be of the order of 1013!
  • Represents a mass the order of 1000 Earths.

Kuiper Belt
  • Second source of comets just beyond the orbit of
  • First object discovered in 1992.
  • Diameter 200 km.
  • Period 300 years.
  • 60 objects found since then.
  • Share orbital resonance with Neptune two orbits
    completed for three by Neptune.
  • Nicknamed Plutinos for this reason.
  • Speculated that Pluto is the largest example of
    this group.
  • They may share a common origin.

Fate of Comets
  • Comets spend nearly all their existence in the
    Oort cloud or Kuiper belt
  • At a temperature near absolute zero.
  • As comet enters the inner Solar System, their
    life changes altogether!
  • If they survive the initial passage near the Sun,
    they return towards the cold aphelia and may
    follow a quasi-stable orbit for a while.
  • May impact the Sun
  • May be completely vaporized as they fly by the
  • May interact with a planet
  • Final impact
  • Speed up and ejection
  • Perturbed into an orbit of shorter period.
  • Each flyby the Sun reduces the size and mass of
    the nucleus of the comets.
  • Some comets end their life catastrophically by
    breaking apart.
  • Shoemaker-Levy 9 broke into 20 pieces when it
    passed close to Jupiter in July 1992.
  • Fragments of Shoemaker-Levy captured into a very
    elongated 2 year around Jupiter In 1994 the
    comet fragments crashed into Jupiter.

Comet Shoemaker-Levy 9
  • Recorded by HST-WFP in different wavelengths

Shoemaker-Levy 9 Impact Site
Sequence of Impact Sites
  • The rotation of Jupiter left a trail of impacts.
  • We see the debris left in the upper atmosphere.
  • Debris fields would cover Earth!

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Time Evolution of Debris Fields
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