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

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Chapter 12 Comets andAsteroids Debris of the
Solar System
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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
  discovered.
• More than 10000 asteroids now have well
  determined orbits.

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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
  Fraknoi

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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

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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.

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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.

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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.

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Asteroid Classification - 1

• Primitive bodies
• Dark asteroids
• Chemically unchanged since beginning of Solar
  System
• Composed of silicates with dark organic carbon
  compounds
• 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

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Asteroid Classification - 2

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

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Asteroid Classification - 3

• Class M
• M stands for metal
• Identification difficult
• Done by radar for the largest asteroids such as
  Psyche.
• 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.

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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
  indefinitely.
• 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.

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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
  composition.
• 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.

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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)

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Near Earth Objects (NEOs)

• 640 NEOs larger than 1km located by the end of
  2002
• 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
  years.
• None of the known NEOs will end up crashing into
  the Earth in the foreseeable future
• Larger impacts likely to generate environmental
  catastrophes
• A good argument towards further investigation of
  NEOs.

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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
  together.

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Comets
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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.

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Comet Orbits

• Scientific study of comets dates back to Newton
  who first recognized their orbits are elongated
  ellipses.
• 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.

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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.

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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
  telescopes.
• 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)

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Comet Structure

• 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 off of the nucleus
• hydrogen cloudhuge (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 tailas much as several hundred million km
  long composed of plasma and laced with rays and
  streamers caused by interactions with the solar
  wind

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Comet Structure
dust tail
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Nucleus and Coma

• Nucleus ancient ice, dust and gaseous core
  material
• 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
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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
  Sun.
• 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.

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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!

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Stardust Mission
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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.

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Kuiper Belt

• Second source of comets just beyond the orbit of
  Pluto.
• 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.

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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
  Sun
• 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.

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Comet Shoemaker-Levy 9
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Impact!

• Recorded by HST-WFP in different wavelengths

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Shoemaker-Levy 9 Impact Site
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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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