Asteroids

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Asteroids Comets
2
Debris of the Solar System

• Asteroids are rocky or metallic objects orbiting
  the Sun that are smaller than a major planet, but
  that show no evidence of an atmosphere and
  contain little volatile (easily evaporated)
  material
• Comets are icy bodies that revolve around the Sun
  and are smaller than a major planet, but that
  contain frozen water and other volatile materials

3
Discovery of Asteroids

• Most asteroid orbits lie in
  the asteroid belt, between
  Mars and Jupiter
• too small to be visible
  without a telescope
• They were first discovered
  when astronomers were
  hunting for a missing
  planet between Mars and Jupiter
• The 1st asteroid, named Ceres and initially
  thought to be the missing planet, was
  discovered by Giovanni Piazzi in 1801
• It orbits at 2.8 AU from the Sun
• The discovery of other minor planets in similar
  orbits followed in subsequent years
• Now, more than 20,000 asteroids are known to have
  well-determined orbits

4
Asteroid Nomenclature

• Asteroids are given both a
  number and a name
• The names were originally
  chosen from Greek/Roman
  goddesses, then other female
  names were used, and
  finally all names go!
• Asteroids 2410 and
  4859 are named
  after Morrison and
  Fraknoi

Gaspra
Ida
Mathilde
5
Asteroid Census

• The total number of asteroids in the solar system
  is very large
• It must be estimated on the basis of systematic
  sampling of the sky
• Studies indicate that there are 106 asteroids
  with diameters greater than 1 km!
• The largest is Ceres, with a diameter of 1000 km
• Pallas and Vesta have diameters of 500 km
• 15 more are larger than 250 km across
• There are 100 times more objects 10-km across
  than 100-km across
• The total mass of asteroids is less than the mass
  of the Moon

6
Asteroid Orbits

• All asteroids revolve around the Sun in
  west-to-east direction, like the planets
• Most of their orbits lie near the plane in which
  the Earth and the other planets circle
• The asteroid belt is defined as the region that
  contains all asteroids with semi-major
  axes in the range from
  2.2
  to 3.3 AU
• Their orbital periods range
  from 3.3 to 6 years
• 75 of known asteroids are
  in the main belt
• But they are not closely
  spaced

7
Asteroid Families

• Japanese astronomer Kiyotsuga Hirayama found in
  1917 that some asteroids fall into families
• The families are groups with similar orbital
  characteristics
• Each family may have resulted from a breakup of a
  larger body, or from the collision of two
  asteroids
• Members of each family have similar speeds
• There are physical similarities among the larger
  members of a given family
• Several dozen families are found

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Asteroid Physical Appearance

• The majority of asteroids are very dark
• They do not reflect much light
• Their reflectivities are only 3 to 4 percent
• There is, however, a sizable group that is not
  very dark
• Its typical reflectivities are 15 to 20 percent
  (similar that of the Moon)
• A few asteroids even have reflectivities as high
  as 60
• To understand the reasons for these differences,
  astronomers performed spectral analysis of the
  light reflected by asteroids

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Asteroid Classification (1)

• The dark asteroids
• are believed to be primitive bodies
• chemically unchanged since the beginning of the
  solar system
• are composed of silicates mixed with dark,
  organic carbon compounds
• include Ceres, Pallas, and most objects in the
  outer third of the asteroid belt
• Most of the primitive asteroids are classed as C
  asteroids
• C stands for carbonaceous (carbon rich)

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Asteroid Classification (2)

• The second most populous group is the S asteroids
  
• S stands for a stony or silicate composition
• They have no dark carbon compounds and hence
  higher reflectivities
• Most S asteroids seem to be also primitive
• The third group is the M asteroids
• M stands for metal
• Their identification is difficult
• done by radar for the largest asteroids such as
  Psyche
• They are much less numerous
• Each may have come from a parent body that had
  earlier differentiated and later shattered in a
  collision
• There is enough metal in a 1-km M-type asteroid
  to supply the world with iron for a long period
  of time

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Asteroid Classification (3)
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Trojan Asteroids

• The Trojans are located far beyond main belt
• at 5.2 AU from the Sun, nearly the same distance
  as Jupiter
• dark, primitive objects, like some other
  asteroids
• They have stable orbits because of Jupiter
• In Jupiters orbit, there are two points near
  which an
  asteroid can
  stay almost indefinitely
• They make equilateral
  triangles with Jupiter
  and the
  Sun
• Since their first discovery
  in 1906, several hundreds
  have been found
• The larger Trojans can be up to 200 km across

13
Animation the Trojans
Green circles indicate main-belt asteroids and
blue dots on the outermost circle are the Trojans
14
Asteroids in Outer Solar System

• There are asteroids with orbits that carry them
  far beyond Jupiter
• They are hard to detect and only a few have been
  found
• Examples
• Chiron is 200 km across and the largest of them,
  with a path carrying it from just inside the
  orbit of Saturn to almost the distance of Uranus
• Pholus, with an orbit that takes it 33 AU from
  the Sun, beyond Neptune, has the reddest surface
  of any object in the solar system, with unknown
  composition
• They are named after centaurs (mythological half
  horse, half human) because these objects have
  some of the properties of both comets and
  asteroids
• In 1988, on its closest approach to the Sun,
  Chirons brightness doubled, much like the comets
• Chiron, however, is much bigger than comets

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Earth-Approaching Asteroids

• Some asteroids may stray far outside the main
  belt and travel inward along paths that come
  close to or cross Earths orbit
• Such asteroids, and other objects that come close
  to the Earth, are collectively known as
  Near-Earth Objects (NEOs)
• Needless to say, they are of great interest to us
• Some of these NEOs have collided with the Earth
  in the past, and some others are likely to do so
  in the future
• In 1994, a 1-km object passed closer than the
  Moon
• By the end of 2002, more than 640 NEOs larger
  than 1 km in diameter had been discovered
• Astronomers have estimated that there are
  probably 500 or so NEOs larger than 1 km in
  diameter that have not yet been found

16
NEOs

• NEOs generally have unstable orbits
• Each NEO will meet one of 2 fates
• collide with one of the terrestrial planets and
  be destroyed
• be ejected gravitationally from the inner solar
  system after a near-encounter with a planet
• The probability for impact is once every 100
  million years
• Hence the likelihood is very remote than any one
  of the known NEOs will end up crashing into the
  Earth in the foreseeable future
• The larger of these impacts will likely generate
  environmental catastrophes for our planet
• This is a good reason for further investigation
  of NEOs

17
An NEO Observation

• A 5-km-long NEO called Toutatis
• approached to within 3 million km of the Earth in
  1992
• which is less than 3 times the distance to the
  Moon
• Radar images indicate that it is a double object
• consisting of 2 irregular lumps, with diameters
  of 3 km and 2 km, squashed
  together
• Animation

Radar images
18
Rendezvous with Eros

• Eros is an Earth-approaching S-type asteroid
• potato-shaped, 34 km long, and 11 km wide
• heavily cratered, suggesting that the surface is
  old
• Movies of Eros captured by the
  NEAR-Shoemaker spacecraft,
  which orbited and then landed
  on it in 2000

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Comets

• They have been observed since
  antiquity
• Typical comets appear as rather
  faint, diffuse spots of light
• smaller than the Moon and many
  times less brilliant
• They are small chunks of icy
  material that develop atmospheres
  as they get closer to
  the Sun
• As a comet gets very close to the Sun, the
  comet may develop a faint, nebulous tail
  extending far from the main body of the comet
• Their appearance is seemingly unpredictable
• Comets typically remain visible for periods from
  a few days to a few months

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Comet Orbits

• The scientific study of comets dates back to
  Newton who first recognized that their orbits
  were very elongated ellipses
• Edmund Halley (a contemporary of Newton) in 1705
  calculated/published 24 cometary orbits
• He noted that the orbits of bright comets seen in
  1531, 1607, and 1682 were quite
  similar and could belong to the
  same comet returning to the
  perihelion every 76 years
• He predicted a return in 1758
• When the comet did appear in 1758,
  it was given the name Comet Halley

21
Comet Halley

• It has been observed
  and recorded on every
  passage near the Sun at intervals from 74
  to 79 years since 239 B.C.
• The period variations are caused by the jovian
  planets
• In 1910 the Earth was brushed by the comets
  tail, causing much needless public concern
• Its last appearance in our skies was in 1986
• met by several spacecraft
• It is predicted to return in 2061
• Its nucleus is approximately
  16 x 8 x 8 km3

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Comet Census

• Records exist for 1000 comets
• Comets are discovered at an average rate of 5 to
  10 per year
• Most of them are visible only on photographs made
  with large telescopes
• Every few years, a comet may appear that is
  bright enough to be seen with the naked eye
• Recent flybys
• Comet Hyakutake, with a very long tail, was
  visible for about a month in March 1996
• Comet Hale-Bopp appeared in 1997

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Comet Components (1)

• Nucleus relatively solid and
  stable, composed mostly of
  ice and gas,
  with a small amount of
  dust and other solids
• Coma a dense cloud of water,
  carbon dioxide, and other neutral
  gases sublimed off of
  the nucleus
• Hydrogen cloud a huge (millions
  of km in diameter), but very
  sparse,
  envelope of neutral H gas
• 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
  wind

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Comet Components (2)
ion tail
dust tail
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Nucleus and Coma of Comet

• The nucleus is composed of ancient ice, dust, and
  gaseous core material
• The nucleus has low gravity
• It cannot keep dust and gas from escaping
• The coma is the bright head of the comet, as 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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Nucleus of Comet Wild 2
The images were captured by NASA's Stardust
spacecraft
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Comets Ion Tail

• The Sun spews out charged particles
• This solar wind occurs along the solar
  magnetic-field lines, extending radially outward
  from the Sun
• Ultraviolet (UV) sunlight ionizes gases in the
  coma
• These ions (charged particles) are pushed by
  solar-wind particles along field lines to form a
  tail millions of km long
• The blue ion tail acts like a "solar" wind-sock
• The 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, the
  
  comets 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

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Origin and Evolution of Comets

• Comets originate from very great distances
• The aphelia of new comets are typically 50,000
  AU
• This clustering of aphelia was first noted by
  Dutch astronomer Jan Oort in 1950
• He then proposed an idea for the origin of those
  comets, which is still accepted by most
  astronomers today
• Oorts model of comet origin
• The Suns sphere of influence extends only a
  little beyond 50,000 AU, or about 1 LY
• Objects in orbit about the Sun at this distance
  can be easily perturbed by the gravity of passing
  stars
• The comets are some of the perturbed objects,
  which take on orbits that bring them much closer
  to the Sun
• The reservoir of ancient icy objects from which
  such comets are presumably derived is called the
  Oort comet cloud

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Oort Comet Cloud

• Astronomers estimate that there may be about a
  trillion (1012) comets in the Oort cloud
• In addition, 10 times this number of comets could
  be orbiting the Sun between the planets and the
  Oort cloud
• Such cometary objects remain undiscovered
  probably because they are too faint to be seen
  directly and because their stable orbits do not
  bring them closer to the Sun
• The total number of comets within the sphere of
  influence of our Sun could therefore be on the
  order of ten trillion (1013)!
• Their total mass would be similar to that of 1000
  Earths
• Cometary material could thus be the most
  important constituent of the solar system after
  the Sun itself

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Kuiper Belt

• Another possible source of comets lies just
  beyond the orbit of Neptune
• The existence of this region was first suggested
  by Gerard Kuiper in 1951
• The first object from this region, now called the
  Kuiper belt, was discovered in 1992
• The object is 200 km across
• Since then, several hundred more Kuiper-belt
  objects (KBOs) have been found
• It appears that these KBOs are heavily influenced
  by the gravity of Neptune
• Many of the known KBOs have orbits like that of
  Pluto
• Some astronomers have, therefore, suggested that
  Pluto can be considered the largest member of the
  Kuiper belt
• For this reason, KBOs are sometimes called
  plutinos

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Fate of Comets

• Most comets probably spend nearly all their
  existence in the Oort cloud or Kuiper belt
• at a temperature near absolute zero
• But once a comet enters the inner solar system,
  its life likely changes dramatically!
• If it survives the initial passage near the Sun,
  it will return towards the cold aphelion
• and may follow a fairly stable orbit for a
  while
• It may impact the Sun, or be completely vaporized
  as it flies by the Sun
• It may interact with one or more planets with 3
  possible fates
• destroyed after impacting a planet
• speeded up and ejected, leaving the solar system
  forever
• perturbed into an orbit of shorter period
• Each time it approaches the Sun, a comet loses
  part of its material
• It may end its life catastrophically by breaking
  apart

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Comet Shoemaker-Levy 9

• Some comets die very spectacularly
• When comet Shoemaker-Levy 9 passed close to
  Jupiter in July 1992, the comet broke into about
  20 pieces
• perhaps due Jupiters tidal forces
• Fragments of the comets then orbited Jupiter
  until July 1994 when they
  crashed into Jupiter,
  experiencing violent
  destruction
• Animation