Asteroids, Comets and Dust

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Asteroids, Comets and Dust
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Remnants of Our Solar Systems Formation

• Asteroids
• Meteorites
• Comets
• Dust
• Gas

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Planetesimals

• Material left over from the Suns formation and
  the construction of the planets are the building
  blocks of the planets called planetesimals which
  include
• Asteroids primarily fused rock and metal with
  some ices in the outer asteroid belt
• Comets primarily ices and dust grains with
  minor amounts rock and metal
• As well as materials in the form of
• Dust
• Gas
• Note the asteroid and comet planetesimals are
  now called Small Solar System Bodies (SSBS)

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Asteroids
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Asteroids

• Asteroids were formed in the inner solar system
  during the early solar system formation period
  following the Suns intense T-Tauri emission
• Collisional heating fused larger and larger
  fragments together by accretion
• Larger fragments pulled surrounding materials to
  build larger planetesimal asteroids
• Heating from impacts fused smaller asteroids, and
  created molten cores in larger asteroids

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Asteroids

• Fused and partially fused rock fragments became
  carbonaceous asteroids
• High-density molten fragments became metallic
  asteroids
• Primarily iron with some nickel

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Asteroids

• Asteroids types
• S-Type (Silicaceous)
• About 17 of asteroids
• Relatively bright and reflective
• Composition is metallic iron mixed with iron- and
  magnesium-silicates
• S-type asteroids dominate the inner asteroid belt
• M-type (Metallic)
• Relatively bright and reflective
• Composed mainly of metallic iron
• M-type asteroids inhabit the belt's middle region

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Asteroids

• Asteroids types
• C-type (Carbonaceous)
• More than 75 of known asteroids
• Very dark and non-reflective
• Solid composition similar to solar system makeup,
  except depleted in hydrogen, helium, and other
  volatiles
• C-type asteroids are found mainly in the belt's
  outer regions
• D-type
• Very dark and non-reflective
• Reddish in color

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Asteroids

• Distribution throughout solar system is dominated
  by gravitational forces from the largest planets
• Jupiter has collected and distributed most
  asteroids in regions of orbital stability or gaps
  relating to instability
• Earlier known as minor planets
• Largest asteroid named Ceres is a dwarf planet

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Asteroids

• Orbit resonances and Jupiters stability points
  create distinct patterns and gaps in asteroid
  distribution
• Mars generates small but significant stability
  boundaries in asteroid distribution near the
  planet
• Note Asteroids are also classified by their
  orbits, and by their reflected spectra

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Asteroid Distributions
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Asteroid Distributions
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Asteroid Distributions
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Asteroids

• Average shapes and sizes
• Asteroids, which are the remnants of the early
  solar system formation, reflect the conditions in
  the early inner solar system
• Size of these solid bodies varies between pebbles
  to irregular-shaped bodies several hundred km in
  diameter
• Ceres, the largest asteroid that has a mean
  diameter of 950 m
• Since asteroids are generally small, their
  gravitational pressure is not enough to force
  them into a spherical in shape

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Asteroids

• The asteroid explorer named Dawn, launched
  September 27, 2007 explored Vesta, the second
  largest asteroid, from July 2011 to September,
  2012
• Dawn was then sent to Ceres for a multi-year
  investigation of the dwarf planet beginning in
  February, 2015
• Dawn spacecraft is powered by three xenon ion
  engines

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Asteroid Examples
With the exception of Ceres, the asteroids are
irregular in shape And although the shapes vary,
all are irregular and cratered
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Asteroid - Impacts

• Impacts and cratering on Earth from comets and
  asteroids is roughly proportional to the object's
  mass since hyperbolic velocity is typically 20-40
  km/s
• Evidence of the type of impact object can be made
  by analysis of the crater rim or ejecta
• High levels of iridium are found in asteroid
  samples and debris
• Diameter Energy of impact Crater diameter
• (Megatons of TNT) (km)
• 20 m 5 0.2
• 100m 100 1.0
• 1 km 10,000 10
• 10 km 10,000,000 100

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Asteroid - Impacts

• Impact Examples
• Tunguska Siberia (1908)
• Comet/carbonaceous asteroid is estimated at 50 m
  diameter
• No crater was generated, which means that it had
  to be a weak (carbonaceous) meteorite or a comet
• 15 M ton energy equivalent
• Air burst explosion devastated 1,000 sq km of the
  surface

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Asteroid - Impacts

• Impact Examples
• Arizona Crater (50,000 years ago)
• 50 m iron/nickel meteorite
• 1.3 km crater
• 200 Megaton energy equivalent

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Asteroid - Impacts

• Impact Examples
• Chicxulub, Yucatan impact (65 M years ago)
• 13 km diameter asteroid
• 130 km crater diameter
• 100,000,000 Mton energy equivalent
• 70 species exterminated on Earth

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Meteorites
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Meteorites

• Meteorites are asteroid materials that survive
  atmospheric entry and can be found on the Earths
  surface
• Meteor visible event of cosmic debris entering
  Earths atmosphere
• Meteorite Cosmic material entering Earths
  atmosphere and surviving to reach Earths surface
• Meteoroid cosmic debris in space (asteroid or
  cometary material)

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Meteorites

• Meteorites are classified primarily by their
  composition similar to asteroids (silicate rock,
  metal, and chondrites)
• Chondrites (85 collected)
• Named after chondrules (spheres of silicate and
  dust materials) found within the solid rock
  bodies
• Achondrites (8 collected)
• Silicate rock, but no chondrules found within the
  solid body
• Iron (5 collected)
• Iron and nickel metal pieces from the interior of
  larger asteroids that formed metal cores

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Meteorites

• Stony-iron (1 collected)
• A mixture of iron-nickel metal and silicate rock
  that may have originated between the mantle and
  core regions of larger asteroids

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Meteorites

• Average composition
• Although the meteorites vary widely in their
  composition, the average is roughly the same
  composition as the inner solar system
• Chemical and physical changes are introduced by
  atmospheric heating and surface impacts

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Asteroids

• Average composition
• The average composition of meteorite samples
  represents inner-belt asteroid composition
• Oxygen       36.3
• Iron             25.6
• Silicon         18.0
• Magnesium 14.2
• Aluminum   1.3
• Nickel          1.4
• Calcium       1.3
• Sodium        0.6

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Comets
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Comets

• Comets and cometary debris is composed primarily
  of ices and dust (Si, C, Al, Fe, etc.)
• Gases are given off (evolved) by the comet
• Sublimation
• Generally a small loss because of the extremely
  low temperatures in space
• 20-50 K
• Solar heating
• Strong inside Earths orbit (1 AU)
• Rapidly age comets because of their porous
  structure

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Comets

• Often described as dirty snowballs
• White-blue-green ice compounds are mixed with
  brown-black-grey dust
• Dust grains include carbon compounds, silicate
  compounds, and iron compounds, including graphite
  which is black
• Most abundant ices are carbon dioxide, ammonia,
  methane, and water

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Comets

• Extend even beyond the heliosphere in a region
  called the Oort cloud
• Another extended region of the planetary disk
  contains more comets and dwarf ice planets called
  the Kuiper Belt

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Comets

• Distant solar system diagram in the horizontal
  plane
• Shown are the positions of objects with
  semi-major axes greater than 5 AU (orbital
  periods greater than 11 years) on 2008 January 1
• The orbits and positions of Earth, Jupiter,
  Saturn, Uranus, Neptune, Pluto, and comets Halley
  and Hale-Bopp are also shown
• Asteroids are yellow dots and comets are
  symbolized by sunward-pointing wedges
• The vernal equinox is to the right along the
  horizontal axis (X direction)

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Comets

• Top view of the distant solar system
• Shown are the positions of objects with
  semi-major axes greater than 5 AU (orbital
  periods greater than 11 years) on 2008 January 1
• The orbits and positions are shown for the
  trans-Neptunian objects Eris, Quaor, Sedna,
  Orcus, Hale-Bopp and others
• The vernal equinox is to the right along the
  horizontal axis (X direction)

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Comets

• Typical composition
• Spectral emission of the Deep Impact collision
  with comet Tempel 1 showing the distinct
  signature of water (as ice)
• Also present are several hydrocarbons, and carbon
  dioxide

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Trans-Neptunian Objects
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Trans-Neptunian Objects

• Recent discoveries of the icy bodies populating
  the outer solar system by the powerful Hubble
  Space Telescope and several new ground-based
  adaptive-optics telescopes have revealed hundreds
  of Pluto-like bodies roughly in the same plane as
  the major planets
• This region that extends beyond Neptune, called
  the Kuiper Belt after astronomer Gerard Kuiper,
  is populated with moon-like comets that resemble
  Pluto, but are generally much smaller

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Trans-Neptunian Objects

• The disk population is confined to a semimajor
  axis of about 55 AU, and clustered at specific
  positions with respect to integer fractions of
  Neptune's orbit period
• Not only does Neptune's gravitation direct the
  motion of these outer bodies, Neptune has
  stabilized many of these icy bodies inside or
  outside the orbital resonance positions
• Because of Neptune's fundamental role in
  shepherding these icy satellites, including
  Pluto, they are classified as Trans-Neptunian
  Objects (TNOs)

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Trans Neptunain Objects
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Dust
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Dust

• Small particles of material left over from the
  solar system formation, and from later collisions
  between planetesimals, are found throughout the
  planetary disk, but are less abundant closer to
  the Sun
• Continual outward pressure from solar radiation
  has pushed most of the small particles outward

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Dust

• Although particle and dust concentrations occur
  around the planets, collections of the dust and
  debris can also be found at the Lagrange
  stability points L4 and L5
• The nearest example is the Earth-Moon system that
  has both L4 and L5 regions populated by
  particles, but low in abundance
• Trojan asteroids, asteroids found in Jupiter's 
  L4-L5 stability points in the Sun-Jupiter system,
  are much larger than the dust in the Earth-Moon
  system because of Jupiter's much larger mass
• Dust accumulations can also be seen in a diffuse
  belt in the planetary plane known as the zodiacal
  belt

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Asteroid Distributions
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Jupiters Lagrange Stable Points
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Dust

• The most problematic material encountered by
  spacecraft are the most abundant particles (much
  larger than atoms and molecules)
• This is the dust and small debris common in
  Earth-orbit
• The damage is not catastrophic, but instead
  accumulates its erosive effects with time

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Dust

• Shielding from this small debris consists of the
  following
• Sensitive instrument or optics use mechanical
  shades or coverings
• Durable coatings are used for spacecraft surface
  integrity
• Thin bumper shielding is commonly used for
  pressurized spacecraft and spacecraft on comet
  flyby missions where particles the size of sand
  can be a serious problem
• A 10 mg sand grain traveling at 40 km/s has the
  destructive energy of a small-caliber bullet

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Dust

• Debris tracked in Low-Earth Orbit (LEO)

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Dust

• Orbital debris larger view showing outline of
  geostationary orbit

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Questions?