Impacting Bodies: Surviving Entry through Earths Atmosphere

1
Impacting Bodies Surviving Entry through
Earths Atmosphere
2
A Few Definitions

• Meteoroids
• Meteors
• Meteorites
• Bolides

3
Once in the Atmosphere

• The atoms and molecules of the Earths atmosphere
  strike like a hot stream of gas
• The air pushes atoms away from the surface of the
  impacting body
• Molecular bonds begin to break
• Kinetic energy ? thermal energy

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Once in the Atmosphere/Contd/

• The impacting body becomes incandescent
• The surface becomes a liquid
• Atoms lose electrons ? plasma
• Eventually body turns into very hot vapor/gas
• Temperature may rise up to 50,000K (90,000 F)

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Descending into atmosphere

• Density of atmosphere increases rapidly (doubles
  for each 5 km of altitude lost)
• Pressure of hot air decelerates body
  (deceleration increases with the square of
  velocity)
• Terminal speed is often reached, as body cannot
  overcome the atmospheric drag
• Increasing T and pressure tend to flatten and
  spread the meteor (i.e. crash it into tiny powder)

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Meteor Storms

• Produced by encounters of Earth with dense dust
  swarms
• Swarms are about the size of a grain of sand
• The grains become incandescent
• Vaporize when they collide with the earths
  atmosphere at huge speed
• Examples Leonids, Andromendids, Taurids, etc

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Leonids
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Factors for survival

• Size Larger the size, deeper the penetration
  into the atmosphere
• Suitable composition Nickel, Iron
• Density High _at_ 8 times that of water
• Entry Speed Low, 11 km/sec being the lowest

• Entry Angle Shallow, if its too steep the
  object will burn up, if its too shallow the
  object will bounce off the top of the atmosphere
• Area-to-Mass-Ratio Objects with larger ratio
  cool more easily, greater chance to survive

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The shape factor

• The slender, low-drag body from fig a) would
  experience more severe heating and deceleration
  loading than the blunt body in figure b). If,
  however, the latter body were oriented in the
  position shown in figure c), a lift force would
  be developed and it would assume a more shallow
  path of descent with less heating and loading.

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Which One Will Survive??

• 40-meter asteroid
• Solid nickel composition
• Same mass as comet
• Might reach the ground
• intact

• 100-meter cometary object
• Icy Composition
• Density ½ that of water
• Would blow up at a high
• altitude

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Reaching the Surface Intact

• A 50-meter asteroid has a 1 chance of reaching
  the surface intact
• A 100-meter asteroid has a strong chance of a
  cratering event
• We expect that a high velocity object (60-70
  km/sec) is less likely to survive to the surface
  than a low speed object (12-15 km/sec)
• Particles smaller than 0.1 millimeters in size
  (100 microns) gradually reach the surface,
  typically we see those 10-100 microns in size

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Reaching the Surface Intact/contd/

• An icy projectile with a size less than 20-meters
  might be expected to survive to the ground if it
  has a very low entry speed
• Similarly, a rocky body smaller than about
  8-meters would survive if it had a low entry
  speed
• Higher speeds require an object to be larger in
  size to survive entry through the atmosphere

• More detailed analysis implies that minimum sizes
  of about 50-meters for rocky bodies and
  100-meters for icy ones
• Smaller, metallic asteroids can penetrate the
  atmosphere, but these comprise only 3 of the
  impactor flux

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What About Venus??

• Earths atmosphere prevents objects roughly less
  than 100-meters in size from reaching the ground
• Venus has an atmosphere that is dense, with a
  surface pressure 100 times that of Earth
• Objects smaller than 1 kilometer do not penetrate
  to the surface
• Not many particles reach the surface because of
  the temperature, pressure (100 bars), carbon
  dioxide atmosphere, and the sulfuric acid
  droplets in the clouds

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The Separated Fragments Model

• Computer simulation developed by researchers
  Bland and
• Artemieva to predict the likelihood of asteroids
  with a diameter
• up to one kilometer exploding in the atmosphere
  or hitting Earths
• surface
• Pancake Model
• -treats the cascade of asteroid fragments as a
  single
• continuous liquid that spreads out over a larger
  area, to form a
• pancake
• New Model
• -more realistic, calculates motion, aerodynamic
  loading, and
• ablation (dissipation of heat generated by
  atmospheric friction), for
• Each separated fragment in a disrupted impactor

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The Experiment

• Performed over 1,000 simulations using both
  models
• Consisted of 16 simulations for stony impactors
  and 16
• for iron impactors, for bodies from 1 to 108 kg,
  repeating
• each simulation for a given mass more than 20
  times to
• derive average impact conditions

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The Results

• Comparing model outputs, both gave similar
  estimates of total surviving material at the
  surface for irons, but the same was not true for
  stones
• Pancake model signifcantly overestimated
  impactor
• survivability for stones over the whole mass
  range
• Simulations also allowed for the determination
  of impact
• survivability rates for iron and stone, which
  corresponded closely with crater records and
  meteorite data

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• In meteorite falls, the proportion of stones
  decreases steadily at
• higher masses
• In the range of 4e4 to 2e8 kg, less than 5 of
  terrestrial impactors
• are stones
• Stony asteroids need to be 1,000 times larger
  than the iron ones
• to enter the atmosphere and make a similar sized
  crater.
• Impactor fragments at least 3 meters across that
  are capable of forming a crater 100 meters wide
  will strike the Earth once every 200 to 400
  years, with more than 95 being made of iron

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The Importance of the Separated Fragments Model

• Suggests that Earth is not as vulnerable to
  extraterrestrial impacts as previously thought
• Similar simulations may provide insight on the
  cratering rates on other celestrial objects with
  atmospheres, such as Venus or Mars

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Historically Very few meteoroids make it
through the atmosphere of earth and impact the
ground. Everyday over 100 tons of meteoroids
come down through the atmosphere toward earth.
Most days we do not even see the tracks of this
debris, but a few times in the past they have
made it through the atmosphere and caused
significant damage. The three main historical
meteor incidents that will be looked from oldest
to most recent are the Chicxulub cater near the
Yucatan Peninsula, the Barringer Meteor in
Arizona, and the Tunguska event in Siberia.
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Chicxulub Impact

• The Chicxulub impact near the Yucatan is now
  thought to have possibly caused the extinction of
  dinosaurs 65 million years ago. It is believed
  that the meteoroid was about 10 km in diameter
  and made a grater roughly 100 km in diameter.
  This meteor was so incredibly large and massive
  that the atmosphere had little effect on it. It
  collided with earth with a force greater than any
  human can imagine. An object of this size is
  estimated to collide with earth about every
  50-100 million years.

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Barringer Meteor

• The Barringer Meteor while much less massive
  then the Chucxulub once was much more recent,
  being dated back to about 50,000 years. It was
  about 50 m in diameter, large enough to cause
  significant damage. An asteroid this size hits
  earth about once every 1000 years.

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Tunguska Event

• On June 30th, 1908 at about 720 A.M., an
  explosion occurred over the Tungus River in
  Siberia. The explosion was so massive that it
  was light at night in London for the next few
  nights. This explosion is thought to be a meteor
  exploding above the ground.
• While the Tunguska meteoroid was roughly the
  same size as the Barringer one, it had a
  different effect. It exploded about 6-10 km
  above Earths surface due to many variables, such
  as velocity, material, mass, and entry angle.
  Some scientists believe that the meteor was stony
  and not made of nickel. In tests done after, in a
  similar scenario, stony asteroids of the same
  mass, velocity, diameter, and angle would explode
  at about 9 km altitude. Others are skeptics and
  believe that it was a UFO, or an Anti-matter
  explosion, not a meteor. Lets not pay attention
  to these theories since we are astronomers not
  quacks.

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Tunguska Event /contd/

• The Tunguska Event played an important role in
  modeling for modern day scientists and
  understanding how meteoroids will approach and
  react with the surface of earth. It thought that
  most meteors with less than a 100 m diameter will
  not strike the Earth with the exception of a few
  heavy metal ones. While the Tunguska Event
  exploded in the atmosphere, a ground collision
  would have produced much more energy and
  devastation.
• There is evidence that there was much radiation
  released from the Tunguska Event and a comet or
  and asteroid exploding should not cause this, how
  did this happen?
• We think of the atmosphere as our shield,
  should we rely on it that much?