Impact Hazards
Can we predict impacts?
• Incomplete inventory of objects
– May be a million km-sized objects
• Initial observations don't permit completely
accurate predictions
• Comets vent gases and change orbits
• The meaning of probability of impact
– Planets don’t “wander”
– Observational uncertainty
Example, Measuring A Lot
• You measure the lot 5 times, getting 99.7,
99.9, 100.1, 100.0 and 100.3 feet.
• Average = 100
• Best estimate but might not be true value
• Any random measurement has even odds
of being too high or low
• P All 5 too high or low = (1/2)5 = 1/32
• P 4 too high or low = 5/32
• P 3 too high or low = (5*4/2)/32 = 10/32
Impact Probability
Impact Probability
The Torino Scale of Impact Hazard
• Named for the city in Italy, not a person
• Assesses both probability of event and
potential effects of impact, so measures
two different things
• Not completely consistent.
The Torino Scale of Impact Hazard
• Low or no hazard
– 0 - No danger, or object too small to penetrate atmosphere
– 1 - Normal. No likelihood of impact
• Merits attention by astronomers
– 2 - Close pass but no cause for concern
– 3 - 1% chance of impact causing local damage
– 4 - 1% chance of impact causing regional damage
• Threatening
– 5 - Close pass by object capable of causing regional damage
– 6 - Close pass by object capable of causing global effects
– 7 - Very close pass by object capable of causing global effects
• Certain Impact
– 8 - Impact capable of causing local damage or tsunami
– 9 - Impact capable of causing regional damage or tsunami
– 10 - Impact with global effects
Torino Scale
Meteorite
Peekskill,
NY 1992
Chondrite
Stony-Iron Meteorite
Iron Meteorite
Meteo-Wrongs
• Meteorites Never:
– Have internal cavities
– Have layers
– Have veins
– Flatten on impact
– Mold around objects
– Almost never light in color outside
• If you “think” it’s magnetic, it’s not
magnetic
Nope
Nope
Uh-uh
No Way
Nope
Nope
Nope
Tektites
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Very silica-rich, water poor glassy rocks
Terrestrial vs. Extraterrestrial origin?
Volcanic vs. Impact origin?
Problems:
– Odd chemistry
– If terrestrial, why are they spread so widely?
– If extraterrestrial, why are they so localized?
• Now considered impact glass
– Atmospheric shock wave evacuates
atmosphere
Tektites
Spectrum of Impact Scenarios
• Atmospheric impact and air burst
(Tunguska, 1908)
• Surface impact causing local damage
• Surface impact with 100 km damage
radius
• Surface impact with 1000 km damage
radius
• Surface impact with global effects
Tunguska, 1908
Tunguska, 1908
Sikhote-Alin
Fall,
February 12,
1947
Mass =
100,000 Kg
Sikhote-Alin Crater
Sikhote-Alin Crater
Sikhote-Alin Crater
Near Miss, August 10, 1972
1972 Near Miss
• Object was about
the size of a bus
• Entered Atmosphere
over Utah, travelling
north, exited over
Canada
• Velocity 15 km/sec
• Missed by 58 km
Returning to Space
Carangas, Peru, 2007
Carangas, Peru, 2007
What happens during impact
• Atmospheric entry
– Microscopic objects gradually decelerate
– Millimeter-sized objects vaporize, seen as meteors
– Meter-sized objects may fragment and survive
passage
– House-sized objects hit with force
• Contact-compression phase
• Transient crater phase
• Rebound and collapse phase
Impact Processes
• Impact releases kinetic energy
instantaneously – Explosion
• Explosion scaling: Volume proportional to
energy
– Radius scales as cube root of energy
• Energy Measures
– Kiloton = 4.2 x 1012 Joules = 1012 calories
– Megaton = 4.2 x 1015 Joules = 1015 calories
– Note: Small “c” calories
Kinetic Energy
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Assume 10 m rocky object
Volume = 1000 m3, Density = 3000 kg/m3
Mass = 1000 m3 x 3000 kg/m3 = 3 x 106 kg
Velocity = 30 km/sec = 30,000 m/sec
K = ½ mv2 = ½(3 x 106 kg)(30,000)2
K = 13.5 x 1014 Joules = 270 Kt
= 13 Hiroshima nuclear weapons
What is an Explosion?
• Instantaneous point release of energy
• Can be mechanical, chemical or nuclear
• Damage is caused by the surrounding
material: air, water or solid
• Explosions would cause little damage in
space
All Large Explosions Make
Mushroom Clouds
Environmental Effects of Impacts
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Radiant heat and flash burns
Blast wave
Seismic waves
Tsunami
Ejecta
Stratospheric dust
Liberated volatiles (carbon dioxide, sulfur,
methane)
• Impact volcanism - a myth
Averting Impact Hazards
• Simplest Strategy: Detection + Diversion
• Destruction too unpredictable
– Can object be destroyed?
– “Cookie crumbs have no calories”
– In real life, the pieces matter
• The longer the lead time, the easier
diversion becomes
• Only need a close miss
• Detection is cheap and off-the shelf
Diversion
“The question is: how to do it? These things must
be done … delicately.”
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Nukes?
Thrusters?
Space tug?
Gravitational?
Solar Sail
Laser?
Asteroid Itokawa