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
MeteoritePeekskill, 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• 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
• 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
• 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.”
• Nukes?• Thrusters?• Space tug?• Gravitational?• Solar Sail• Laser?
Asteroid Itokawa