# Impact Hazards. Can we predict impacts? Incomplete inventory of objects –May be a million km-sized objects Initial observations don't permit completely.

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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

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

Uh-uh

No Way

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

Sikhote-Alin Fall, February 12, 1947 Mass = 100,000 Kg

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

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 10 12 Joules = 10 12 calories –Megaton = 4.2 x 10 15 Joules = 10 15 calories –Note: Small “c” calories

Kinetic Energy Assume 10 m rocky object Volume = 1000 m 3, Density = 3000 kg/m 3 Mass = 1000 m 3 x 3000 kg/m 3 = 3 x 10 6 kg Velocity = 30 km/sec = 30,000 m/sec K = ½ mv 2 = ½(3 x 10 6 kg)(30,000) 2 K = 13.5 x 10 14 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

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