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