Defending Planet Earth

Mike A’Hearn

Chixculub

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Goddard Symposium

Tunguska

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Airburst of oblique impactor 5-30 Mtons of TNT (20-130 Pj) 30-100m diameter impactor Everything flattened over >2000 km2 Possibly one every 200 years

1927 vs. 2008

Size Distribution & Frequency

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Finding Them

• From Space vs. from Earth– Earth-trailing orbit is fastest– From Earth’s surface is cheapest

• IR vs. Visible– IF! In space, IR seems better

• Goal of G. E. Brown survey– 90% of >140m

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Goddard Symposium

Kill Curves

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Annual Deaths

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LP Comets ignored ~10%, not important LP Comets ignored ~50% Whoops!

Mitigation (per NRC)

• 4 approaches depending on circumstances– Civil defense (evacuation, sheltering, first aid, etc.

• Up to 50m diameter?– Slow Push-Pull (tug, solar heating, albedo change, gravity

tractor, et al.)• Needs decades to operate (plus time to build, etc.)• Max size 300-600 m diameter• Gravity tractor closest to ready and least dependent on

properties of NEO– Kinetic Impacts (Super Deep Impact)

• sensitive to porosity of top meters to tens of meters• momentum transfer efficiency not known• much wider range of applicability (max size 1 to 1.5 km,

shorter warning for small ones)– Nuclear blast

• standoff blast best• works up to 10 km and relatively short warning

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Mitigation Applicability

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What Next?

• Don Quixote-like mission– a rendezvous spacecraft at a small NEO followed by

a large impactor– biggest gain in knowledge directly related to

mitigation– Guess $1.5G; Good for international collaboration

• Gravity tractor demonstration– fewer unknowns other than engineering– second priority– Apophis a possible target but any small NEO will do

• Can’t predict which might need to be used first– Small NEO is by far the most likely– Warning time very uncertain but short warnings are

likely at ~100m diameter or less

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Research Needs

• Momentum transfer• Warning times• Tails of orbital distribution• NEO properties - spin & shape, multiplicity, & especially

porosity and density– lots for science but not so much for hazard mitigation– hence radar programs at Arecibo and Goldstone

• Physics of airbursts• search strategy - death plunge vs. full survey• Effects of the wide range of NEO properties on the

mitigation strategy• tsunami generation• NEO disruption

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Incomplete Mitigation

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Risk of incomplete mitigation could lead to paranoia or even retaliationRisk of retaliation could lead to total inactionRequires official, working level close cooperation

Bottom Line

• Insurance prevents destruction rather than compensating to rebuild

• How much insurance do we want?

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Backup Slides