Defending Planet Earth
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Transcript of Defending Planet Earth
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
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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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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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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