Defending Planet Earth

Defending Planet Earth
Mike A’Hearn

Chixculub
31-3-11
Goddard Symposium
mfa - 2
Every 50-100 Myears

Tunguska
31-3-11
Goddard Symposium
mfa - 3
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
31-3-11
Goddard Symposium
mfa - 4

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
31-3-11
Goddard Symposium
mfa - 5

Kill Curves
31-3-11
Goddard Symposium
mfa - 6

Annual Deaths
31-3-11
Goddard Symposium
mfa - 7
LP Comets ignored ~10%, notimportant
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
31-3-11
Goddard Symposium
mfa - 8

Mitigation Applicability
31-3-11
Goddard Symposium
mfa - 9

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
31-3-11
Goddard Symposium
mfa - 10

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
31-3-11
Goddard Symposium
mfa - 11

Incomplete Mitigation
31-3-11
Goddard Symposium
mfa - 12
Risk of incomplete mitigation could lead to paranoia or even retaliation
Risk of retaliation could lead to total inaction
Requires official, working level close cooperation

Bottom Line
Insurance prevents destruction rather than compensating to rebuild
How much insurance do we want?
31-3-11
Goddard Symposium
mfa - 13

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Defending Planet Earth

by American Astronautical Society

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