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Different classes of impact.
Risk assessment.
Impact mitigation.
Much material borrowed from NASA “Hazard
Impacts” presentation, October 2000.

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Global catastrophe (>1 million Mt)
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Global environmental damage, threatening civilization.
Average interval for whole Earth: 1-100 Myr.
Major risk relative to other natural disasters
(earthquakes, etc.).
Regional catastrophe (~10,000 Mt)
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Destroys area equivalent to small country.
Average interval: ~100,000 yr.
Moderate risk relative to other disasters.

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Tunguska-class impact (~15 Mt)
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Damage roughly equivalent to nuclear bomb (city killer).
Average interval for whole Earth: ~100-
1000 yr.
Minor risk relative to other natural disasters.
Bolides (<5 Mt)
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Great fireworks display!
No damage.

year century
Hiroshima
Tunguska million yr.
K/T billion yr.
0.01
1 100 10,000 million
TNT equivalent yield (Mt)
100 million

Log[Impact Energy, MT]
10
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
-1
9
8
7
6
Harris 2002 (LINEAR)
Brown et al. 2002 (Annual bolide event)
Discovered as of Jan. 28, 2002
Werner et al. 2001 (p
V
Werner et al. 2001 (p
V
Constant power law
= 0.25)
= 0.11)
Bottke et al. 2001
Stuart 2001 (LINEAR)
Rabinowitz et al. 2000 (NEAT)
Rabinowitz et al. 2000 (SW)
D'Abramo et al. 2001 (LINEAR)
0
1
2
3
5
4
4
5
3
6
2
7
1
8
Absolute Magnitude, H
0
30 25 20 15 10
0.01
0.1
1
Diameter, Km
10

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Statistical risk of death from impacts is 1 in a million per year, or
~1:20,000 over an average lifetime.
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Much less (in U.S.) than auto accidents, shootings, etc.
Comparable with other natural hazards (e.g. earthquakes, floods, etc.).
Near threshold for hazards most people are concerned about.
Well above threshold for U.S. governmental or regulatory action.

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Severity of major disasters (billions of people killed) is greater than any other known hazard we face.
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Apparently unique in its threat to civilization.
Places this disaster in a class by itself.
Average interval between disasters
(hundreds of millennia) is larger than for any other hazard we face.
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Causes some to question credibility of hazard.

Average Annual Risk of Death in Parts per Million
1 Total impact risk
0.1 Regional impact (D < 2 km)
0.01 Tunguska impact (D < 300 m)
50 Bangladesh (mainly floods)
25 China (floods & earthquakes)
20 Middle East (earthquakes)
15 Japan (earthquakes)
10 Carribean & C. America (storms, earthquakes, volcanoes)
<1 Europe
<0.1 USA/Canada

Average Annual Risk of Death in Parts per Million
300 Accidents (not motor vehicle)
200 Homicide and suicide
160 Motor vehicle accidents
10 Fire
5 Electrocution
1 Airplane accidents
0.5 Total impacts (global threshold = 1 million Mt)
0.3 Storms and floods (declining)
0.1 Earthquakes (poor statistics), regional impacts
<0.01 Nuclear accidents (design goals), Tunguska impacts

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How can this risk be credible when no one has been killed?
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The small, frequent events (such as meteorite falls) are not a significant hazard; only the large, rare events are important.
If Tunguska events happen so often, why are there no other historical records of such an event?
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Most impacts happen over water or unpopulated areas, and our history goes back only a few millennia.

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Why do mass extinctions occur less often than global catastrophes?
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A mass extinction means every member of a species is killed. The global catastrophe we have defined requires only crop loss.
If the interval between major impacts is so long, why not let future generations deal with it?
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Spacing between impacts is random; the probability of impact is as great for next year as for any later year.

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Originally proposed by NASA in 1992.
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Supported by US Congress in 1995.
Adopted as NASA goal in 1998 (with USAF).
Objective: Discover and track 90% of
NEAs (with D > 1 km) by 2008.
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Estimated number with D > 1 km: ~1100.
Number found by 2003: ~660.
Estimated completion date: 2008?
Most NEAs found by LINEAR.
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Two USAF 1-m telescopes; funded by NASA.

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NEAT (Near-Earth Asteroid Tracking)
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Joint JPL/USAF effort similar to LINEAR.
Spacewatch
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Discovered many of the smaller NEAs.
LONEOS (Lowell Observatory Near-
Earth Object Search)
Catalina Sky Survey (CSS)
Japanese Spaceguard Association
(JSGA)
Asiago DLR Asteroid Survey (ADAS)

So far only one
NEA ranks above zero on the Torino scale: 1997 XR2
(rank 1 on the
Torino scale).

The House Committee on Science and Technology believes that it is imperative that the detection rate of
Earth-orbit-crossing asteroids must be increased substantially, and that the means to destroy or alter the orbits of asteroids when they do threaten collisions should be defined and agreed upon internationally. The chances of the Earth being struck by a large asteroid are extremely small, but because the consequences of such a collision are extremely large, the Committee believes it is only prudent to assess the nature of the threat and prepare to deal with it.
NASA Authorization Bill, 1991

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Impacts are preventable natural disasters.
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Modern technology can deflect or disrupt an incoming object.
Long lead time is required (as provided by
Spaceguard).
Deflection (change of orbit) is preferred approach.
Disruption may be possible if warning time is less.

1. Nuclear explosions
•
Destroy, or deflect by vaporizing surface.
PRO: Technology already exists.
PRO: Can be rapidly deployed.
PRO: Can work even for very close hazards.
CON: Results not predictable or controllable.
- Could split asteroid into large pieces.
SUMMARY: Last resort only!

2. Kinetic impact
•
Smash spacecraft into asteroid at speed.
PRO: Technology exists.
CON: Needs to be precisely aimed.
- Want all KE to go into changing orbit.
CON: Splitting is also a concern.

3. Mass driver
•
Hurl rocks from asteroid, use recoil.
PRO: More control than explosions/impacts.
PRO: No spacecraft propellant required.
CON: Robotic installation very challenging.

4. Ablation
•
Use laser or reflected sunlight to vaporize surface, deflecting asteroid.
PRO: Controllable.
PRO: Asteroid spin not a concern.
CON: Lots of fuel needed to keep laser/mirror in position.
CON: Optical elements vulnerable to coating.

5. Solar pressure
•
Coat asteroid in reflective paint, use solar heating to alter asteroid’s course.
PRO: Thermal forces have now been shown to be effective at altering asteroid orbits.
CON: A lot of paint required!
CON: Applying paint to surface is challenging.

6. Land and push
•
The “asteroid tug” (Sci. Am. 11/03).
PRO: Fully controllable.
CON: Necessary propulsion system still under development.
- Prometheus Project (nuclear propulsion).
CON: Maneuvering and attaching spacecraft to asteroid is challenging.
- But NEAR/Shoemaker worked.

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Short-period comets constitute
~1% of NEO impact hazard.
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Mitigation strategies may need to be different for such objects.
Long-period comets cannot be detected very far in advance!
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Probably the least preventable of the most dangerous impact risks.
Design early warning system far beyond the planets?…