The NEO Impact Hazard: Cometary Component, and Recent Developments in Mitigation Strategies Duncan S

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The NEO Impact HazardCometary Component, and
Recent Developments in Mitigation
Strategies Duncan SteelBall Solutions Group,
Canberra, ACTAlso Vice-President, The
Spaceguard Foundation, Rome, ItalyBSG is the
Australian subsidiary of Ball Aerospace
Technologies Corp, the manufacturer of the Deep
Impact spacecraft

• Deep Impact Workshop
• Australian Centre for Astrobiology
• Macquarie University
• 2004 September 6

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• Gradual recognition of the hazard posed by
  asteroids comets examples
• Edmond Halley (1690s) comets
• Benjamin Franklin (1757)
• Should a comet in its course strike the Earth,
  it might instantly beat it to pieces. But our
  comfort is, the same great Power that made the
  Universe, governs it by his providence. And such
  terrible catastrophes will not happen till 'tis
  best they should
• William Herschel (1802) asteroids
• Lord Byron (1822)

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Was Byron the inventor of cosmic impact
mitigation studies?
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• Discovery of Eros (1898)
• Discovery of Aten, Apollo Adonis (1930s)
• Recognition of terrestrial impact craters
  (1930s-)
• Lunar landings
• Mass extinctions linked to impacts
• (e.g. Nininger, 1941 Urey, 1973
• Napier Clube, 1979 Alvarez et
  al., 1980)

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Comet dangers in the mass media
Punch, 1907 (Comet Halley due in 1910)
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(No Transcript)
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Tunguska event (1908) an asteroid
(devolatilised comet?) ca.60m in size Not
recognised until 1927 ( still argued about)
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Brazilian Tunguska in 1930
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Terrestrial impact craters
Aorounga (Chad) 3 ? 10 km
Manicouagan (Canada) 100 km
Silverpit (North Sea, UK) 3 km
Gosses Bluff (Australia) 6 km central uplift,
22 km ring
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The SL9 impacts on Jupiter in 1994 led to much
modelling of effects of impacts on that planet
and Earth
Atlantic impact south of Long Island
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Tsunamis from oceanic impacts
An actual event
A hypothetical event
Courtesy Steve Ward, UC Santa Cruz
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An impact killing the dinosaurs has become part
of public consciousness
From the New Yorker
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Media stories and public interest (and alarm)
have led to the development of various reference
scales for communicating the level and nature of
specific feasible events. Note that a comet
could not become a red (level 8,9,10) object
until very soon before an impact
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and of course
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The true level of activity on NEOs is becoming
apparent to some in the media
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Orbits of all known near-Earth asteroids as at
January 2000
Map from
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Asteroid trajectories (impacts) are predictable
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What about comets?
While asteroid trajectories/impacts are
predictable comet impacts are not, due to
non-gravitational forces

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Are asteroids or comets the more dangerous?
There is a relatively small number of short- or
intermediate-period comets cf. NEA map
Prograde Retrograde
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• What fraction of the NEO impact hazard is due
  to comets?
• Problem tackled by various researchers
  (Shoemaker, Weissman, Steel).
• Major uncertainty is the relation between
  apparent comet brightness and nucleus mass
• Answers range from 2 to 40 (for 1-2 km
  objects producing 10-30 km craters)
• Suitable baseline would be 15 (10 from
  SP/IP comets, 5 from LP/parabolic comets)
• An implication is that once the NASA Spaceguard
  goal is reached (90 of cis-jovian NEAs larger
  than 1 km), comets become the major hazard!
• Higher energy impacts (causing mass extinctions)
  predominantly due to big comets
• Lower energy impacts (causing local effects)
  predominantly due to small asteroids

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Mean impact probability is 2.2 x 10-9 for a
spherical Oort cloud uniform q-distribution
From B.G.Marsden D.I.Steel, in Hazards Due to
Comets and Asteroids, ed. T. Gehrels (1994)
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From B.G.Marsden D.I.Steel, in Hazards Due to
Comets and Asteroids, ed. T. Gehrels (1994)
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• Open Questions
• How many NEAs are actually extinct/dormant
  comets?
• Are there many asteroids in IP/LP/retrograde
  orbits?
• Role of Centaurs other outer planetary system
  objects unclear What would be the effect of a
  100-km minor planet entering an inner solar
  system orbit? (Time-scale is ca.105 years)
• Finding objects in the outer planetary region
• LP/parabolic comet impacts not only
  unpredictable, but also the comets are not
  easily discoverable until 1-2 years prior to
  event

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From B.G.Marsden D.I.Steel, in Hazards Due to
Comets and Asteroids, ed. T. Gehrels (1994)
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From B.G.Marsden D.I.Steel, in Hazards Due to
Comets and Asteroids, ed. T. Gehrels (1994)
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But what could we do if we did find an object on
a collision course with Earth?
From the New Yorker
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Techniques for asteroid impact mitigation?
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A variety of techniques have been suggested for
pushing asteroids off a collision course with
Earth, but all acknowledge that a far better
knowledge of the physical characteristics of such
bodies is needed first
(cf. Erice meeting,
August 2004)
Sci Am, November 2003
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US DoD Clementine 2 mission cancelled in
1997. Spacecast 2020 report in 1998
PREPARING FOR PLANETARY DEFENSE Detection and
Interception of Asteroids on Collision Course
with Earth Available from http//www.au.af.mil/S
pacecast/Spacecast.html
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United Nations NEO conference (1995), and ongoing
policy issues
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AIAA Seville workshop in 2001 highlighted need
for mitigation studies
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UK Government report, 2001
No real action (yet), but led to developments
within the OECD Global Science Forum
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OECD Global Science Forum workshop, 2003
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Legal implications of mitigation attempts
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(No Transcript)
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ICSU proposal, 2003
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Arlington meeting, September 2002
Papers presented available at http//www.noao.edu
/meetings/mitigation/eav.html
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Book resulting from the meeting is also now
available
SCIENTIFIC REQUIREMENTS FOR THE MITIGATION OF
HAZARDOUS COMETS AND ASTEROIDS (Cambridge
University Press, 2004) Chapter 16   IMPACTS
AND THE PUBLIC COMMUNICATING THE NATURE OF THE
IMPACT HAZARD  David Morrison, Clark R. Chapman,
Duncan Steel Richard Binzel   ABSTRACT Public
support is required for both asteroid survey
programs and eventual hazard mitigation (if an
impact is predicted). Yet the impact hazard is
difficult to understand, because of its relative
unfamiliarity and the extremely long intervals
between impacts. At the same time, any prediction
of an impact makes excellent press and generates
widespread public interest. Over the past few
years asteroid scientists have been coping with
varying success with the twin challenges of
explaining the general nature of the impact
hazard and of responding to sometimes exaggerated
media reports of impending disaster. We must
balance between the poles of complacency and
alarmist reactions. This chapter explores a
variety of facets of the communications
challenge, including a history of early ideas,
the development of the Torino scale, lessons that
can be learned from media flaps during the past
five years, and scenarios that can be expected
for the future.  
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Belton (2003)
  Towards a National Program to Remove the Threat
of Hazardous NEOs Michael J.S. Belton Belton
Space Exploration Initiatives, LLC   I consider
issues associated with the establishment of a
national program in the United States to prevent
asteroidal collisions with the Earth. I take the
position that costs associated with future damage
to social infrastructure rather than potential
loss of life will stimulate public
representatives to begin work on a system to
mitigate the possibility of an asteroidal
collision. With some uncertainty, there is a 0.3
percent chance of a 50-meter, or larger, sized
asteroid impacting United States territory in the
lifetime of its current population (100 years).
I show how a probable lack of concern for this
small probability might be offset by the cost of
the damage that could be caused by the large
energy release (gt10 Megatons of TNT) on impact.
I outline four conditions, focused on the
interests of United States citizens, that I
believe will need to be met before the start of a
national mitigation program is viable. These
reflect issues of public concern, feasibility,
cost, timing, and security. Establishment of a
public consensus on how well these conditions
have been met and some modestly detailed
preplanning are probably prerequisites for the
initiation of a national program. I outline a
planning roadmap that indicates what a national
program might look like up to the point where
work on a practical mitigation project directed
at a specific target could begin. I also indicate
how responsibilities for the task might be
divided up between different government agencies.
Rough estimates of the time to complete these
preliminary activities (25 yr), and a rough
estimate of the cost (5B) are given.  
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Michael Belton (Sept 2003), Towards a National
Program to Remove the Threat of Hazardous NEOs
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The Deep Impact mission might be thought of as
the vanguard of (comet) target characterization
Mike AHearn (U MD), Deep Impact PI
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Deep Impact after launch
Note that P/Tempel 1 is not an NEO,
formally-speaking
From U of Maryland/JPL Deep Impact web site
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• Asteroid comet impacts
• a spectrum of implications
• Individual death probabilities
• Downfall of national/global economic systems
• Downfall of civilisation (new Dark Age)
• Extinction of the human species and
• Termination of the potential to spread DNA
  throughout the galaxy.