Stones from the sky

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Stones from the sky?
In the Aristotelian model of the Universe,
planetary orbits are separated by crystalline
spheres. It is consequently impossible for
stones to fall from heaven (a view also held by
Newton).
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In 1795 farm labourers in Yorkshire (N.England)
reported that during a severe thunderstorm a
stone had fallen from the sky and buried itself
in the ground. One farm labourer had been so
close that he was hit by mud and debris. The
stone created an impact crater about 1 m in
diameter, and had to be dug out of the
ground.The local squire (Capt. Topham) exhibited
the stone in London (entrance fee 1 shilling),
and provided testimonials from locals who had
heard or seen it fall. Sir Joseph Banks (the
President of the Royal Society) compared it to
other imputed meteorites from Italy, India, and
Paraguay, and concluded that stones could,
indeed, on occasion fall from the sky.
Captain Edward Topham
Tophams monument to the meteorite (1799)
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Asteroid/Comet Impacts

• Geog 312
• Ian Hutchinson

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TOPICS1. The Threat Direct and Indirect
Effects2. Risk Analysis Calculating the
probabilities3. Protection? NASA to the
rescue?
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Asteroids

• Asteroid orbits continuously modified by
  gravitational perturbation of asteroid belt.
• About 2000 asteroids currently have orbits that
  cross that of Earth ( NEOs Near Earth
  Objects).
• Orbital trajectories of 200 NEOs are known
  i.e. the paths of 90 of the asteroids that
  threaten Earth are unknown.
• Largest NEOs have diameters of about 8 km the
  orbits of about 35 of asteroids gt5 km diameter
  are known.

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Asteroid size-frequency relations
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Comets

• About 10-20 of comets (piles of rubble and ice
  with tail coma) are in Earth-crossing orbits.
• Some 700 long-period comets (Tgt200 yrs) known.
• Periodic comets (T200 yrs) - 95 have lost their
  coma ( stealth comets) 25 known, 1500 gt 1 km
  diameter may exist.
• Our first warning is likely to be their initial
  entry into Earths atmosphere.

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Effects

• Direct (predominantly local)Impact crater plus
  blast-wave and firestorm
• Indirect effects (may be global)Dust veil (large
  impactors)Acid rain (large impactors)Tsunami
  (oceanic impacts)

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Impactors

• lt10 m diameter - burnup in atmosphere.
• Category 1 10-100 m diameter - disintegrate in
  atmosphere exploding fragments create airburst
  (e.g. Tunguska event).
• Category 2 100 m - 1 km diameter - capable of
  striking surface, forming impact craters, effects
  local (e.g. Meteor Crater, AZ).
• Category 3 gt 1 km in diameter may cause severe
  global effects (e.g. Chicxulub impactor, Mexico)

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Impact craters on Mercuryindicative of the
protective effects of Earths atmosphere
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Category 1 Tunguska

• 50-60 m diameter stony meteor? exploded in June
  1908 above central Siberia. Energy release
  10-30 MT TNT (1 000 - 3 000 Hiroshima bombs)
• Radius of destruction 25 km ( 2 000 km2).
• Recorded by seismograms in Irkutsk and barograms
  in London.

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First photos of Tunguska fireball were taken by a
Russian expedition in 1920s, more than a decade
after the event.
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Category 2 Meteor (a.k.a. Barrington) Crater,
AZ.
Impact occurred about 50 000 years ago it is
likely that all plant and animal life within 10
km of the impact site was vapourized.
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Category 3
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Category 3
Crater 10 - 15x diameter of impactor
Veil of dust in atmosphere for months/years
Reduced sunlight
Reduced photosynthesis
Lowered global temperature
Polar and temperate areas uninhabitable
Food chain collapses
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Category 3
Very high temperatures at impact site
Firestorm spreads from impact site
N2 in atmosphere burns
Intense smokefrom firestorm reduced sunlight,
etc.
Nitric acid produced acidic precipitation
Reduced photosynthesis food chain collapses
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Rock hammer for scale
Sandstone
Tertiary
Clay
Coal
Cretaceous
Shale
Asteroid impact dust deposit (clay layer) marking
K-T boundary at 65 Ma BP in Colorado, 2500 km
from impact site.
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Hazardclassification
The scale value PS is given by PS log10 PI /
(fB . DT), where PI is the impact probability
of the event in question and DT is the time until
the potential event, measured in years. The
annual background impact frequency, fB 0.03 .
E-4/5 is the annual probability of an impact
event with energy (E, in megatons of TNT) at
least as large as the event in question.
The Palermo scale was developed to categorize
potential impact risks. Intended for use by
specialists.
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Hazardclassification
The scale was devised by delegates to an
international symposium in Torino (Turin Italy)
in 1999 as a means of communicating risk to the
public.
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Potential impactor (2002 NT7 Feb 01/2019?)
Initial reports based on on only a handful of
observations of NT-7s orbit in 2002
2002 NT7 is 2 km in diameter
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The NT7 scare 2002


based on an assumed initial velocity of 25 km/s
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Current top three NEOs(ranked by Palermo scale)
N.B. 2002 NT7 no longer features on the list of
potential impactors.
as of Aug. 15, 2008 (http//neo.jpl.nasa.gov/ri
sk/)
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What is the probability that an inhabited area or
city will be hit?
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Computing annual probability of
impacts (Tunguska 300 yr recurrence 0.003
annual probability)
Tunguska
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Impact probability (P)
P P(D) P(A)

• where

D projectile diameter P(D) annual
frequency of projectile D P(A) probability
of hitting target area of
target/surface area of Earth
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Annual probability (P) of a Tunguska event
impacting
1) an inhabited area (10 Earth area) P 0.003t
0.1 1 3 300 2) a city (1 Earth area) P
0.003t 0.01 1 33 000 3) Fraser lowlands
(0.01 Earth area) P 0.003t 0.00001 1 33
million
assumes 300 yr return interval for Tunguska
event (estimates range from 50-500 yr
recurrence)
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Tunguska impact area from a local perspective
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NT7
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Oceanic Impactsthe tsunami hazard
Tsunamis reach all amphi-oceanic areas within 10
hours of impact
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airbursts
NT7
after Ward and Asphaug (2000)
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Simulation of 500 m diameter asteroid impact into
5 km deep ocean
vi 20 km /sri 3.3 g/cm3 t 25 s
After Crawford and Mader (1998)
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Ocean impact tsunami
Source www.lanl.gov/worldview/news/tsunami.mov
(Stephen Ward)
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after Ward and Asphaug (2000)
vi impactor velocity ri impactor density h
water depth
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1000-year probabilities () of impact tsunami
exceeding critical wave height at typical coastal
and mid-ocean sites in the Pacific Ocean
after Ward and Asphaug (2000)
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Impact tsunamis bathymetric effects
N. America
Impact site
abyssal canyons up to five-fold increase in
wave height at coastline
Africa
Europe
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Deep Impact Project

• NASA detonated a 370 kg impactor ( 5 T of
  dynamite) in a near-Earth comet (9P/TEMPEL-1) on
  July 4, 2005.
• The primary purpose was to study cometary
  structure (which proved to be less icy and
  dustier than expected), but the experiment may
  illustrate the effects of trying to deflect or
  fragment such objects before they reach Earth.

View of the nucleus of the comet 9P/Tempel-1 from
impactor

• BUT - is it advisable to create numerous
  projectile fragments?

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Spacewatch Project

• Initiated at the University of Arizona in early
  1980s, the Spacewatch project involves automated
  searches of the sky for 20 nights per month for
  new asteroids (particularly NEOs) and
  short-period comets. Now includes cooperative
  efforts with other observatories in North
  America, Europe and Australia.

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Topic One
Graphics courtesy of University of Bologna,
NASA, Don Davis, US Geological Survey, Natural
Resources Canada http//fernlea.tripod.com/
woldcottage.html