In the Aristotelian model of the Universe, planetary orbits are separated by crystalline spheres. It...
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Transcript of In the Aristotelian model of the Universe, planetary orbits are separated by crystalline spheres. It...
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).
Stones from the sky?
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
Topham’s monumentto the meteorite (1799)
TOPICS
1. The Threat Direct and Indirect Effects
2. Risk Analysis Calculating the probabilities
3. Protection? NASA to the rescue?
Asteroids
• Asteroid orbits continuously modified by gravitational perturbation of asteroid belt.
• About 2000 asteroids currently have orbits that cross that of Earth (= NEO’s :Near Earth Objects).
• Orbital trajectories of 200 NEO’s are known; i.e. the paths of 90% of the asteroids that threaten Earth are unknown.
• Largest NEO’s have diameters of about 8 km; the orbits of about 35% of asteroids >5 km diameter are known.
Comets
• About 10-20% of comets (piles of rubble and ice with tail =“coma”) are in Earth-crossing orbits.
• Some 700 long-period comets (>200 yrs) known.
• Periodic comets (≤200 yrs) - 95% have lost their coma (= “stealth comets”) 25 known, 1500 > 1 km diameter may exist.
• Our first warning is likely to be their initial entry into Earth’s atmosphere.
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)
Impactors
• <10 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: > 1 km in diameter may cause severe global effects (e.g. Chicxulub impactor, Mexico)
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.
First photos of Tunguska fireball were taken by a Russian expedition in 1920’s, more than a decade after the event.
Category 2: Meteor (a.k.a. Barrington) Crater, AZ.
1200 m wide, 180 m deep
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.
Category 3
Veil of dust in atmosphere for months/years
Crater 10 - 15x diameter of impactor
Reduced sunlight
Food chain collapses
Reduced photosynthesis Lowered global temperature
Polar and temperate areas uninhabitable
Category 3
Firestorm spreads from impact site
Very high temperatures at impact site
Intense smokefrom firestorm:
reduced sunlight, etc.
Reduced photosynthesis; food chain collapses
N2 in atmosphere burns
Nitric acid produced;acidic precipitation
Clay
Sandstone
Coal
Asteroid impact dust deposit (clay layer) marking K-T boundary at 65 Ma BP in Colorado, 2500 km from impact
site.
Shale
Tertiary
Cretaceous
Rock hammer for scale
Hazardclassification
The Palermo scale was developed to categorize potential impact risks. Intended for use by specialists.
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.
Hazardclassification
The scale was devised by
delegates to an international
symposiumin Torino (Turin; Italy) in 1999 as
a means of communicating
risk to the public.
Potential impactor: (2002 NT7: Feb 01/2019?)
2002 NT7 is 2 km in diameter
Initial reports based on on
only a handful of observations of NT-7’s orbit
in 2002
NEO Year rangePotentia
l impacts
Probability of
impact
Velocity (km/s)
Diam.
(km)
Palermo scale
Torino Scale
2007 VK184
2048-2057 4 1.0e-04 15.630.13
0-1.82 1
2004 MN4 2036-2069 3 2.2e-05 5.870.27
0-2.41 0
2004 XY130
2009-2107 87 5.0e-07 3.060.50
3-2.73 0
Current* top three NEOs(ranked by Palermo scale)
* as of Aug. 15, 2008 (http://neo.jpl.nasa.gov/risk/)
Extremely unlikely to collide with Earth in this period
The probability of impact during this time is 0.0001 (~1:10,000)
VK184 will cross the Earth’s orbit four times between AD2048 and 2057.
N.B. 2002 NT7 no longer features on the list of potential impactors.
Tunguska
Computing annual probability of impacts
(Tunguska ~300 yr recurrence; = 0.003 annual probability)
Impact probability (P)
where:
P = P(D) * P(A)
D = projectile diameter;P(D) = annual frequency of projectile D;
P(A) = probability of hitting target ; = area of target/surface area of Earth
1) an inhabited area (10% Earth area) P = 0.003 * 0.1 = 1 : 3 300
2) a city (1% Earth area) P = 0.003 * 0.01 = 1 : 33 000
3) Fraser lowlands (0.01% Earth area)P = 0.003 * 0.00001 = 1 : 33 million
*assumes 300 yr return interval for Tunguska event (estimates range from 50-500 yr recurrence)
Annual probability (P) of a “Tunguska event*” impacting:
After Crawford and Mader (1998)
vi = 20 km /si = 3.3 g/cm3
t = 25 s
Simulation of 500 m diameter asteroid impact into 5 km deep ocean
Ocean impact tsunami
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
Source: www.lanl.gov/worldview/news/tsunami.mov (Stephen Ward)
1000-year probabilities (%) of impact tsunami exceeding critical wave height at typical coastal and mid-ocean sites in the
Pacific Ocean
Waves Tokyo,Japan
Hilo,Hawaii
5m 4.2 8.310m 1.6 2.325m 0.4 0.550m 0.1 0.2
after Ward and Asphaug (2000)
Impact tsunamis: bathymetric effects
Impact site
“Barriers” =ridges
“Fingers of God”
N. America
AfricaEurope
=abyssal canyons;up to five-fold
increase in wave height at coastline
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.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
•BUT - is it advisable to create numerous projectile fragments?
View of the nucleus of the comet 9P/Tempel-1
from impactor
Spacewatch Project
• Initiated at the University of Arizona in early 1980’s, 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.