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)

Asteroid/Comet Impacts

Geog 312Ian Hutchinson

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.

Asteroid size-frequency relations

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)

Impact craters on Mercury

indicative of the protective effects of Earth’s atmosphere

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

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

based on an assumed initial velocity of 25 km/s

*

*

The NT7 scare [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.

What is the probability that an inhabited area or city will be hit?

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:

Tunguska impact areafrom a local perspective

NT7

Oceanic Impacts:the tsunami hazard

Tsunamis reach all amphi-oceanic areas within 10 hours of impact

airbursts

after Ward and Asphaug (2000)

NT7

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)

vi =impactor velocity; i = impactor density; h = water depth

after Ward and Asphaug (2000)

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.

Topic One

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