ES 1111

1
ES 1111

• Lecture 3 Big Bangs!
• Asteroids, Comets, and Nukes

2
Collisions from Space

• Hazardous life-threatening collisions come from
  two sources
• Asteroids
• Comets
• Collisions are relatively rare because space is
  big and the Earth is a small target!
• Unfortunately, when collisions do happen with
  large bodies, it is catastrophic.

3
Factors Influencing Destruction

• Impact speed
• Objects mass
• Composition
• Land/Ocean collision
• A 1-mile asteroid hitting Earth at 12 miles per
  second would unleash an explosive yield of
  240,000 megatons (20 million times the yield of
  the Hiroshima bomb and 3,600 times higher than
  the most powerful hydrogen bomb ever developed)

4
Craters

• Craters generally are 10-20 times the size of the
  impactor.
• Over 200 impact craters have been identified
  worldwide
• 35 Million years ago Southern Chesapeake Bay
• 65 Million years ago Yucatan Peninsula
• 120 miles wide
• Wiped out dinosaurs and many other living things
• 250 Million years ago Australia

5
Earths Craters

• Figure showing the location of impact craters on
  the Earth

6
Asteroids

• Asteroid rocky, small minor planets
• 100,000 are bright enough to be seen from
  Earth-based telescopes
• Most are smaller than 1 km across
• Most orbit the sun at distances between 2 and 3.5
  AU in the asteroid belt
• 3 asteroids have a diameter 300 km
• 30 asteroids have a diameter between 200 and 300
  km
• 200 asteroids are between 100 and 200 km.
• First asteroid discovered on January 1, 1801
• Ceres diameter of 918 km
• Accounts for 30 of the mass of all asteroids
  combined

7
Near Earth Objects

• Some asteroids have orbits that cross Earths
  orbit
• Near-Earth Object any asteroid or comet that
  can come close to the Earths orbit (within 1.3
  AU)
• NEA Near Earth Asteroid
• The threshold size for an asteroid to cause
  global catastrophe is 1-2 kilometers
• Smaller objects (down to tens of meters diameter)
  can cause severe local damage
• Astronomers have been busy trying to catalog and
  chart all NEAs (Spaceguard)

8
Near Earth Asteroids

• Figure showing the positions of Near Earth
  Asteroids that pose a threat for us

9
As of 2003

• Approximately 63 of suspected NEAs have been
  located
• 2,600 known Near Earth Asteroids
• 691 NEAs are about 1 km in size
• 131 classified as Potentially Hazardous Asteroids
  (larger than 1 km)

10
Near Misses!

• 2004 FH
• March 18th, 2004 closest near miss that we know
  of
• 30 meter diameter asteroid
• 26,500 miles away (3.4 Earth diameters)
• 2001 YB5
• Passed within 375,000 miles
• 500 meters in diameter
• Tunguska asteroid was only about 60 meters
• Regional destruction would have been massive
• 1997 XF11
• Largest near-miss (hopefully) that we know of
  currently
• 1 mile-wide asteroid
• Guesses are it will miss Earth by just 30,000
  miles on October 26, 2028, but it is not entirely
  known how close it will come (margin of error in
  estimate is 180,000 miles!)

11
Just How Often?

• Asteroid 50 meters in size will hit Earth every
  100 years
• Asteroid 1 kilometer in size will hit Earth every
  100,000 years

12
Comets

• A chunk of ice with imbedded rock fragments
• A comet has the following parts
• Nucleus the solid dirty snowball (few to
  tens of kilometers in size)
• Coma fuzzy, luminous ball of gases surrounding
  the nucleus
• Tail luminous gases streaming out away from the
  Sun
• Can be longer than the distance from the Earth to
  the Sun
• Each comet actually has two tails an ion tail
  and a dust tail
• Comets can break apart and leave a trail of
  debris that results in annual meteor showers
  (Perseid meteor shower is due to Comet
  Swift-Tuttle)
• In 2126, Comet Swift-Tuttle will miss earth by
  just two weeks

13
Reservoirs of Comets

• Kuiper belt
• Lies in the plane of the ecliptic from Pluto to
  500 AU
• 300 objects have been discovered there from Earth
• Jupiter-family comets orbit the sun in fewer
  than 20 years
• Oort cloud
• Spherical collection from Kuiper belt to 50,000
  AU
• No objects discovered
• Estimated to have 6 trillion comet nuclei
• Intermediate-period comets 20-200 year orbital
  periods
• Comet Halley (orbits the Sun in opposite
  direction than planets)
• Long-period comets 1-30 million years

14
Kuiper Belt

• Figure showing the location of the Kuiper Belt in
  the Solar System

15
Oort Cloud

• Figure showing the location of the Oort Cloud in
  the Solar System

16
What is the Risk?!

• There is a greater chance of an individual dying
  due to an asteroid striking our planet than in a
  jetliner crash!
• Asteroid impacts dominate all other natural
  disasters in terms of odds of a person dying!
• The reason is, although very rare, deadly
  asteroid impacts will kill the majority of living
  things at that time
• Asteroids are a greater risk than comets because
  asteroids have a smaller orbit than comets (more
  chances for a collision over time)

17
Nuclear Winter

• Premise a large-scale nuclear war would
  generate large amounts of smoke and dust which
  would prevent solar radiation from reaching the
  surface and warming the atmosphere
• The theory was born in the early-80s during the
  Cold War
• Because the nuclear war must be massive, it
  involved a nuclear exchange between the United
  States and the former Soviet Union
• A large nuclear exchange between the U.S. and the
  former U.S.S.R. would involve a total nuclear
  yield in excess of 100 megatons

18
Smoke

• Smoke from oil, plastics, and asphalt fires would
  be very black and sooty
• Sooty smoke absorbs solar radiation very well
  (black color)
• Allows terrestrial radiation to pass through
• If the majority of the injected smoke consists of
  submicron-sized soot or elemental carbon
  particles, their small size and low fall
  velocities would result in a long residence time
  in the atmosphere
• No insolation combined with loss of terrestrial
  radiation to space would drastically cool Earth

19
Amount of Smoke Depends on

• Fuel open to the air or buried beneath rubble?
• Fire burning quickly versus smoldering?
• Nature of fuel?
• Magnitude and type of bomb used?
• Meteorological conditions?
• Onset of vigorous convective storms to send smoke
  to very high altitudes?
• Estimates by US Nuclear Regulatory Commission on
  a large-scale war
• 360 Tg (1012 grams) of smoke
• 65-70 Tg of strongly absorbing soot particles

20
Firestorms

• Intense, large fire that produce vigorous
  whirling winds on the scale of tens of kilometers
• Strong winds in the firestorm helps to ventilate
  the fires, making them stronger
• Vertical updrafts in a firestorm are low
• Therefore, less than 10 of the smoke would be
  lofted into the stratosphere

21
Vertical Distribution of Smoke

• Vital in determining how long the smoke would
  remain in the atmosphere.
• If most remains below 2-3 km
• Residence time is only a week or so
• Example were the Kuwaiti oil fires after the Gulf
  War
• Turbulent mixing, cloud and precipitation
  scavenging, and sedimentation of particles
  removes the smoke and soot quickly
• If smoke and soot rise to upper troposphere
• The lower amounts of turbulence, cloud water
  content, and precipitation rate results in a
  longer residence time (several weeks to a month)
• If smoke and soot reach the stratosphere
• the stability and lack of clouds would result in
  a residence time of months to a year or more
  (just like volcanic aerosols following an
  eruption)

22
Water Vapor

• Water vapor would also be transported vertically
  (due to convergence of moist air and the burning
  of substances containing water
• It is anticipated that the increase in water
  vapor aloft along with widespread smoke release
  would result in persistent cirrus and altostratus
  cloud decks which would substantially alter the
  radiation balance (counteract the cooling via
  Greenhouse Effect)
• A delay in surface cooling is possible due to
  cloud formation in the mid-to-upper troposphere.
  However, if the clouds are optically thick, then
  a positive feedback to the cooling would result
  due to the increased albedo and scattering of
  sunlight

23
Other Uncertainties

• What mesoscale and regional scale responses to
  smoke and water injections will take place after
  large-scale nuclear warfare?
• The upper troposphere is likely to become very
  stable due to the heated and lofted smoke layer
  termed the smokeosphere
• The smokeosphere would greatly extend the
  survival of the smoke aloft (due to the stability
  and very little vertical mixing allowed)

24
Nuclear Winter

• Old calculations on the degree of the nuclear
  winter had a 35 degree Celsius decrease in
  summertime surface temperatures
• Winter would most likely not have as big of a
  change since the insolation amount is low to
  begin with
• Due to a reduction in radiation and winter-like
  conditions even in the summer, photosynthesis
  would suffer and many plants would die
• More recent estimates using more complicated
  models had a land surface temperature change of
  only 5-15 degrees Celsius
• This prompted some scientists to refer to this as
  nuclear fall
• The level of uncertainty in estimates of climatic
  responses to large-scale nuclear war makes the
  ramifications to the biosphere difficult to
  predict
• Crucial question Will the temperature,
  precipitation, and radiation anomalies fall
  outside the envelope of expected natural
  variability that agricultural systems cope with?

25
Consequences

• Crippled transportation industry would compound
  the problem of food distribution
• Beyond climate responses, the remaining biosphere
  would have to cope with subsequent radiation
  doses
• Nuclear winter would result in a lower amount of
  smoke and debris in the stratosphere than an
  impact caused by an asteroid or comet.
  Therefore, nuclear winter is not as long-lasting
  as the impact scenario
• Due to legitimate uncertainty in the climate
  response, it is not known to what degree nuclear
  winter or nuclear fall will take place.
  Therefore, it is not known whether the biosphere
  will take a major hit or just a blow that falls
  within normal natural variability

26
Decrease in the Threat!

• Due to the end of the Cold War and the present
  age of nuclear disarmament by Russia and the
  United States, the nuclear winter threat is not
  what it once was
• While the increased threat of a nuclear attack by
  a rogue nation or terrorists can be debated, the
  nuclear winter scenario relies upon a massive
  exchange of nuclear armaments
• Rogue nations or terrorists do not have that
  capability now, and hopefully never will!