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PTYS 214 – Spring2011
Announcements
 Homework #7 available for download from the class website
Due Tuesday, Mar. 29
 Class website:
http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/
Useful Reading: class website  “Reading Material”
http://en.wikipedia.org/wiki/Near-Eart_object
http://neo.jpl.nasa.gov/stats/
http://users.tpg.com.au/users/tps-seti/swaprock.html
Extra Credit Presentation
Sara Cohen
Viktoriya Stoeva
Where did the K/P impactor come from?
Asteroids: small bodies that are made of rock
- Located in the Asteroid Belt (between Mars and Jupiter)
Comets: small bodies made of rock and ice (“dirty snowball”)
- Located in the Oort Cloud and in the Kuiper Belt
Outer SS
Inner SS
Jupiter
Comets
 Among the oldest bodies in the
solar system
 Origin: Kuiper Belt or Oort Cloud
(outskirts of the Solar System)
 Contain organic material
 Very porous objects rich in ices
Comet Wild 2
We do not know a lot about them
First samples: 2006 Stardust Mission!
Comet dust resembles asteroid material
A natural experiment:
Impact of Comet SL9
HST, May 1994
 Comet Shoemaker-Levy 9 was torn into pieces as a result of a
close approach to Jupiter in July 1992
 Discovered in 1993, it collided with Jupiter at a speed of 60 km/s
(135,000 mi/hr!) during the third week of July 1994
Plumes thousands of km high!
Dark “scars” lasted for months
HST, July 27, 1994
Known asteroids
Jupiter’s orbit
Ecliptic
Near Earth Objects
(NEOs)
Main Belt
Mars’ orbit
Near Earth Objects (NEOs)
Asteroids in the
neighborhood of
the Earth, called
Near Earth
Objects (or NEOs)
NEOs rarely get
close to Earth
enough to be
considered a major
hazard
But the possibility exists!
Peekskill Meteor, 9 Oct. 1992 – 40 seconds of glory!
NEOs are potentially hazardous
Peekskill
Meteorite
June 30, 1908
The Tunguska Event
Early morning:
A big fireball raced
through the dawn sky
over Siberia (Russia)
It exploded in the
atmosphere over the
Tunguska region with an
estimated force of 1,000
Hiroshima bombs
- The atmospheric shock wave knocked people off their feet
and broke windows up to 650 km (400 miles) away
- For few weeks, night skies were so bright that one could
read in their light
Tunguska: No crater!
1927: The first expedition to the site found a region of
scorched trees about 50 km across and no crater!
- Most trees had been knocked down pointing away
from the center (“ground zero”)
What happened?
 It was the airburst of a meteor 6 to 10 kilometers above the
Earth's surface
 Near ground zero, trees were knocked down by the shock
wave produced by a large explosion, similar to the effects
observed in atmospheric nuclear tests in the 1950s and 1960s
 Alternate Explanation: the Tunguska event is the result of an
exploding alien spaceship or an alien weapon going off to
"save the Earth from an imminent threat"
No evidence was ever found by UFO sympathizers…
Asteroids Hazard
Bolides (energy <5 MT; D< 30 m ) – no crater
 Great fireworks display (“shooting stars”), no damage
Small Impact (<15Mt; D< 50 m) – crater ~1 km
 Damage similar to large nuclear bomb (city-destroyer)
 Average interval for whole Earth: >1,000 years
Local catastrophes (<10,000 MT; D<250 m) – crater ~10km
 Destroys area equivalent to small country
 Average interval for whole Earth: >100,000 years
Global catastrophe (>106 MT; D>1 km) – crater >50 km
 Global environmental damage, threatening civilization
 Average interval for whole Earth: >1 million years
1 MT= 1 Mton TNT equivalent= 4.21015 J
Terrestrial Impact Frequency
year
10,000 years
Hiroshima
Time
century
Tunguska
Meteor
Crater
million yr
Global catastrophe
(for human civilization)
End-Cretaceous
billion yr
0.01
1
100
10,000
million
TNT equivalent yield (MT)
1 MT= 1 Mton TNT equivalent= 4.1861015 J
100 million
Spaceguard Program
In the United States it is funded by NASA
Goal:
Find 90% of NEAs
with
D > 1 km
by the end of 2008
Mar. 16, 2011:
822 discovered
(>85%)
D>1 km
Comparison with Other Risks
Statistical risk of death from impacts: 1 in a million per year
(about 1:20,000 over a lifetime)
Much less than auto accidents, shootings (in U.S.)
Comparable with other natural hazards (earthquakes, floods)
It is a different kind of risk!
 Average interval between major impact disasters is larger
than for any other hazard we face (millions years)
 A single event can kill millions of people (and other living
things) !
 Unique as major threat to civilization (comparable to a
global nuclear war)
Impact cratering is normally regarded
as a destructive process, dangerous
for life…
… but is it always that way?
Impacts eject material at high speed
Could an impact eject material into space?
Could it eject rocks with LIFE into space?
Near-surface rocks can
be ejected at high
speed without serious
damage (low shock)
Could microbes contained
in ejected rocks be ejected
alive?
YES!
Nicholson et al. (2009) Bacterial spores survive hypervelocity launch by
spallation: Implicatons for lithopanspermia, 27th Lunar Planet. Sci. Conf.
Microbes could be transferred from
one large body to another, but they
must survive a host of hazards!
1. Launch
2. Space
exposure
3. Landing
Material ejected from
planets in large
impacts wanders
around the solar
system, rather than
traveling directly from
planet to planet
Reaching another
planet may take tens
of millions of years!
Can life survive
this long without
nurishment?
Spore-forming bacteria are tough
Spore-forming bacteria Bacillus
Subtilis survived a 6-years
spaceflight, experiencing
vacuum, cold, lack of water and
radiation
Dormant microbes
may survive for tens
of millions of years
(can contamination
really be ruled out?)
Science 268 (1995) 1060-1064
Some micro-organisms can
tolerate a lot of cosmic
radiation, but not much UV,
so they have to hide in rocks
Deinococcus Radiodurans
Listed in The Guinness Book of
World Records as the world’s
toughest bacterium
Surviving reentry and
landing is difficult
…yet meteorites contain
fragile organic molecules
like amino acids !
So, It is highly probable that viable microbes could
be carried from Earth to Mars or vice versa
Quiz Time !
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