PTYS 214 – Fall 2019
Midterm #4 in one week!
Extra Credit! Steward public lecture: 11/4; 7:30pm; Steward Obs. N210
There's a New SAGUARO under the Desert Skies: Finding the Sources of Gravitational Waves
Michael Lundquist
Take notes and get them stamped!
Announcements
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Mass extinctions
K/Pg extinction
• Impact theory -- evidence?
• Other possible causes
Other extinctions
Previously
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Where did the K/Pg 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
Inne
r S
S
Ou
ter
SS
Jupiter
Pluto
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Comets Among the oldest bodies in the
solar system
Origin: Kuiper Belt or Oort Cloud (outskirts of the Solar System)
Very porous objects rich in ices
Contain organic material
We do not know a lot about them
First samples: 2006 Stardust Mission! Comet dust resembles asteroid material
Comet Wild 2
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What kinds of orbits would Earth-approaching comets have?
Kepler's Laws
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1. Planetary orbits are ellipses with Sun at one focus ellipticity is described by eccentricity e (0--1)
2. Equal areas swept out in equal times:
i.e., planets move fastest near periapse and slowest near apoapse
3. Square of orbital period P proportional to cube of semimajor axis a:
•i.e., larger orbits have longer periods
••periapse•apoapse
67p/Churyumov-Gerasimenko (CG) Jupiter-family comet
Q=5.689 AU; q=1.2432 AU
4.1 x 4.5 km
Visited by Rosetta spacecraft in 2014 Aug
Philae Landing: 11/12/2014
Comet Wild 2
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67p/Churyumov-Gerasimenko
2014 MU69
(Ultima Thule)
Trans-Neptunian comet
Q=46.442 AU; q=42.721 AU
31 x 19 x 14 km
Imaged by New Horizons 2018 Aug
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A natural experiment: Impact of Comet SL9 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
HST, July 27, 1994
Plumes thousands of km high! Dark “scars” lasted for months
HST, May 1994
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Near Earth Objects (NEOs)
Known asteroidsJupiter’s orbit
Mars’ orbit
Ecliptic
Main Belt
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Trojans
Trojans
What kinds of orbits would Earth-approaching asteroids have?
A/2017 U1 (Oumuamua) -- An interstellar object
Object discovered 10/19/2017 after perihelion, perigee
Passed within 0.25 AU of Sun; 0.16 AU of Earth
~ 230 x 35 m; eccentricity ~ 1.15 ==> unbound! (barely)
11comet or asteroid?
C/2019 Q4 (2I/Borisov) – Another one!
Discovered 2019 August prior perihelion
Perihelion 7 Dec.; ~2 AU
Eccentricity ~ 3 ==> very fast!
12Looks like a comet!
Near Earth Objects (NEOs)
NEOs rarely get close to Earth enough to be
considered a major hazard...
Asteroids in the neighborhood of the Earth, called
Near Earth Objects (or NEOs)
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But the possibility exists!
Peekskill Meteor, 9 Oct. 1992 – 40 seconds!Peekskill Meteor, 9 Oct. 1992 – 40 seconds!14
NEOs are potentially hazardous
Peekskill Meteorite
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1980 Chevy Malibu
Some definitions
Asteroid – Rock in space, orbits the sun, not a planet
Meteoroid – Very small asteroid
Meteor – Fireball seen during meteoroid entry
Meteorite – Former meteoroid found on the ground
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June 30, 1908The Tunguska Event
- 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 (in London!)
Early morning:
A big fireball appeared over Siberia (Russia)
It exploded in the atmosphere over the Tunguska region with an estimated force of 1,000 Hiroshima bombs
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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”)
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What happened? It was the airburst of an object 6 to 10 kilometers above the
Earth's surface
Size estimates range from 60 to 200 m across
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
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February 15, 2013: Chelyabinsk
- The shock wave injured ~1500 people
- Largest object known to have struck Earth since Tunguska.
- Captured by numerous cameras.
Early morning:
Another fireball over Russia.
As with Tunguska, it exploded in the atmosphere.
Smaller than Tunguska: ~20--30 Hiroshima bombs.
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Chelyabinsk “crater” 6-m-wide hole in the ice covering lake Chebarkul.
- Created by 700-kg fragment
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What happened?Like Tunguska, it was the airburst of a meteor in the atmosphere. Diameter: 20 m; Altitude: ~24 km
Near ground zero, several loud explosions were heard and windows were shattered in thousands of buildings. Some structural damage was reported as well.
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Impact Hazards
1 MT= 1 Mton TNT equivalent= 4.21015 J
Bolides (energy <5 MT; D< 30 m ) – no crater Great fireworks display (“shooting stars”), “no” damage Average interval for whole Earth: ~100 years?
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Impact Hazards
1 MT= 1 Mton TNT equivalent= 4.21015 J
Bolides (energy <5 MT; D< 30 m ) – no crater Great fireworks display (“shooting stars”), “no” damage Average interval for whole Earth: ~100 years?
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
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Impact Hazards
1 MT= 1 Mton TNT equivalent= 4.21015 J
Bolides (energy <5 MT; D< 30 m ) – no crater Great fireworks display (“shooting stars”), “no” damage Average interval for whole Earth: ~100 years?
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 catastrophe (<10,000 MT; D<250 m) – crater ~10km Destroys area equivalent to small country Average interval for whole Earth: >100,000 years
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Impact Hazards
1 MT= 1 Mton TNT equivalent= 4.21015 J
Bolides (energy <5 MT; D< 30 m ) – no crater Great fireworks display (“shooting stars”), “no” damage Average interval for whole Earth: ~100 years?
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 catastrophe (<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
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Terrestrial Impact Frequency
year
century
million yr
billion yr
10,000 years
100 millionmillion10,00010010.01
Hir
osh
ima
Tunguska
End-Cretaceous
TNT equivalent yield (MT)
Global catastrophe(for human civilization)
Fre
qu
ency
Meteor Crater
1 MT= 1 Mton TNT equivalent= 4.1861015 J28
Chelyabnsk
Statistical Independence
Random events are not affected by history!
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: ??
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: ??
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
Probability of rolling a six after rolling a six: ??
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
Probability of rolling a six after rolling a six: 1/6 !!
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
Probability of rolling a six after rolling a six: 1/6 !!
Probability of rolling a six after rolling 99 sixes: ??
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
Probability of rolling a six after rolling a six: 1/6 !!
Probability of rolling a six after rolling 99 sixes: 1/6 !!
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
Probability of rolling a six after rolling a six: 1/6 !!
Probability of rolling a six after rolling 99 sixes: 1/6 !!
Probability of rolling 100 sixes: ?? !!
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Statistical Independence
Random events are not affected by history!
Probability of rolling a six: 1/6
Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36
Probability of rolling a six after rolling a six: 1/6 !!
Probability of rolling a six after rolling 99 sixes: 1/6 !!
Probability of rolling 100 sixes: (1/6)100 ~ 10-78 !!
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Are impacts truly a random process?
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Known Earth-crossing asteroids as of 2013
i.e., can they be predicted?
Spaceguard Program
D>1 km
Goal: Find 90% of NEAs
with D > 1 km
by the end of 2008
As of 2013: 864 discovered
(90%<)
In the United States it is funded by NASA
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Activity: Death Risks
Rank the following causes of death in terms of risk from lowest to highest:
A) Asteroid? B) Lightning? C) Terrorist attack? D) War? E) Shark attack? F) Car accident? G) Earthquake? H) Selfie?
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Activity: Death Risks
Answers*: E) Shark attack 1:8,000,000 H) Selfie 1:2,500,000 ** D) War 1:250,000 A) Asteroid 1:200,000 G) Earthquake 1:132,000 C) Terrorist attack 1:90,000 B) Lightning 1:74,000 F) Car accident 1:84
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*CDC statistics, probably subject to large uncertainties**Anecdotal: 259/6 yr * 75 yr/life ~ 3000 → 3000/7.5d9 ~ 1: 2d6 J. Family Med Prim Care. 2018 Jul-Aug; 7(4): 828–831
Comparison with Other RisksStatistical risk of death from impacts: about 1:200,000
Much less than auto accidents (in U.S.)Comparable with other natural hazards (earthquakes, floods)
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Yet no one has ever died by asteroid!
Comparison with Other RisksStatistical risk of death from impacts: about 1:200,000
Much less than auto accidents (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)
45Analogous to human-induced climate change!
Impact cratering is normally regarded as a destructive process, dangerous
for life…
… but is it always that way?
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Impacts eject material at high speed
Could an impact eject material into space?
Could it eject rocks containing LIFE into space?47
Homework #17 available shortly on the web site
Homework
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