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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



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




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




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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Probability of rolling a six: ??

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Probability of rolling a six: 1/6

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Probability of rolling two sixes in a row: ??

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Probability of rolling two sixes in a row: 1/6 x 1/6 = 1/36

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Probability of rolling a six after rolling a six: ??

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Probability of rolling a six after rolling a six: 1/6 !!

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Probability of rolling a six after rolling 99 sixes: ??

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Probability of rolling a six after rolling 99 sixes: 1/6 !!

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Probability of rolling 100 sixes: ?? !!

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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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Announcements - lpl.arizona.edu · G) Earthquake 1:132,000 C) Terrorist attack 1:90,000 B)...

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