Midterm! Part I (Take home exam, including 10 points from Mastering Astronomy, 50 pts) is available, due October 26 th, noon Next week, Part II (in class

Midterm!• Part I (Take home exam, including 10 points

from Mastering Astronomy, 50 pts) is available, due October 26th, noon

• Next week, Part II (in class exam, 50 pts.)– Taken in 3rd hour (week of 10/22 to 10/25)– Bring SCANTRON (882 form) and #2 pencil– Based on “Review Questions” handout, available

now!

• Also: 10 of the 25 extra credit points are due by October 26th, noon.

© Sierra College Astronomy Department

2

Lecture 8: Asteroids, Comets and Dwarf Planets

Solar System Debris• Apart from the Sun (a large object) and the

planets and larger moons (medium-sized objects), most of the other objects in the solar system can be classified as Solar System debris – a collection of ice and rock fragments.

• Solar system debris comes in a number of forms, including asteroids, meteoroids, comets, dust, and Kuiper Belt Objects or Trans-Neptunian Objects).


3

• History: Bode’s Law and the “missing planet”• There are currently over 150,000 identified asteroids also known

as minor planets.• Ceres, at 1,000 km (600 mi) in diameter, is the largest asteroid

and makes up 30% of the mass of all asteroids. It large enough to be round and is therefore considered to be a dwarf planet (see later).

• Pallas and Vesta have diameters greater than 500 km. • About 23 more asteroids have diameters between 200 and 500

km. • About 100 are larger than 100 km an all the rest are under 100 km

in diameter.• There are probably more than a million asteroids with a diameter

greater than 1 km.• If you put all the asteroids together they would produce an object

barely over half the size of the Moon.


Asteroids Belt 2

Vesta CeresVesta

Belt

http://cfa-www.harvard.edu/iau/lists/NumberedMPs.html

Gaspra Mathilde

Ida and DactylOther Asteroids

ItokawaFromHaybusa

Itokawaorigin


6

The Orbits of Asteroids• The asteroids revolve around the Sun

in a counterclockwise direction like the planets.

• Most asteroids orbit in or near the plane of the ecliptic.

• Most asteroids orbit the Sun at distances from 2.2 to 3.3 AU (between Mars and Jupiter) in what is called the asteroid belt.


Asteroids

Asteroidorbits


7

• Apollo asteroids are some 50 asteroids with diameters larger than 1 km that have eccentric orbits that cross the Earth’s orbit (e.g. Eros).

• Asteroids are not evenly distributed across the asteroid belt.

• At certain distances - 2.5 and 3.28 AU - gaps appear and are related, respectively, to 1/3 and 1/2 of Jupiter’s orbital period (resonances).

• These Kirkwood gaps are due to synchronous tugs (resonances) from Jupiter.


Asteroids

Kirkwood

Eros

Belt 2


8

The Origin of the Asteroids

• Astronomers originally thought the asteroids were due to an exploded planet, but there is no known mechanism for making a planet explode.

• Most likely the asteroids are primordial material that never formed into a planet because of Jupiter’s gravitational influence.


Asteroids


9


Meteors

• Meteoroid is an interplanetary chunk of matter smaller than an asteroid.

• Meteor is the phenomenon of a streak in the sky caused by the burning of a rock or dust particle as it falls into our atmosphere.

• Meteorite is an interplanetary chunk of matter after it has hit a planet or moon.


10

Meteors• A meteoroid’s typical speed is 50 km/s, so

when it hits the Earth’s atmosphere, it heats up and begins to vaporize.

• What actually lands on the Earth?– Micrometer sized objects float down to the Earth– millimeter-sized particles burn up in mesosphere

(shooting star)– Centimeter-sized particles burn up as a fireball

(rare)– Meter-sized particle strike ground (very rare)

• It is estimated that 1,000 tons of meteoritic material hit the Earth every day.


Meteors


11

Meteoroids• It is estimated that only 1 in 1 million

meteoroids that hit the atmosphere survives to reach the surface.

• Unlike most asteroids, meteoroids may orbit the Sun in any orientation.

• It is thought that many small meteoroids are debris from asteroid collisions.

• Many other meteors come from material evaporated from a comet’s nucleus.


Meteors


12

Types of Meteorites• In every case that someone has been able to track or film a meteor

as it fell to the ground, the meteors have been discovered to originate from the asteroid belt

• There are two basic types of meteorites:– Primitive: simple mixtures of rock and metal, sometimes also

containing carbon compounds and small amounts of water– Processed: these appear to have undergone differentiation and have

a core/mantle/crust structure. Some are made mostly of iron, suggesting they came from a core of a shattered asteroid. These are generally younger than the primitive meteorites.

• Both types are informative about the early history of the solar system.

• Some meteorites have come from the Moon and Mars.


Meteors types


13

• Edmund Halley, a friend of Newton, used Newton’s methods, his own observations, and prior comet descriptions to calculate orbits for a number of comets.

• He correctly surmised that these prior comets were in fact the same comet. He correctly predicted the next return of the comet that was then named in his honor.

• Comet Halley is probably the most famous periodic comet.


Comets


14

• The planes of revolution of comets are not limited to the ecliptic but are randomly oriented.

• Consequently, comets sweep past the Sun from all directions.

• Periods of revolution vary from a few years to millions of years.


Comets

Comet Composition• Head:

– nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solid

– coma: dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleus

• hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen

• Tail:– ion tail: as much as 1 AU long composed of plasma and

laced with rays and streamers caused by interactions with the solar wind.

– dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases this is the most prominent part of a comet to the unaided eye and is caused by interaction from solar radiation pressure.

Cometanatomy

Cometanatomy2


16

• Fred Whipple proposed in 1950 that the nucleus of a comet is essentially a dirty snowball, as opposed to a “traveling gravel bank”.

• The composition of the nucleus is water ice, frozen carbon dioxide, other ices, and small solid grains.

• The nucleus also has a significant fraction of organic material.


Comets


17

• Giotto, a European spacecraft, revealed that Halley’s coma is billions of times less dense than the atmosphere of the Earth at sea level.

• The nucleus of Comet Hale-Bopp is about 15 km across and spins once every 12 hours (very similar to what Giotto found for Halley’s comet).

• As Hale-Bopp’s nucleus spun, material was ejected from it in geysers and spiraled away from it.


Comets

HB-not working

Recent Comets• Becomes bright/prominent when close to

the Sun–Dozens of faint ones seen per year–A bright one seen once in ~ 10 years

• Hale-Bopp (1997), Hyakutake (1996), West (1976)

• Halley (1986)• Semi-bright NEAT, LINEAR and Bradfield (2004)• Semi-bright Machholz (2004-05)• Semi-bright SWAN (2006)• The Great Comet McNaught (January 2007)

Hale-Bopp March 1997

Hale-Bopp and The Andromeda Galaxy

Dust Tail

Ion Tail

Comet Hale-Bopp (April 1997)

Comet Hyakutake (March-April 1996)

Comet West (1976)

Comet Halley and the Galactic Center

Halley’s Comet (1986)

The head of Comet Halley (1910)

Halley’sNucleus (1986)

Nucleus of Halley’s Comet

Comet Machholz (C/2004 Q2) and the Pleiades (M45)

7 January 2005

Jan 9Montana

McNaught (C/2006 P1), the Great Comet of

2007

Jan 17Cape Town, South Africa

Jan 20Siding Spring Obs.

NSW, Australia

Comet Wild 2 as seen by Stardust (2004)

Comet is 5 km across

Stardust collected samples and has returned to Earth

Results

http://stardust.jpl.nasa.gov/mission/webcam.html

http://stardust.jpl.nasa.gov/



“Deep Impact” to Comet Tempel 1

• Objective: Determine the structure and composition of comet Tempel 1

• Significance of results– Comets formed in outer regions of the solar

system– Early planetary material still frozen inside– We can learn much about the formation of the

solar system by analyzing the composition of “pristine” comets

– Tempel 1 is a “pristine” or “well-preserved” comet

“Deep Impact” Mission Results• Mission Results

– July 3, 2005 • 820 lb “Hammer” Impacts Comet 9P/Tempel 1

– Ejected fluffy “powder” like talcum powder• Mostly dust (silicates), steam and carbon dioxide

• Other compounds include carbonates, aromatic hydrocarbons

• Some ice found on surface (surprise!)

– Preliminary results indicate nucleus composition more like a “fluff ball” than an “ice cube”

– Scientific analysis ongoing

DeepImpact


33

Comet Tails• A comet’s tail always points away from the Sun

(and thus does not always follow the comet’s head).

• After passing the Sun, a comet’s tail actually leads the head.

• The comet’s “straight” (ion) tail consists of charged molecules (ions) which are dynamically influenced by the Solar Wind.

• The curved diffuse (gas) tail is caused by dust in the coma being pushed away by solar radiation pressure.


Comets

Cometmotion2

Cometmotion


34

• Öort cloud: aphelia of billions of comets lie about 10,000 – 100,000 AU from the Sun,–Proposed by Jan Öort in 1950– Icy chunks of material ejected by Jovian

planets in early solar system formation–Today, a few are Perturbed by nearby stars

and brought in• Kuiper belt: comets which lie just outside

Neptune– Includes Trans-Neptunian objects/Plutinos–Pluto may be king of these objects

Oort

Oort2

KuiperBelt


Comet Origins


35


Comet Origins

What is the fate of a comet?• It can impact a planet or Sun

– Like Shoemaker-Levy 9 into Jupiter

• It can get ejected out of the solar system

• It can get put into a shorter orbit– Eventually “burns-out” from repeated close

encounters with the solar wind near perihelion which cause evaporation of nucleus and/or volatile material


36

Lecture 8c: Pluto and the Kuiper Belt

The Discovery of Pluto

• Clyde Tombaugh used a blink comparator to compare two photos of the sky taken a few days apart.

• In 1989 Pluto was as close to the Earth as it had been for 248 years. (From 1979 to 1999 Pluto was inside Neptune’s orbit.)

• Pluto’s average distance from the Sun is 40 AU, but its eccentric orbit causes it to vary in distance from 30 AU to 50 AU.

Pluto and Charon (1994)

Pluto closeup (1996)

Hubble Space Telescope (HST) pictures


38

• Stellar occultations indicate that Pluto has a thin nitrogen, carbon monoxide and methane atmosphere. At aphelion, it is probably too cold for Pluto to maintain an atmosphere.

• Pluto’s atmosphere limits an accurate determination of its size, which probably ranges from 2,362 to 2,412 km.

• In 1978, J. Christy discovered that Pluto has a moon, now named Charon (KAIR en or SHAHR en).


Pluto


39


Pluto and Charon• New Horizons Mission to Pluto

– Launched: January, 2006– Jupiter Flyby: 28 Feb 2007– First Flyby Opportunity: July 2015– Other Targets: Centaurs and

KBOs– The two newest moons of Pluto

(Nix and Hydra) were named not only because of there mythological connection to the underworld, but because the their initials are the same as those of the spacecraft mission to Pluto.


40


Pluto• Charon’s diameter (1,200 km) is about half that of Pluto• Pluto’s mass is about 12 times that of Charon but only 1/5 that of the

moon• Charon density 1.2-1.3 g/cm3; Pluto 1.8-2.1 g/cm3

• Charon is less than 9 Pluto diameters away from Pluto (compare: Moon ¼ diameter of Earth, and 30 Earth diameters away from Earth)

• Charon orbit is tilted 119o to Pluto’s orbit around the Sun (i.e. it is in the equatorial plane of Pluto)

• Despite their small size, they are tidally looked in a 1:1 resonance with Charon orbiting Pluto every 6.4 days, the same as Pluto’s rotation

• But wait there’s more!

PlutoSurface Map

More… Named


41

A Former Moon of Neptune?• Because Pluto is small and has an eccentric

orbit, some theorize that it a former moon of Neptune that was somehow ejected.

• The discovery of Charon (and now 2 other moons) made it seem less likely that Pluto was once Neptune’s moon.

• Also, the large difference in density between Charon and Pluto points to Charon’s capture by Pluto.


Origins of Pluto


42

Planet, Asteroid, or ???• Pluto doesn’t fit the asteroid

classification since its density and composition is more consistent with a satellite of Jovian planet

• In the 1990s Pluto was proposed to be the largest member of the “plutino” class objects found in the Kuiper belt.


Origins of Pluto KuiperBelt


43

Kuiper Belt ???• In addition to the Asteroid Belt, the Solar System appears to

have a second belt, now called the Kuiper belt:– Support for this comes from the detection of about 600

small, presumably icy, bodies orbiting near and beyond Pluto (first object discovered was 1992QB1).

– Extent of belt is unknown, but statistical analysis indicates that the Kuiper belt may have an total mass far greater than that found in the asteroid belt.

– Objects in the belt are sometimes referred to as KBOs or plutinos, and Quaoar (found in 2002 with a diameter of 1250 km), Sedna (discovered in 2003 with a diameter of 1600 km), and 2004DW are among its largest members.


The Kuiper Belt

AsteroidBelt

KuiperBelt

“Sedna”: was the furthest object in solar system ever seen (13 billion km away) Detected Nov 2003

Likely size: 1300 to 1700 km Bigger than Charon but smaller than Pluto

The Inuit goddess of the ocean

Sedna Orbit

• Orcus was discovered in 2004 and is a bit smaller than Sedna• 2003 EL61 was announced in July 2005 is likely bigger than Sedna (and has a moon)


45


A New Planet?• July 2005: Mike Brown and

associates announced the discovery of 2003UB313 as the “tenth planet”

• 2003UB313 is just a bit larger than Pluto– How can they know that?

• Discovered some 97 AU from the Sun, near its aphelion– It’s the furthest object detected in our solar

system

• Huge 557-year eccentric orbit takes it within 38 AU of Sun

• It was discovered to have a moon! But what to name these objects ….

KBOComp

Xena and Gabrielle

2003 UB313 and Friend

orbit

Eris and Dysnomia

http://skyandtelescope.com/news/article_1603_1.asp

http://skyandtelescope.com/news/article_1603_1.asp


47

Associations• Triton’s orbit is “backwards” and is highly tilted with

respect to Neptune’s equator – Triton is perhaps a captured planetesimal from the Kuiper belt

• Short-period comets are now believed to be icy nuclei from the Kuiper belt

• The centaurs, of which Chiron is a well-known example, appear to originate from the Kuiper belt.

• 2003 UB313 (Eris) is a KBO larger than Pluto, in an orbit that crosses that of Pluto, and has a moon (Gabrielle?)

• Should Pluto still be considered a planet or a member of the Kuiper belt?


The Kuiper Belt


48

Good-bye Planet Pluto• Recently, Pluto has been demoted the ranks of

planethood at the International Astronomical Union Meeting last August

• One initial committee suggested that not only was Pluto a planet, but Charon, Eris, and asteroid Ceres were also planets

• After much debating however….• A planet is officially defined as an object …

– that is in orbit about the sun– has sufficient mass for its self gravity to overcome rigid-body

forces so that it assumes [a nearly round] shape.– has cleared the neighborhood around its orbit


49

What happens when something strikes Earth?• We have evidence of things hitting the Earth

– Craters, meteorites

• As a general rule the craters made by meteors are 10 times bigger than the impactor

• The most prominent impact crater on Earth is Meteor Crater near Winslow, Arizona.

• There may have been impacts which affected life significantly– Chicxulub meteor which landed off the Yucatán

Peninsula may have wiped out the dinosaurs

Lecture 13: Solar System Debris

Collisions

MeteorCrater


50

Have we ever witnessed a major impact?• We have not witnessed a major impact on a

solid body, but in 1994 Comet Shoemaker-Levy 9 (SL9) impacted into Jupiter.

• This event had 2 effects:– It was one of the best examples of international

cooperation– It made the public awareness of current nature

of giant collisions in our solar system


Collisions

SL9

SL9 intoJupiter

SL9 scars


51

Did an impact kill the dinosaurs?• We have identified more than 150 impact craters

on the Earth• One impact, off the coast of the Yucatan

Peninsula, may have wiped out the dinosaurs 65 million years ago

• Clues such as the deposit of iridium sediment (coming from an asteroid) at the right geological depth in the soil helps verify such a claim

• This meteor impact lead to a mass extinction where 99% (and 75% of the species) were extinguished


Collisions

Iridiumline

Yucatan


52

Is the impact threat real?• There are certainly many objects that could hit us: we

have detected over 800 asteroids over 1 km in size which pass near the Earth’s orbit.

• The threat is real, but the chances of something big hitting us in our lifetime is small

• Nevertheless, we were hit by a comet or asteroid in the region of Tunguska, Siberia in 1908 resulting in a tremendous explosion. A hit like this over a major city would be devastating.

• If a big asteroid were headed for us, could we prevent the impact?


Collisions

Tunguska

ImpactEffectsgraph


53

The End

Documents

Midterm! Part I (Take home exam, including 10 points from Mastering Astronomy, 50 pts) is available, due October 26 th, noon Next week, Part II (in class