Minor Planets: Small WorldsRemnants of Rock and Ice

Asteroids, Comets, and the Kuiper Belt

Asteroids

• Our goals for learning• What are asteroids like?• Why is there an asteroid belt?

Asteroids are:

– Made of metal and rock, – Not round, – Covered in craters, – Have no atmosphere, – Main surface process is mass wasting

– Rocky leftovers of planet formation.

Asteroids

Asteroid Facts

• Largest is Ceres, diameter ~1,000 km (now a Dwarf planet)• 150,000 in catalogs, and probably over a million with

diameter >1 km.• Small asteroids are more common than large asteroids.• All the asteroids in the solar system wouldn’t add up to

even a small terrestrial planet.

Density of Asteroids• Measuring orbit of

asteroid’s moon tells us asteroid’s mass

• Mass and size tell us asteroid’s density

• Some asteroids are solid rock; others just piles of rubble

Kleopatra -spinning rubble pile

Meteorite Types1) Primitive: Unchanged in composition since

they first formed 4.6 billion years ago.

Meteorite Types2) Processed: younger; have experienced

processes like volcanism or differentiation

Why is there an asteroid belt?

• Most asteroids orbit in a belt between Mars and Jupiter

• Trojan asteroids follow Jupiter’s orbit

• Orbits of near-Earth asteroids cross Earth’s orbit

Origin of Asteroid Belt• Rocky planetesimals

between Mars and Jupiter did not accrete into a planet.

• Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

What have we learned?

• What are asteroids like?– They are battered rocky leftovers from the era

of planet formation – Chemistry of primitive meteorites show they

are remnants from solar nebula.

• Why is there an asteroid belt?– Orbital resonances with Jupiter prevented

planetesimals between Jupiter and Mars from forming a planet

What are comets like?

Two regions for small icy worlds:

1) Kuiper belt, KBOs

have orderly orbits from 30-100 AU in disk of solar system

2) Oort Cloud, worlds have random orbits out to >50,000 AU

Sources for comets.

A comet occurs when a KBO or oort cloud object gets kicked into the inner solar system.

Kuiper belt comets: orbits in plane of solar system, Short period <200 years

Oort cloud comets: random orbits, long period 1000s of years

Nucleus of Comet

• A “dirty snowball”

• Source of material for comet’s tail

Comets Visited By Spacecraft

Comets are Dirty Snowballs!• KBOs are covered in dark organic molecules.

They would look black in ordinary light, but appear paler against the blackness of space

67P/Churyumov–Gerasimenko• Visited by Rosetta in 2014, orbiter & lander• Comet is a short period Earth-crosser

67P/Churyumov–GerasimenkoClose-ups

Anatomy of a Comet• Coma is atmosphere

that comes from heated nucleus

• Plasma tail is gas escaping from coma, pushed by solar wind

• Dust tail is pushed by photons

Growth of Tail

Comet Hale-Bopp 1999

Far from the Sun. Tail starting to form

Close to the Sun. Tail streams away from Solar Wind

What have we learned?

• What are comets like?– Comets are like dirty snowballs– Most are far from Sun and do not have tails– Tails grow when comet nears Sun and nucleus

heats up• Where do comets come from?– Comets in plane of solar system come from

Kuiper Belt– Comets on random orbits come from Oort

cloud

Extra-terrestrial Hazard

Our goals for learningWhat crosses Earth's path?Have we ever witnesses a large impact?Did an impact really kill the dinosaurs?Are imapcts a real threat or media hype?

Extra-Terrestrial Hazard

• Earth regularly crosses the path of several asteroids and comets.

• Called Near-Earth-Objects or NEOs

• There are a variety of sizes of material; only those over 100m and <0.05 AU away are considered a serious threat

• Currently over 1500 known.

Asteroids and comets eject small particles that follow the body around in its orbit. Meteor showers occur when Earth crosses the small body's orbit hitting the particles.

Meteor Showers

Meteor Terminology

• Meteorite: A rock from space that falls through Earth’s atmosphere and hits the ground.

• Meteor: The bright trail left by a meteorite (doesn’t hit the ground)

Have we ever witnessed a major impact?

• The only modern large impact that we have witnessed is the 1994 Shoemaker-Levy 9 comet

• It broke up on approach and hit Jupiter in pieces

Meteor Crater, Arizona• Hit Earth about 50,000 yrs ago• Small hole- “only” one mile across• Iron asteroid about 10-50m across• Energy of 10-20 Megatons of TNT

• Crater made by the impact of a 1–2 meter object in Peru, 2007

Airbursts - Close Calls

• Tunguska, Siberia 1908– A ~40-meter object disintegrated and exploded in

the atmosphere. Tree damage

• Chelyabinsk, Russia 2013– Slow-moving 20m fragment broke up 18 miles up.

No deaths, over 1100 injured

http://www.youtube.com/watch?v=dpmXyJrs7iU

Did an Impact Really Kill the Dinosaurs?

• Dinosaurs went extinct 65 millions yrs ago.• Extra-terrestrial materials found world-wide at

the extinction layer.• Third largest crater on Earth,180km across, at

Chicxulub, Mexico is dated to ~65 million yrs old. Impactor waould have been ~10km

Impacts- Real threat or media hype?

• Asteroids and comets have hit Earth.

• A major impact is only a matter of time: not IF but WHEN.

• Major impacts are very rare.

• Extinction level events happen millions of years apart.

• Major damage happen tens to hundreds of years apart.

BUT

• Minor impacts may do little environmental damage, but massive damage to our civilization.

Frequency of Impacts

• Small impacts happen almost daily.

• Impacts large enough to cause mass extinctions happen many millions of years apart.

Influence of the Jovian Planets

• Gravity of a jovian planet (especially Jupiter) can redirect a comet.

• Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud.

What can we do about it?

• Deflection is more probable with years of advance warning.

• Control is critical: Breaking a big asteroid into a bunch of little asteroids is unlikely to help. (i.e. in movies like Armeggedon!)

• Critical to track the paths of hazardous asteroids in order to plan ahead.

What have we learned?

• What crosses Earth's path?– There are thousands of small Near-Earth-

Objects crossing the Earth's orbit– Fine dust from these objects' orbits falls on us

every year (meteor showers)

• Have we ever witnessed a large impact?– The largest impact observed by humans

occurred to Jupiter–We have had several close calls with smaller

impactors

What have we learned?

• Did an impact really kill the dinosaurs?– The dinosaurs died at the time a 10km asteroid stuck

the Earth– Exact cause and effect are still being investigated

• Are impacts a real threat or media hype?– Impacts are inevitable, but large ones are rare– Mass extinction events are predicted about once

every 100 million years– More serious threat to individual cities or societies