Chapter 9Asteroids, Comets, and Dwarf Planets

Their Nature, Orbits, and Impacts

Asteroid Facts

• Asteroids are rocky leftovers of planet formation.• The largest is Ceres, diameter ~1,000 km.• There are 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.

Asteroids with Moons• Some large asteroids

have their ownmoon.

• Asteroid Ida has atiny moon namedDactyl.

Asteroid Orbits• Most asteroids orbit

in a belt betweenMars and Jupiter.

• Trojan asteroidsfollow Jupiter’sorbit.

• Orbits of near-Earthasteroids crossEarth’s orbit.

Which explanation for the asteroid beltseems the most plausible?

1. The belt is where all the asteroidshappened to form.

2. The belt is the remnant of a largeterrestrial planet that used to be betweenMars and Jupiter.

3. The belt is where all the asteroidshappened to survive.

Asteroids

1. Are rocky and small–typically the size of a grainof rice or a marble

2. Are rocky, with a wide range of sizes, up tohundreds of miles in diameter

3. Have only thin atmospheres4. Are made mostly of metals5. Are mostly found in the inner solar system

If you keep pushing a person on a swing with littlepushes, at just the right frequency, they will swing

very high. This is an example of:

1. Newton’s second law2. Newton’s first law3. Energy conservation4. Resonance5. Conservation of angular momentum

Orbital Resonances• Asteroids in orbital

resonance withJupiter experienceperiodic nudges.

• Eventually thosenudges moveasteroids out ofresonant orbits,leaving gaps in thebelt.

Orbits of asteroids in the asteroidbelt

1. Often intersect planets2. Are mostly between Mars and Jupiter3. Are grouped into patterns by resonances with

Jupiter4. Are mostly inside the frost line5. All except # 1

Meteorites come from

1. Stars–they are falling stars2. Destroyed planets3. Asteroids4. The Moon and Mars5. Volcanic ejecta

A typical meteorite is1. About the size of a house, and makes a crater

when it lands2. About the size of a pea or grain of rice and is

invisible when it lands3. About the size of a pea or grain of rice. It makes

a bright streak in the sky and burns up4. Made of ice

Meteor Terminology

• Meteorite: A rock from space that fallsthrough Earth’s atmosphere.

• Meteor: The bright trail left by a meteorite.

Meteorites from Moon and Mars

• A few meteorites arrive from the Moon andMars

• Composition differs from the asteroidfragments

• A cheap (but slow) way to acquire moon rocksand Mars rocks

How do comets get their tails?

Comet Facts

• Formed beyond the frost line, comets are icycounterparts to asteroids.

• The nucleus of a comet is like a “dirty snowball.”• Most comets do not have tails.• Most comets remain perpetually frozen in the

outer solar system.• Only comets that enter the inner solar system

grow tails.

Anatomy of a Comet• Coma is atmosphere

that comes fromheated nucleus.

• Plasma tail is gasescaping from coma,pushed by solarwind.

• Dust tail is pushedby light.

Growth of Tail

If Earth passed through the tail of acomet, what would happen?1. People would die from the gasses such as

methane and ammonia2. It depends on if it was the gas tail or the dust tail3. Earth might be knocked out of its orbit or its axis

might get tilted4. Nothing. Halley’s comet did this and nothing

happened

What is plasma (in astronomy)?

1. An element commonly found in space2. A constituent of blood3. An ionized or charged gas, made when atoms

lose one or more electrons4. Another name for the solar wind

Why do comet tails always point away from theSun?

1. They are left behind as the comet moves2. Newton’s third law: Comet goes one way, tail

goes the other3. The solar wind blows on them4. They don’t; this is just a perspective effect of

how we view them

Comets eject small particles that follow the comet around in itsorbit and cause meteor showers when Earth crosses the comet’sorbit.

How big can a comet be?

Is Pluto a Planet?

• Much smaller than the eight major planets• Not a gas giant like the outer planets• Has an icy composition like a comet• Has a very elliptical, inclined orbit• Pluto has more in common with comets than with the

eight major planets.

Discovering Large Iceballs

• In summer 2005,astronomersdiscovered Eris, aniceball even largerthan Pluto.

• Eris even has amoon: Dysnomia.

Other Kuiper Belt Objects

• Most have been discovered very recently solittle is known about them.

• NASA’s New Horizons mission will studyPluto and a few other Kuiper Belt objects ina planned flyby.

Have we ever witnessed a majorimpact?

Comet SL9 caused a string ofviolent impacts on Jupiter in1994, reminding us thatcatastrophic collisions stillhappen.

Tidal forces tore it apart during aprevious encounter with Jupiter.

This crater chain on Callisto probably came from anothercomet that tidal forces tore to pieces.

Dusty debris at an impact site

Artist’s conception of SL9 impact

Several impact sites

Impact sites in infrared light

Did an impact kill the dinosaurs?

Mass Extinctions

• Fossil record shows occasional large dips inthe diversity of species: mass extinctions.

• The most recent was 65 million years ago,ending the reign of the dinosaurs.

Iridium Layer

Dinosaur fossils inlower rock layers

No dinosaur fossilsin upper rock layers

Thin layercontaining the rareelement iridium

Consequences of an Impact

• A meteorite 10 km in size would send largeamounts of debris into the atmosphere.

• Debris would reduce the amount of sunlightreaching Earth’s surface.

• The resulting climate change may havecaused mass extinction.

Likely Impact Site• Geologists found a

large subsurfacecrater about 65million years old inMexico.

Comet orasteroidabout 10km indiameterapproachesEarth

Is the impact threat a real dangeror just media hype?

Facts About Impacts

• Asteroids and comets have hit the Earth.• A major impact is only a matter of time: not IF but

WHEN.• Major impacts are very rare.• Extinction level events ~ millions of years• Major damage ~ tens to hundreds of years

Tunguska, Siberia: June 30, 1908A ~40 meter object disintegrated and exploded in the atmosphere

Meteor Crater, Arizona: 50,000 years ago (50 meter object)

Frequency of Impacts• Small impacts

happen almost daily.

• Impacts largeenough to causemass extinctions aremany millions ofyears apart.

The Asteroid with Our Name on It

• We haven’t seen it yet.• Deflection is more probable with years of

advance warning.• Control is critical: breaking a big asteroid

into a bunch of little asteroids is unlikely tohelp.

• We get less advance warning of a killercomet.

What are we doing about it?

• Stay tuned tohttp://impact.arc.nasa.gov

Surprising discovery?: A small asteroid that orbitswithin the asteroid belt has an active volcano.

1. Plausible. Several small objects in the solar system have activevolcanoes (e.g. Io).

2. Plausible. Several asteroids are known to be composed of basaltic(lava) material.

3. Implausible. Only planets, not moons or asteroids, havevolcanoes.

4. Implausible. Asteroids are too small to be geologically active now.

Surprising discovery?: A small asteroid thatorbits

within the asteroid belt has an active volcano.

1. Plausible. Several small objects in the solar systemhave active volcanoes (e.g. Io).

2. Plausible. Several asteroids are known to be composedof basaltic (lava) material.

3. Implausible. Only planets, not moons or asteroids, havevolcanoes.

4. Implausible. Asteroids are too small to be geologicallyactive now.