Chapter 9
Asteroids, 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 own
moon.
• Asteroid Ida has a
tiny moon named
Dactyl.
Asteroid Orbits
• 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.
Which explanation for the asteroid belt
seems the most plausible?
1. The belt is where all the asteroids
happened to form.
2. The belt is the remnant of a large
terrestrial planet that used to be between
Mars and Jupiter.
3. The belt is where all the asteroids
happened to survive.
Asteroids
1. Are rocky and small–typically the size of a grain
of rice or a marble
2. Are rocky, with a wide range of sizes, up to
hundreds of miles in diameter
3. Have only thin atmospheres
4. Are made mostly of metals
5. Are mostly found in the inner solar system
If you keep pushing a person on a swing with little
pushes, at just the right frequency, they will swing
very high. This is an example of:
1.
2.
3.
4.
5.
Newton’s second law
Newton’s first law
Energy conservation
Resonance
Conservation of angular momentum
Orbital Resonances
• Asteroids in orbital
resonance with
Jupiter experience
periodic nudges.
• Eventually those
nudges move
asteroids out of
resonant orbits,
leaving gaps in the
belt.
Orbits of asteroids in the asteroid
belt
1. Often intersect planets
2. Are mostly between Mars and Jupiter
3. Are grouped into patterns by resonances with
Jupiter
4. Are mostly inside the frost line
5. All except # 1
Meteorites come from
1.
2.
3.
4.
5.
Stars–they are falling stars
Destroyed planets
Asteroids
The Moon and Mars
Volcanic ejecta
A typical meteorite is
1. About the size of a house, and makes a crater
when it lands
2. About the size of a pea or grain of rice and is
invisible when it lands
3. About the size of a pea or grain of rice. It makes
a bright streak in the sky and burns up
4. Made of ice
Meteor Terminology
• Meteorite: A rock from space that falls
through Earth’s atmosphere.
• Meteor: The bright trail left by a meteorite.
Meteorites from Moon and Mars
• A few meteorites arrive from the Moon and
Mars
• Composition differs from the asteroid
fragments
• A cheap (but slow) way to acquire moon rocks
and Mars rocks
How do comets get their tails?
Comet Facts
• Formed beyond the frost line, comets are icy
counterparts 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 from
heated nucleus.
• Plasma tail is gas
escaping from coma,
pushed by solar
wind.
• Dust tail is pushed
by light.
Growth of Tail
If Earth passed through the tail of a
comet, what would happen?
1. People would die from the gasses such as
methane and ammonia
2. It depends on if it was the gas tail or the dust tail
3. Earth might be knocked out of its orbit or its axis
might get tilted
4. Nothing. Halley’s comet did this and nothing
happened
What is plasma (in astronomy)?
1. An element commonly found in space
2. A constituent of blood
3. An ionized or charged gas, made when atoms
lose one or more electrons
4. Another name for the solar wind
Why do comet tails always point away from the
Sun?
1. They are left behind as the comet moves
2. Newton’s third law: Comet goes one way, tail
goes the other
3. The solar wind blows on them
4. They don’t; this is just a perspective effect of
how we view them
Comets eject small particles that follow the comet around in its
orbit and cause meteor showers when Earth crosses the comet’s
orbit.
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,
astronomers
discovered Eris, an
iceball even larger
than Pluto.
• Eris even has a
moon: Dysnomia.
Other Kuiper Belt Objects
• Most have been discovered very recently so
little is known about them.
• NASA’s New Horizons mission will study
Pluto and a few other Kuiper Belt objects in
a planned flyby.
Have we ever witnessed a major
impact?
Comet SL9 caused a string of
violent impacts on Jupiter in
1994, reminding us that
catastrophic collisions still
happen.
Tidal forces tore it apart during a
previous encounter with Jupiter.
This crater chain on Callisto probably came from another
comet 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 in
the diversity of species: mass extinctions.
• The most recent was 65 million years ago,
ending the reign of the dinosaurs.
Iridium Layer
No dinosaur fossils
in upper rock layers
Thin layer
containing the rare
element iridium
Dinosaur fossils in
lower rock layers
Consequences of an Impact
• A meteorite 10 km in size would send large
amounts of debris into the atmosphere.
• Debris would reduce the amount of sunlight
reaching Earth’s surface.
• The resulting climate change may have
caused mass extinction.
Likely Impact Site
• Geologists found a
large subsurface
crater about 65
million years old in
Mexico.
Comet or
asteroid
about 10
km in
diameter
approaches
Earth
Is the impact threat a real danger
or 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, 1908
A ~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 large
enough to cause
mass extinctions are
many millions of
years 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 to
help.
• We get less advance warning of a killer
comet.
What are we doing about it?
• Stay tuned to
http://impact.arc.nasa.gov
Surprising discovery?: A small asteroid that orbits
within the asteroid belt has an active volcano.
1. Plausible. Several small objects in the solar system have active
volcanoes (e.g. Io).
2. Plausible. Several asteroids are known to be composed of basaltic
(lava) material.
3. Implausible. Only planets, not moons or asteroids, have
volcanoes.
4. Implausible. Asteroids are too small to be geologically active now.
Surprising discovery?: A small asteroid that
orbits
within the asteroid belt has an active volcano.
1.
2.
3.
4.
Plausible. Several small objects in the solar system
have active volcanoes (e.g. Io).
Plausible. Several asteroids are known to be composed
of basaltic (lava) material.
Implausible. Only planets, not moons or asteroids, have
volcanoes.
Implausible. Asteroids are too small to be geologically
active now.