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ASTR-1010
Planetary Astronomy
Day - 37
Titan and Dione
Course Announcements
Next Lab: 51 Pegasi: Discovery of a New Planet
Homework Chapter 11: Due Friday April 23.
Homework Chapter 12: Due Wednesday April 28.
Homework Chapter 21: Due Wednesday April 28.
-this is extra credit.
The last 1st Quarter moon observing night is:
Thursday (April 22) - 8:00.
Exam 4 and “Final”: Friday, April 30 – 1030 am
Orbits of Asteroids
• Most are in the asteroid belt between Mars
and Jupiter.
• Orbits are prograde, in the same direction
as the planets orbit.
• Orbits have small tilts from the plane of the
Solar System.
• Orbits are circular or somewhat elongated.
• Near-Earth asteroids have orbits that
cross that of the Earth.
Asteroid
Orbits
Concept Quiz  Asteroids
Asteroids are made of
A.
B.
C.
D.
dust and gas.
ices and dust.
rock and metal.
frozen carbon dioxide (“dry” ice).
Comets
• Nucleus is an ice/rock mix.
• Size of nucleus is typically ~ 5 km.
• When near Sun, comets are “active”:
– Nucleus
– Coma (head)
– Ion tail
– Dust tail
• Comet tails point away from the Sun.
Active Comets
© Terry Acomb
Comet Nuclei
NASA/JPL/Caltech
NASA/JPL-Caltech/UMD
Orbits of Comets
• Orbits are large and often elongated.
• Short-period comets:
– Periods < few centuries.
– Near ecliptic plane.
– Prograde orbits, circular or somewhat
elongated.
• Long-period comets:
– Periods 1000 to above 100,000 years.
– Prograde or retrograde orbits.
– Large tilts from the ecliptic, very elongated
orbits.
Comet Families
• Short-period comets come from the Kuiper
Belt.
– 30-1000 AU from the Sun.
– Many planetesimals orbit there (< 50 AU).
– These are called Kuiper Belt Objects (KBOs).
• Long period comets are from the Oort
Cloud (100s to 1000s AU).
Many comets come from
the Kuiper Belt
Some comets come from
the Oort Cloud
Concept Quiz – Short-period
Comets
Short-period comets are associated with
what class of objects?
A.
B.
C.
D.
Kuiper Belt objects
Terrestrial planets
The Oort Cloud
The giant planets
Concept Quiz – Distant Comets
A distant comet at its farthest point from
the Sun would have which of the
following?
A.
B.
C.
D.
Dust tail
Ion tail
Nucleus
Coma
Comets and Dust
• Comets lose dust and gas near the Sun.
• Small grains make most meteors.
• Material left behind by a comet makes
meteor showers.
• Dust in the plane of the Solar System
makes the zodiacal light.
Comets are the debris of
the Outer Solar System
When a comet approaches the inner
solar system the ice evaporates
The gas and flaked off dust
form a coma and tail
A comet’s
tail always
points away
from the sun
Comet tails
can be
millions of
kilometers
long
The tail can break off due to
“gusts” in the solar wind
Comet orbits are tilted from the
ecliptic and very eccentric
Comets “die” in one of three ways
1: They fall in to the sun
2: They break-up and fizzle out
3: They collide with a planet or moon
Collisions with the Earth
• Small collisions (dust grains) are frequent.
• Large collisions (comets, asteroids) less
so.
• Large collisions can be devastating.
• The biggest collisions have caused mass
extinctions.
The Tunguska Event
Courtesy of the Wolbach Library, HarvardSmithsonian Center for Astrophysics,
Cambridge, MA
Big rocks DO fall from the
sky!
A bad day for the dinosaurs
Chicxulub Impact Site
The
Dinosaur
Killer!
The crater is buried
several hundred meters
under the surface and is
over 200 km in diameter
Wells Creek Impact Basin
Meteor Shows are the result of
Earth passing through a debris trail
left by a comet
A Meteor
Shower
Tony Hallas/Science Faction
Meteor Showers come at
regular times of the year
Meteorites are classified as
Stones, Irons or Stony-irons
The most common meteorite,
stones look like ordinary
rocks with burnt crust
The most common “find” is
an iron meteorite
Widmanstätten Patterns
are iron crystals that take
millions of years to form
Stony-Irons are
intermediate between
stones and irons
Carbonaceous chondrites
are from the earliest age
of the solar system
The Zodiacal
Light
Courtesy of Joe Orman