Small Bodies of the Solar System
Pluto, Comets, Asteroids,
Meteors and Zodiacal Light
PLUTO
God of the Underworld
Pluto Physical Data
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Discovered by Clyde W. Tombaugh in 1930
Diameter: 2294 km (0.18 Dearth)
Mass: 1.2x1025 g (0.002 Mearth)
Density: 1.84 g/cm3
Rotation Period: 6.39 days
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Tilt of Axis: 96 (retrograde!)
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Surface Temperature: 43 K (-382 F)
Pluto Physical Data
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Orbital Semi-Major Axis: 39.44 AU
Orbital Period: 247.7 years
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Orbital Inclination: 17.2
Orbital Eccentricity: 0.250
Surface Gravity: 0.06 Earth gravity
Satellites: 1
Magnetic Field: unknown
The Discovery of Planet X
• Discovered Feb 8,
1930 by then 24-year
old Clyde Tombaugh
• Its existence was
predicted by Percival
Lowell using the same
techniques that
Leverrier used to find
Neptune
The Discovery of Planet X
January 23, 1930
January 29, 1930
The Plates on which Clyde Tombaugh found Pluto
Pluto’s Surface
• This image was taken
by the Hubble Space
Telescope and is the
best surface map yet
made
• Only contrasts are
seen, no features
• Pluto has not yet been
visited by spacecraft
Pluto’s Atmosphere
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Very thin atmosphere (when closest to Sun)
Envelopes its moon too
Mostly made of Nitrogen and Methane
Atmosphere condenses and snows to the
surface of Pluto and its moon when they are
farther from the Sun
Pluto’s Moon
CHARON:
• Discovered by James
Christy in 1978
• Saw a bump move from
one side of Pluto to the
other about every six
days
• About half the size of
Pluto, orbits retrograde
One of Christy’s images
Pluto’s Moon
• As seen by Hubble Space Telescope
Charon
Pluto
19,700 km
Pluto’s Strange Orbit
• Pluto’s Orbit is highly eccentric (0.250),
highly inclined to the plane of the solar
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system (17.2 ), and its orbit crosses
Neptune’s!
• Since Jan 23, 1979 and until March 15,1999
Pluto is closer to the Sun the Neptune!
Pluto’s Orgin
• Since Pluto and Charon are thought to have
the same composition as Neptune’s Triton,
it is suggested that they might be escaped
satellites of Neptune (dynamicists say no)
• Or Triton, Pluto and Charon all formed near
Neptune and only Triton was captured
Asteroids
The Asteroid Belt
Theory 1:
• Material between Jupiter and Mars tried to form a
planet when the Solar System was forming, but
Jupiter’s gravitational influence wouldn’t let it
happen
Theory 2:
• A planet did form between Jupiter and Mars
(Asteroidia), and some catastrophic event
destroyed the planet, leaving the asteroids
The Discovery of Asteroids
• On the first day of the 19th C. (Jan 1,1801),
Ceres (the largest known asteroid) was
discovered by Giuseppe Piazzi
• By the end of the 19th C., several hundred
were known
• We now know of more than 7000
Particular Asteroids
CERES:
• The largest asteroid by far
• Diameter : 914 km
• Contains 25% of the mass of all the asteroids combined
The next largest are Pallas, Vesta, and Hygiea which are
between 400 and 525 km in diameter
All other known asteroids are less than 340 km across
Particular Asteroids
IIDA and DACTYL:
• Ida was encountered by
the Galileo spacecraft in
1993
• Ida was found to have a
small satellite, Dactyl
• Dactyl’s size is 1.6 x 1.2
km, and orbits 90 km
above Ida
Comets
Comet West
Comet Basics
• Comets are mostly (~50%) water ice
• Comets are referred to as “dirty snowballs”
• They are a mixture of water ice, frozen
gases, and silicate materials
• Comets have very eccentric orbits
• They only have tails when close to the Sun
• The tails are made of gas and dust released
by the comet.
Anatomy of a Comet
Direction
of Comet’s
Motion
Ion Tail
Coma
Nucleus
To Sun
Hydrogen Cloud
Dust Tail
History of Comets
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Comet Halley
• Orbits Sun every 76
years
• Furthest point in its
orbit is just beyond the
orbit of Neptune
• Nucleus: 15 x 8 x 8 km
• Rotates every 7.6 days
Recent Comets
Comet Hyakutake in March of 1996
Image taken by J. De Buizer and J. Radomski of the University of Florida Department of Astronomy
Recent Comets
Comet Hale-Bopp in
March 1997
Image taken by J. DeBuizer and
J.Radomski of the University of
Florida Department of
Astronomy
Meteors, Meteorites, and Meteor
Showers
Meteor Nomenclature
Meteoroids - interplanetary debris
Meteor - Also called “shooting star”
When a meteorite has entered the
atmosphere creating a streak of light
Meteorite - Those few meteoroids that make
it to the Earth’s surface
Types of Meteorites
IRONS:
• Mostly made of iron
with about 9% nickel
STONES:
• Primarily silicates
similar to Earth rocks
STONY-IRONS:
• Mixture of the above
two types
Meteor Showers
• Associated with debris left behind by
comets
• Typically very small meteoroids, so no
meteorites are produced during a shower
• Can be as many as 100 meteors per hour in
a good shower
Meteor Impacts
• One catastrophic meteor impact every 26
million years
• Might have been responsible for dinosaur
extinction
Barringer crater in Arizona (1200 m in diameter, 200 m deep)
Zodiacal Light and Gegenschein
• Zodiacal light is due to
dust, concentrated in the
plane of the solar system,
that reflects the light of
the Sun towards our eyes
• Reflection from the dust
also causes a patch of light
Horizon
directly opposite the Sun,
called the gegenschein
Ecliptic
Zodiacal
Light
Sun