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Solar System Debris
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Solar System Debris
Solar System Debris
• Apart from the Sun (a large object) and the
planets and larger moons (medium-sized
objects), most of the other objects in the
solar system can be classified as Solar
System debris – a collection of ice and rock
fragments.
• Solar system debris comes in a number of
forms, including asteroids, meteoroids,
comets, dust, and Kuiper Belt Objects or
Trans-Neptunian Objects).
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Solar System Debris
Asteroids
• History: Bode’s Law and the “missing planet”
• There are currently over 150,000 identified asteroids also
known as minor planets.
• Ceres, at 1,000 km (600 mi) in diameter, is the largest
asteroid and makes up 30% of the mass of all asteroids. It
large enough to be round and is therefore considered to be a
dwarf planet (see later).
• Pallas and Vesta have diameters greater than 500 km.
• About 23 more asteroids have diameters between 200 and
500 km.
• About 100 are larger than 100 km an all the rest are under
100 km in diameter.
• There are probably more than a million asteroids with a
diameter greater than 1 km.
• If you put all the asteroids together they would produce an
object barely over half the size of the Moon.
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Solar System Debris
Asteroids
The Orbits of Asteroids
• The asteroids revolve around the Sun
in a counterclockwise direction like the
planets.
• Most asteroids orbit in or near the plane
of the ecliptic.
• Most asteroids orbit the Sun at
distances from 2.2 to 3.3 AU (between
Mars and Jupiter) in what is called the
asteroid belt.
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Solar System Debris
Asteroids
• Apollo asteroids are some 50 asteroids with
diameters larger than 1 km that have eccentric
orbits that cross the Earth’s orbit (e.g. Eros).
• Asteroids are not evenly distributed across the
asteroid belt.
• At certain distances - 2.5 and 3.28 AU - gaps
appear and are related, respectively, to 1/3 and
1/2 of Jupiter’s orbital period (resonances).
• These Kirkwood gaps are due to synchronous
tugs (orbital resonances) from Jupiter.
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Solar System Debris
Asteroids
The Origin of the Asteroids
• Astronomers originally thought the asteroids
were due to an exploded planet, but there is
no known mechanism for making a planet
explode.
• Most likely the asteroids are primordial
material that never formed into a planet
because of Jupiter’s gravitational influence.
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Solar System Debris
Meteors
• Meteoroid is an interplanetary chunk of
matter smaller than an asteroid.
• Meteor is the phenomenon of a streak
in the sky caused by the burning of a
rock or dust particle as it falls into our
atmosphere.
• Meteorite is an interplanetary chunk of
matter after it has hit a planet or moon.
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Solar System Debris
Meteors
Meteors
• A meteoroid’s typical speed is 50 km/s, so
when it hits the Earth’s atmosphere, it heats
up and begins to vaporize.
• What actually lands on the Earth?
– Micrometer sized objects float down to the Earth
– millimeter-sized particles burn up in mesosphere
(shooting star)
– Centimeter-sized particles burn up as a fireball
(rare)
– Meter-sized particle strike ground (very rare)
• It is estimated that 1,000 tons of meteoritic
material hit the Earth every day.
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Solar System Debris
Meteors
Meteoroids
• It is estimated that only 1 in 1 million
meteoroids that hit the atmosphere survives
to reach the surface.
• Unlike most asteroids, meteoroids may orbit
the Sun in any orientation.
• It is thought that many small meteoroids are
debris from asteroid collisions.
• Many other meteors come from material
evaporated from a comet’s nucleus.
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Solar System Debris
Meteors
Meteor Showers
• Meteor shower is the phenomenon of a large group
of meteors seeming to come from a particular area of
the celestial sphere. The Earth actually passes
through a swarm of small meteoroids.
• Radiant of a meteor shower is the point in the sky
from which the meteors of a shower appear to
radiate.
• Most of the major meteor showers are associated
with comets.
• Some showers change in intensity from year to year
because the swarm of particles that cause the
shower clump together in one region of the comet’s
orbit.
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Solar System Debris
Meteors
Types of Meteorites
• In every case that someone has been able to track or film a
meteor as it fell to the ground, the meteors have been discovered
to originate from the asteroid belt
• There are two basic types of meteorites:
– Primitive: simple mixtures of rock and metal, sometimes also
containing carbon compounds and small amounts of water
– Processed: these appear to have undergone differentiation and have
a core/mantle/crust structure. Some are made mostly of iron,
suggesting they came from a core of a shattered asteroid. These are
generally younger than the primitive meteorites.
• Both types are informative about the early history of the solar
system.
• Some meteorites have come from the Moon and Mars.
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Solar System Debris
Comets
• Edmund Halley, a friend of Newton, used
Newton’s methods, his own observations,
and prior comet descriptions to calculate
orbits for a number of comets.
• He correctly surmised that these prior
comets were in fact the same comet. He
correctly predicted the next return of the
comet that was then named in his honor.
• Comet Halley is probably the most famous
periodic comet.
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Solar System Debris
Comets
• The planes of revolution of comets are
not limited to the ecliptic but are
randomly oriented.
• Consequently, comets sweep past the
Sun from all directions.
• Periods of revolution vary from a few
years to millions of years.
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Solar System Debris
Comet Composition
• Head:
– nucleus: relatively solid and stable, mostly ice and gas with a small
amount of dust and other solid
– coma: dense cloud of water, carbon dioxide and other neutral gases
sublimed off of the nucleus
– hydrogen cloud: huge (millions of km in diameter) but very sparse envelope
of neutral hydrogen
• Tail:
– ion tail: as much as 1 AU long composed of plasma and laced with
rays and streamers caused by interactions with the solar wind.
– dust tail: up to 10 million km long composed of smoke-sized dust
particles driven off the nucleus by escaping gases this is the most
prominent part of a comet to the unaided eye and is caused by
interaction from solar radiation pressure.
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Solar System Debris
Comets
• Fred Whipple proposed in 1950 that the
nucleus of a comet is essentially a dirty
snowball, as opposed to a “traveling gravel
bank”.
• The composition of the nucleus is water ice,
frozen carbon dioxide, other ices, and small
solid grains.
• The nucleus also has a significant fraction of
organic material.
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Solar System Debris
Comets
• Giotto, a European spacecraft, revealed that
Halley’s coma is billions of times less dense
than the atmosphere of the Earth at sea
level.
• The nucleus of Comet Hale-Bopp is about
15 km across and spins once every 12 hours
(very similar to what Giotto found for Halley’s
comet).
• As Hale-Bopp’s nucleus spun, material was
ejected from it in geysers and spiraled away
from it.
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Solar System Debris
Recent Comets
• Becomes bright/prominent when close to the Sun
– Dozens of faint ones seen per year
– A bright one seen once in ~ 10 years
• Hale-Bopp (1997), Hyakutake (1996), West (1976)
• Halley (1986)
• Semi-bright NEAT, LINEAR and Bradfield (2004)
• Semi-bright Machholz (2004-05)
• Semi-bright SWAN (2006)
• The Great Comet McNaught (January 2007)
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Solar System Debris
“Deep Impact” to Comet Tempel 1
• Objective: Determine the structure and
composition of comet Tempel 1
• Significance of results
– Comets formed in outer regions of the
solar system
– Early planetary material still frozen inside
– We can learn much about the formation of
the solar system by analyzing the
composition of “pristine” comets
– Tempel 1 is a “pristine” or “well-preserved”
comet
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Solar System Debris
“Deep Impact” Mission Results
• Mission Results
– July 3, 2005
• 820 lb “Hammer” Impacts Comet 9P/Tempel 1
– Ejected fluffy “powder” like talcum powder
• Mostly dust (silicates), steam and carbon dioxide
• Other compounds include carbonates, aromatic
hydrocarbons
• Some ice found on surface (surprise!)
– Preliminary results indicate nucleus composition
more like a “fluff ball” than an “ice cube”
– Scientific analysis ongoing
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Solar System Debris
Comets
Comet Tails
• A comet’s tail always points away from the Sun
(and thus does not always follow the comet’s
head).
• After passing the Sun, a comet’s tail actually
leads the head.
• The comet’s “straight” (ion) tail consists of
charged molecules (ions) which are dynamically
influenced by the Solar Wind.
• The curved diffuse (gas) tail is caused by dust in
the coma being pushed away by solar radiation
pressure.
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Solar System Debris
Comet Origins
• Öort cloud: aphelia of billions of comets lie
about 10,000 – 100,000 AU from the Sun,
– Proposed by Jan Öort in 1950
– Icy chunks of material ejected by Jovian
planets in early solar system formation
– Today, a few are Perturbed by nearby stars
and brought in
• Kuiper belt: comets which lie just outside
Neptune
– Includes Trans-Neptunian objects/Plutinos
– Pluto may be king of these objects
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Solar System Debris
Comet Origins
What is the fate of a comet?
• It can impact a planet or Sun
– Like Shoemaker-Levy 9 into Jupiter
• It can get ejected out of the solar
system
• It can get put into a shorter orbit
– Eventually “burns-out” from repeated close
encounters with the solar wind near
perihelion which cause evaporation of
nucleus and/or volatile material
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Solar System Debris
The Discovery of Pluto
• Clyde Tombaugh used a blink
comparator to compare two photos
of the sky taken a few days apart.
• In 1989 Pluto was as close to the
Earth as it had been for 248 years.
(From 1979 to 1999 Pluto was
inside Neptune’s orbit.)
• Pluto’s average distance from the
Sun is 40 AU, but its eccentric orbit
causes it to vary in distance from 30
AU to 50 AU.
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Solar System Debris
Pluto and Charon
• Stellar occultations indicate that Pluto has a
thin nitrogen, carbon monoxide and
methane atmosphere. At aphelion, it is
probably too cold for Pluto to maintain an
atmosphere.
• Pluto’s atmosphere limits an accurate
determination of its size, which probably
ranges from 2,362 to 2,412 km.
• In 1978, J. Christy discovered that Pluto has
a moon, now named Charon (KAIR en or
SHAHR en).
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Solar System Debris
Pluto and Charon
• New Horizons Mission to Pluto
– Launched: January, 2006
– Jupiter Flyby: 28 Feb 2007
– First Flyby Opportunity: July
2015
– Other Targets: Centaurs and
KBOs
– The two newest moons of
Pluto (Nix and Hydra) were
named not only because of
there mythological connection
to the underworld, but
because the their initials are
the same as those of the
spacecraft mission to Pluto.
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Solar System Debris
Pluto and Charon
• Charon’s diameter (1,200 km) is about half that of Pluto
• Pluto’s mass is about 12 times that of Charon but only
1/5 that of the moon
• Charon density 1.2-1.3 g/cm3; Pluto 1.8-2.1 g/cm3
• Charon is less than 9 Pluto diameters away from Pluto
(compare: Moon ¼ diameter of Earth, and 30 Earth
diameters away from Earth)
• Charon orbit is tilted 119o to Pluto’s orbit around the Sun
(i.e. it is in the equatorial plane of Pluto)
• Despite their small size, they are tidally looked in a 1:1
resonance with Charon orbiting Pluto every 6.4 days,
the same as Pluto’s rotation
• But wait there’s more!
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Solar System Debris
Origins of Pluto
A Former Moon of Neptune?
• Because Pluto is small and has an eccentric
orbit, some theorize that it a former moon of
Neptune that was somehow ejected.
• The discovery of Charon (and now 2 other
moons) made it seem less likely that Pluto
was once Neptune’s moon.
• Also, the large difference in density between
Charon and Pluto points to Charon’s
capture by Pluto.
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Solar System Debris
Origins of Pluto
Planet, Asteroid, or ???
• Pluto doesn’t fit the asteroid
classification since its density and
composition is more consistent with a
satellite of Jovian planet
• In the 1990s Pluto was proposed to be
the largest member of the “plutino” class
objects found in the Kuiper belt.
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Solar System Debris
The Kuiper Belt
Kuiper Belt ???
• In addition to the Asteroid Belt, the Solar System appears to
have a second belt, now called the Kuiper belt:
– Support for this comes from the detection of about 600
small, presumably icy, bodies orbiting near and beyond
Pluto (first object discovered was 1992QB1).
– Extent of belt is unknown, but statistical analysis indicates
that the Kuiper belt may have an total mass far greater than
that found in the asteroid belt.
– Objects in the belt are sometimes referred to as KBOs or
plutinos, and Quaoar (found in 2002 with a diameter of
1250 km), Sedna (discovered in 2003 with a diameter of
1600 km), and 2004DW are among its largest members.
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Solar System Debris
A New Planet?
• July 2005: Mike Brown and
associates announced the discovery
of 2003UB313 as the “tenth planet”
• 2003UB313 is just a bit larger than Pluto
– How can they know that?
• Discovered some 97 AU from the Sun,
near its aphelion
– It’s the furthest object detected in our solar
system
• Huge 557-year eccentric orbit takes it
within 38 AU of Sun
• It was discovered to have a moon! But
what to name these objects ….
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Solar System Debris
The Kuiper Belt
Associations
• Triton’s orbit is “backwards” and is highly tilted with
respect to Neptune’s equator – Triton is perhaps a
captured planetesimal from the Kuiper belt
• Short-period comets are now believed to be icy nuclei
from the Kuiper belt
• The centaurs, of which Chiron is a well-known example,
appear to originate from the Kuiper belt.
• 2003 UB313 (Eris) is a KBO larger than Pluto, in an orbit
that crosses that of Pluto, and has a moon (Gabrielle?)
• Should Pluto still be considered a planet or a member of
the Kuiper belt?
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Solar System Debris
Good-bye Planet Pluto
• Recently, Pluto was “demoted” from the ranks of
planethood at the International Astronomical Union
Meeting in August, 2006, to the status of a dwarf
planet.
• A planet is officially defined as an object
– that is in orbit about the sun.
– has sufficient mass for its self gravity to overcome rigid-body
forces so that it assumes [a nearly round] shape.
– has cleared the neighborhood around its orbit.
• A dwarf planet is officially defined as an object
– that meets the first two criteria of a planet, but not the third.
• Pluto, Eris, and Ceres are now dwarf planets – others
will most certainly follow.
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Solar System Debris
Collisions
What happens when something strikes Earth?
• We have evidence of things hitting the Earth
– Craters, meteorites
• As a general rule the craters made by meteors are
10 times bigger than the impactor
• The most prominent impact crater on Earth is
Meteor Crater near Winslow, Arizona.
• There may have been impacts which affected life
significantly
– Chicxulub meteor which landed off the Yucatán
Peninsula may have wiped out the dinosaurs
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Solar System Debris
Collisions
Have we ever witnessed a major impact?
• We have not witnessed a major impact on a
solid body, but in 1994 Comet ShoemakerLevy 9 (SL9) impacted into Jupiter.
• This event had 2 effects:
– It was one of the best examples of international
cooperation
– It made the public awareness of current nature
of giant collisions in our solar system
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Solar System Debris
Collisions
Did an impact kill the dinosaurs?
• We have identified more than 150 impact craters
on the Earth
• One impact, off the coast of the Yucatan
Peninsula, may have wiped out the dinosaurs 65
million years ago
• Clues such as the deposit of iridium sediment
(coming from an asteroid) at the right geological
depth in the soil helps verify such a claim
• This meteor impact lead to a mass extinction
where 99% (and 75% of the species) were
extinguished
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Solar System Debris
Collisions
Is the impact threat real?
• There are certainly many objects that could hit us: we
have detected over 800 asteroids over 1 km in size
which pass near the Earth’s orbit.
• The threat is real, but the chances of something big
hitting us in our lifetime is small
• Nevertheless, we were hit by a comet or asteroid in
the region of Tunguska, Siberia in 1908 resulting in a
tremendous explosion. A hit like this over a major city
would be devastating.
• If a big asteroid were headed for us, could we prevent
the impact?
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The End
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