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Remnants of Our Solar System’s Formation
Asteroids
 Meteorites
 Comets
 Dust
 Gas

Planetesimals
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Material left over from the Sun’s formation and the
construction of the planets are the building blocks of
the planets called planetesimals which include:
 Asteroids – primarily fused rock and metal with
some ices in the outer asteroid belt
 Comets – primarily ices and dust grains with
minor amounts rock and metal
As well as materials in the form of:
 Dust
 Gas
Note: the asteroid and comet planetesimals are now
called Small Solar System Bodies (SSBS)
Asteroids
Asteroids were formed in the inner solar system during
the early solar system formation period following the
Sun’s intense T-Tauri emission

Collisional heating fused larger and larger fragments
together by accretion
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Larger fragments pulled surrounding materials to
build larger planetesimal asteroids

Heating from impacts fused smaller asteroids, and
created molten cores in larger asteroids
Asteroids

Fused and partially
fused rock fragments
became
carbonaceous
asteroids

High-density molten
fragments became
metallic asteroids
 Primarily iron with
some nickel
Asteroids
Asteroids types

S-Type (Silicaceous)
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About 17% of asteroids
Relatively bright and reflective
Composition is metallic iron mixed with iron- and
magnesium-silicates
S-type asteroids dominate the inner asteroid belt
M-type (Metallic)
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Relatively bright and reflective
Composed mainly of metallic iron
M-type asteroids inhabit the belt's middle region
Asteroids
Asteroids types

C-type (Carbonaceous)
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More than 75% of known asteroids
Very dark and non-reflective
Solid composition similar to solar system
makeup, except depleted in hydrogen, helium,
and other volatiles
C-type asteroids are found mainly in the belt's
outer regions
D-type
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Very dark and non-reflective
Reddish in color
Asteroids

Distribution throughout solar system is
dominated by gravitational forces from the
largest planets

Jupiter has collected and distributed most
asteroids in regions of orbital stability or
gaps relating to instability

Earlier known as minor planets

Largest asteroid named Ceres is a dwarf
planet
Asteroids

Orbit resonances and Jupiter’s stability
points create distinct patterns and gaps in
asteroid distribution

Mars generates small but significant stability
boundaries in asteroid distribution near the
planet
Note: Asteroids are also classified by their
orbits, and by their reflected spectra
Asteroid Distributions
Asteroid Distributions
Asteroid Distributions
Asteroids
Average shapes and sizes

Asteroids, which are the remnants of the early
solar system formation, reflect the conditions in
the early inner solar system

Size of these solid bodies varies between
pebbles to irregular-shaped bodies several
hundred km in diameter
 Ceres, the largest asteroid that has a mean
diameter of 950 m

Since asteroids are generally small, their
gravitational pressure is not enough to force
them into a spherical in shape
Asteroids

The asteroid explorer named Dawn, launched
September 27, 2007 explored Vesta, the second
largest asteroid, from July 2011 to September,
2012

Dawn was then sent to Ceres for a multi-year
investigation of the dwarf planet beginning in
February, 2015

Dawn spacecraft is powered by three xenon ion
engines
Asteroid Examples
With the exception
of Ceres, the
asteroids are
irregular in shape
And although the
shapes vary, all are
irregular and
cratered
Asteroid - Impacts

Impacts and cratering on Earth from comets and
asteroids is roughly proportional to the object's mass
since hyperbolic velocity is typically 20-40 km/s

Evidence of the type of impact object can be made by
analysis of the crater rim or ejecta
 High levels of iridium are found in asteroid samples
and debris
Diameter
20 m
100m
1 km
10 km
Energy of impact
(Megatons of TNT)
5
100
10,000
10,000,000
Crater diameter
(km)
0.2
1.0
10
100
Asteroid - Impacts
Impact Examples
Tunguska Siberia (1908)
 Comet/carbonaceous asteroid is estimated at 50 m diameter
 No crater was generated, which means that it had to be a weak
(carbonaceous) meteorite or a comet
 15 M ton energy equivalent
 Air burst explosion devastated 1,000 sq km of the surface
Asteroid - Impacts
Impact Examples
Arizona Crater (50,000 years ago)
 50 m iron/nickel meteorite
 1.3 km crater
 200 Megaton energy equivalent
Asteroid - Impacts
Impact Examples
Chicxulub, Yucatan
impact (65 M years
ago)
 13 km diameter
asteroid
 130 km crater diameter
 100,000,000 Mton
energy equivalent
 70% species
exterminated on Earth
Meteorites
Meteorites are asteroid materials that survive
atmospheric entry and can be found on the
Earth’s surface
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Meteor – visible event of cosmic debris
entering Earth’s atmosphere
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Meteorite – Cosmic material entering Earth’s
atmosphere and surviving to reach Earth’s
surface
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Meteoroid – cosmic debris in space (asteroid
or cometary material)
Meteorites
Meteorites are classified primarily by their
composition similar to asteroids (silicate rock, metal,
and chondrites)

Chondrites (85% collected)
 Named after chondrules (spheres of silicate and
dust materials) found within the solid rock bodies

Achondrites (8% collected)
 Silicate rock, but no chondrules found within the
solid body
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Iron (5% collected)
 Iron and nickel metal pieces from the interior of
larger asteroids that formed metal cores
Meteorites
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Stony-iron (1% collected)
 A mixture of iron-nickel metal and silicate rock
that may have originated between the mantle and
core regions of larger asteroids
Meteorites
Average composition
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Although the meteorites vary widely in their
composition, the average is roughly the
same composition as the inner solar system
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Chemical and physical changes are
introduced by atmospheric heating and
surface impacts
Asteroids
Average composition
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The average composition of meteorite
samples represents inner-belt asteroid
composition
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Oxygen
36.3%
Iron
25.6%
Silicon
18.0%
Magnesium 14.2%
Aluminum 1.3%
Nickel
1.4%
Calcium
1.3%
Sodium
0.6%
Comets

Comets and cometary debris
is composed primarily of ices
and dust (Si, C, Al, Fe, etc.)

Gases are given off (evolved)
by the comet
 Sublimation
 Generally a small loss
because of the
extremely low
temperatures in space
 20-50 K

Solar heating
 Strong inside Earth’s
orbit (1 AU)
 Rapidly age comets
because of their porous
structure
Comets

Often described as dirty snowballs
 White-blue-green ice compounds are
mixed with brown-black-grey “dust”
 Dust grains include carbon compounds,
silicate compounds, and iron
compounds, including graphite which is
black

Most abundant ices are carbon dioxide,
ammonia, methane, and water
Comets
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Extend even beyond
the heliosphere in a
region called the
Oort cloud
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Another extended
region of the
planetary disk
contains more
comets and dwarf ice
planets called the
Kuiper Belt
Comets
Distant solar system diagram in
the horizontal plane

Shown are the positions of
objects with semi-major axes
greater than 5 AU (orbital
periods greater than
~11 years) on 2008 January 1
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The orbits and positions of
Earth, Jupiter, Saturn,
Uranus, Neptune, Pluto, and
comets Halley and Hale-Bopp
are also shown
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Asteroids are yellow dots and
comets are symbolized by
sunward-pointing wedges
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The vernal equinox is to the
right along the horizontal axis
(+X direction)
Comets
Top view of the distant solar
system

Shown are the positions
of objects with semi-major
axes greater than 5 AU
(orbital periods greater
than ~11 years) on
2008 January 1
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The orbits and positions
are shown for the transNeptunian objects Eris,
Quaor, Sedna, Orcus,
Hale-Bopp and others
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The vernal equinox is to
the right along the
horizontal axis (+X
direction)
Comets
Typical
composition
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Spectral emission
of the Deep Impact
collision with
comet Tempel 1
showing the
distinct signature
of water (as ice)
Also present are
several
hydrocarbons, and
carbon dioxide
Trans-Neptunian Objects

Recent discoveries of the icy bodies
populating the outer solar system by the
powerful Hubble Space Telescope and
several new ground-based adaptive-optics
telescopes have revealed hundreds of
Pluto-like bodies roughly in the same plane
as the major planets

This region that extends beyond Neptune,
called the Kuiper Belt after astronomer
Gerard Kuiper, is populated with moon-like
comets that resemble Pluto, but are
generally much smaller
Trans-Neptunian Objects

The disk population is confined to a semimajor
axis of about 55 AU, and clustered at specific
positions with respect to integer fractions of
Neptune's orbit period
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Not only does Neptune's gravitation direct the
motion of these outer bodies, Neptune has
stabilized many of these icy bodies inside or
outside the orbital resonance positions

Because of Neptune's fundamental role in
shepherding these icy satellites, including Pluto,
they are classified as Trans-Neptunian Objects
(TNOs)
Trans Neptunain Objects
Dust

Small particles of material left over from
the solar system formation, and from
later collisions between planetesimals,
are found throughout the planetary disk,
but are less abundant closer to the Sun
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Continual outward pressure from solar
radiation has pushed most of the small
particles outward
Dust
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Although particle and dust concentrations occur around
the planets, collections of the dust and debris can also
be found at the Lagrange stability points L4 and L5
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The nearest example is the Earth-Moon system that
has both L4 and L5 regions populated by particles,
but low in abundance
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Trojan asteroids, asteroids found in Jupiter's L4-L5
stability points in the Sun-Jupiter system, are much
larger than the dust in the Earth-Moon system
because of Jupiter's much larger mass
Dust accumulations can also be seen in a diffuse belt in
the planetary plane known as the zodiacal belt
Asteroid Distributions
Jupiter’s Lagrange Stable Points
Dust
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The most problematic
material encountered
by spacecraft are the
most abundant
particles (much larger
than atoms and
molecules)
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This is the dust and
small debris common
in Earth-orbit
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The damage is not
catastrophic, but
instead accumulates
its erosive effects with
time
Dust
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Shielding from this small
debris consists of the
following
 Sensitive instrument or
optics use mechanical
shades or coverings
 Durable coatings are
used for spacecraft
surface integrity
 Thin bumper shielding
is commonly used for
pressurized spacecraft
and spacecraft on
comet flyby missions
where particles the size
of sand can be a
serious problem

A 10 mg sand grain
traveling at 40 km/s has the
destructive energy of a
small-caliber bullet
Dust
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Debris tracked in LowEarth Orbit (LEO)
Dust
Orbital debris – larger view
showing outline of
geostationary orbit