Download Interplanetary Vagabonds

Interplanetary Vagabonds
Astronomy 1-1
Lecture19-1
The Vagabonds
Asteroids
Rocky objects primarily between Mars and Jupiter
Comets
Objects in highly eccentric orbits
Meteoroids
Bright objects streaking across the sky
Beyond Neptune
Astronomy 1-1
Lecture19-2
Asteroids
Primarily between the orbits of Mars and Jupiter
Upwards of 200,000 asteroids have been
discovered and the numbers of charted ones
grow by 150 to 200 per year
Measured sizes range from hundreds of
kilometers down to kilometer size objects
Smaller ones exist
The large asteroids tend to be spherical, while the
smaller ones are jagged in shape
Are in the plane of ecliptic and most have
eccentric orbits
Astronomy 1-1
Lecture19-3
Asteroids
Vary in composition and reflectivity
Composition varies from metal free to
those containing metallic compounds
Three-fourths of the larger ones are
carbonaceous, while most of the rest are
silicon bearing
Astronomy 1-1
Lecture19-4
Asteroids
Astronomy 1-1
Lecture19-5
Asteroids
Are classified in types:
C-type (common in outer asteroid belt)
Extremely dark – low reflectivity
75% of all asteroids
No evidence of high mineral content
Carbon rich
S-type (silicates – inner belt)
Spectral evidence for olivine – a silicate mineral
M-type (rare)
Metallic - iron and nickel
Astronomy 1-1
Lecture19-6
Asteroids
Two small S-type asteroids, Gaspra and Ida, were
visited by the Galileo probe
Gaspra (left) is in false color; it is really gray
Ida (right) has a small moon, Dactyl.
Astronomy 1-1
Lecture19-7
Asteroids
Mathilde, a C-Type asteroid, like many other
asteroids, has a very low density and is probably
not solid
Astronomy 1-1
Lecture19-8
Asteroids
Eros does seem to
be solid
Astronomy 1-1
Lecture19-9
Apollo Asteroids
Some asteroids have very eccentric orbits so that they
cross Earth’s orbit
They are called Apollo asteroids and raise the
concern of a possible collision with the Earth
4400 such asteroids have been discovered so far, of
which about 800 have been designated as potentially
hazardous, due to their size
Another group of asteroids that only intersect the
orbit of Mars – Amor Asteroids
Astronomy 1-1
Lecture19-10
Asteroid Collisions
Asteroids also collide with each other breaking into
fragments
Fragments then travel together following the original
orbits
The groups of fragments are known as
Hirayama Families
If the relative velocity is high enough then the resulting
fragments can be kicked out of asteroid area
These fragments are known as meteoroids when traveling
through the atmosphere
If they survive to hit the ground they are known as
meteorites
Astronomy 1-1
Lecture19-11
Trojan Asteroids
Some asteroids, called
Trojan asteroids, orbit
at the L4 and L5
Lagrange points of
Jupiter’s orbit
Astronomy 1-1
Lecture19-12
Distribution of Asteroids
Not uniformly distributed
between Mars and
Jupiter
Gaps in the radial positions
of the asteroids
The gaps are not
random
Gaps were explained by
Kirkwood
They are due to orbital resonance effects with Jupiter
Orbital period of asteroid is an integer multiple of Jupiter's
period
Astronomy 1-1
Lecture19-13
Meteoroids, Meteors and
Meteorites
A meteoroid is a small asteroid, which becomes a meteor or
shooting star as it penetrates the Earth’s atmosphere
While there is no precise definition for the size of an meteoroid,
objects smaller than about 100 m across are typically termed
meteoroids
Most vaporize completely before striking the ground
A meteorite is a portion of a meteor reaching the ground intact,
possibly producing an impact crater
A meteor shower occurs when the Earth moves through the
debris left behind by a comet
Carbon dating shows that the oldest meteorites are about 4½
billion years old; i.e. the same age as the solar system
Astronomy 1-1
Lecture19-14
Meteoroids
On an average dark night, you can see a few meteors every hour
The flash is caused by heating
Most meteors do not survive to reach the ground
Astronomy 1-1
Lecture19-15
Types of Meteorites
Stony meteorites (94% of all falls)
ordinary chondrites (~80%)
chondrules - silicate balls
other inclusions - Al, Mg, S
achondrites (~10%)
stones with no inclusions
carbonaceous chondrites (~ 4 %)
carbon-rich inclusions
complex organic molecules (amino acids)
water-rich chondrules - never melted!
Iron meteorites (5% of all falls)
nearly pure iron/nickel alloy
large crystals - very slow cooling
Stony- Iron meteorites (1% of all falls)
mixture of iron and slicates
Astronomy 1-1
Lecture19-16
Meteoroids
Are defined as being less than 100 m
in diameter
Most of the smaller ones are the
remnants of comets that have broken
up
If the Earth’s orbit intersects the
comet’s, meteor showers will occur
every year on the same date, until the
meteoroids have burned out
Astronomy 1-1
Lecture19-17
80 -Ton Asteroid Hits the Nubian Desert
In October 2008, an asteroid (which could also be called a meteorite) about 12 ft
across with a mass of about 80,000 kg (80 metric tons) exploded at an
altitude of about 37 km, the parts scattering in the Nubian desert of Sudan
By a lucky accident, asteroid 2008 TC3 became the first such object to be
observed before impact
Fifteen fragments were later recovered over a region of length 29 km
The black jagged rocks collected contained metals, such as iron and nickel,
graphite, and “nanodiamonds”
Astronomy 1-1
Lecture19-18
Stony Meteorite Found in Texas
Stony meteorites look like ordinary rocks, often covered
with a dark crust, caused by the melting of the outer
surface during its descent through the atmosphere
Astronomy 1-1
Lecture19-19
Iron Meteorite Found in Australia
Iron meteorites contain iron-nickel compounds
Astronomy 1-1
Lecture19-20
Impact Crater Formed by a 12 kg Meteorite
Astronomy 1-1
Lecture19-21
Meteor Crater, Arizona
Caused some 50,000 years ago by a meteoroid about 50 m
in diameter
Meteor crater is about 1.2 km across and 200 m deep
Astronomy 1-1
Lecture19-22
Meteoroids
Larger meteoroids are usually loners from the asteroid belt and
have produced most of the visible craters in the Solar System
The Earth has about 100 craters more than 0.1 km in diameter
Erosion has made most of them hard to discern
One of the largest is in
Canada:
Astronomy 1-1
Lecture19-23
Meteoroids
Here are the major meteor showers
Astronomy 1-1
Lecture19-24
Meteoroids
Meteoroids that burn up in the Earth’s atmosphere have
densities of 500 to 1000 kg/m3 and are probably comet-like in
composition
Meteoroids that reach the surface have densities around 5000
kg/m3 and are similar to asteroids
Astronomy 1-1
Lecture19-25
Meteoroids
Most meteorites are rocky (left)
Some are iron (right):
Astronomy 1-1
Lecture19-26
Comets
Comets differ from asteroids in several ways
Asteroids are in the plane of the ecliptic
Comets are randomly oriented with respect to
the ecliptic plane
Asteroids orbits largely confined to be between
Mars and Jupiter
Comets have highly elliptical orbits, coming
close to the Sun
Comets contain ices and dust particles
When they do come close to the Sun, they begin to
glow
Astronomy 1-1
Lecture19-27
Comets
Comets that come close enough to the sun to be
detectable from Earth have very eccentric orbits
Astronomy 1-1
Lecture19-28
Nucleus
Comet Structure
Solid part of the comet
From here come the ices and dust that
vaporize
Size ranges from a few km to 40 km
Composition variable, though H20, CO,
CO2 are seen
Coma
The glowing ball that is seen
Caused by the vaporization of the outer
layers of the comet’s nucleus
Interaction with solar radiation and transform chemically
Anywhere from 105 to 106 km in size
Also surrounded by a cloud of hydrogen gas
This hydrogen cloud could be ten times larger
Astronomy 1-1
Lecture19-29
Comet’s Tail
Two tails to a comet
1. Tail that is pushed by the
solar wind is pointed directly
away from the Sun and it is
ionized
2. Tail that is pushed by the
radiation pressure is curved
and contains both dust
particles and ions
Astronomy 1-1
Comet Hale-Bopp
Lecture19-30
Comets
As the comet approaches the Sun
The incident sunlight heats the
surface of the comet
The heat energy warms the
nucleus
The surface becomes warm
enough that ices sublimate
into gases
A dusty crust forms around
the nucleus, insulating the
nucleus and regulating the
sublimation process
Astronomy 1-1
Lecture19-31
Halley’s Comet
One of the most famous
It has a period of 76 years and has been observed since
antiquity
Its most recent visit, in 1986, was not spectacular
Left: The comet in 1910, as seen with the naked eye
Right: The comet in 1986, as seen through a telescope
Astronomy 1-1
Lecture19-32
Halley’s Comets
Has a shorter period than most comets, but its orbit is not in
the plane of the solar system, probably due to an encounter
with a larger object
Astronomy 1-1
Lecture19-33
Mark Twain — 1835-1910
“I came in with Halley's Comet
in 1835. It is coming again next
year (1910), and I expect to go
out with it.”
Astronomy 1-1
Lecture19-34
Comets
Typical cometary mass: 1012 to 1016 kg
Each trip close to the Sun removes some material
Halley’s comet, for example, is expected to last about another
40,000 years
Sometimes a comet’s nucleus can disintegrate violently, as
comet LINEAR did
Astronomy 1-1
Lecture19-35
Stardust Mission
Flew through the tail of comet Wild-2,
gathering dust particles in detectors
and returning them to Earth for
analysis
Findings indicate the formation of at
least some comets may have included
materials ejected from the inner solar
system to the far and cold outer edge of
the solar nebula
Found very high-temperature minerals
Comets are not composed entirely of volatile rich materials
but rather are a mixture of materials formed at all
temperature ranges, at places very near the early sun and at
places very remote from it
Astronomy 1-1
Lecture19-36
Deep Impact Mission
Mission slammed a projectile into
comet Tempel 1
Studied the material expelled in
order to analyze the composition of
the comet
Material excavated by the impact
Contained more dust and less ice
than had been expected
Finer than expected
Included clays, carbonates, sodium,
and crystalline silicates
Comet was about 75% empty
space
Astronomy 1-1
Lecture19-37
Comets
Most comets that enter the inner solar system reside in the
Kuiper belt outside the orbit of Neptune
Occasionally a comet from the far larger Oort cloud wanders
into the inner solar system as well
Astronomy 1-1
Lecture19-38
Comet Knowns and Unknowns
Comet Facts
What We Still Don't Know
Comets have the most primitive,
We do not know what is hidden below
accessible material in the solar system.
the evolved surface layers.
Comets must become dormant.
We do not know whether ice is
exhausted or sublimation is inhibited.
There must be many dormant comets
masquerading as asteroids.
We do not know how to identify these
bodies.
We know more chemical and physical
details than for other small bodies.
We do not know how to use these
details to constrain our models for
comets.
Abundance of gases in the coma is
widely used to infer the ices in the
protoplanetary disk.
We do not know the relationship
between coma abundances and those in
the nucleus.
Comets break apart under small
stresses.
Nothing is known about variation of
material strength with scale.
Astronomy 1-1
Lecture19-39
Pluto
Pluto was discovered in 1930
It was thought to be needed to explain
irregularities in the orbits of Uranus and Neptune,
but it turned out that there were no such
irregularities
Astronomy 1-1
Lecture19-40
Pluto Data
Average distance from Sun: 39.5 AU
Mass: 0.2% of Earth (0.002 ME)
Diameter: 2370 km (19% of Earth, 49% of Mercury)
Average density: 36% Earth density
Orbital eccentricity: 0.25
Siderial revolution period: 248 Earth years
Rotation period: 6.4 Earth days (retrograde)
For 20 years of its 248 year orbit, Pluto is closer to the Sun than
Neptune, to which it is locked in a 3:2 orbital resonance
Astronomy 1-1
Lecture19-41
Pluto’s Orbit
There is an angle of
17o between the orbits
of Pluto and Neptune
Orbit also crosses the orbit of
Neptune
Astronomy 1-1
Lecture19-42
About Pluto and Charon
Pluto’s orbit is significantly more elliptical and more tilted
with respect to the ecliptic than is any other planet
Pluto’s satellite Charon has a diameter of 1190 km (just half
that of Pluto), and its distance from Pluto is less than 5% of
the Moon-Earth distance
Pluto’s surface contains frozen nitrogen, methane and carbon
monoxide (CO), while Charon’s may be covered with water
ice
Pluto and Charon move in a unique form of synchronous
rotation, in which each has a face locked to the other body
Thus, observed from Pluto, Charon, would appear to hover in
the sky, and vice-versa
Astronomy 1-1
Lecture19-43
Pluto
Observations of eclipses of Pluto and Charon allowed
measurement of orbital details
Astronomy 1-1
Lecture19-44
Kuiper Belt
The solar system appears to contain two comet reservoirs
The Kuiper Belt, estimated to contain over a hundred million
comets, begins near Neptune in the plane of the ecliptic,
extending from 30 AU to at least 55 AU from the Sun
For reasons not yet known, Kuiper Belt objects show a wide
range of color, from slightly blue to very red
The dwarf planets, Pluto, Eris, Makemake and Haumea are
Kuiper Belt objects
Eris, which is about 50% larger than Pluto, is further from the
Sun, with an elliptical orbit, of period about 650 years
Other large objects in the Kuiper Belt are Sedna and Quaoarh
Astronomy 1-1
Lecture19-45
Kuiper Belt
Comparison of several trans-Neptunian objects with
Earth and its moon
Astronomy 1-1
Lecture19-46
Kuiper Belt
Pluto is the second largest known object in the Kuiper belt
Astronomy 1-1
47
Lecture19-47
Oort Cloud
The Oort cloud is a spherical distribution of icy bodies
orbiting the Sun, which extends from about 10,000 AU to
about 100,000 AU
100,000 AU is about 9.3 trillion miles (or 1.6 ly), which is
almost 40% of the way to the nearest star, Proxima
Centauri
The periods of most comets from the Oort cloud may be
millions of years, compared to about 100 years for the
Kuiper belt comets
The Oort cloud is estimated to contain trillions of comets, only
very few of which get gravitational pushes into the inner
solar system, where there become short-period comets with
altered orbits, such as Halley’s Comet
Astronomy 1-1
48
Lecture19-48
The Oort Cloud
Astronomy 1-1
49
Lecture19-49