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Vagabonds of the Solar System
– Asteroids
– Comets
– Meteoroids,
Meteors, Meteorites
Asteroids
• As solar system was forming, matter with too much angular
momentum to fall into the sun coalesced at varying distances from the
sun into planetesimals.
Asteroids
• Many collided, forming planets and larger moons.
• Others were captured whole by planets as small, irregularly shaped
moons
• Many still orbit the Sun. These are asteroids.
Discovery of asteroids Part 1
•
Ceres was the first asteroid
discovered on New Year’s Day in
1801.
– It was discovered because it
changed position every night
– Ceres is huge.
– Ceres accounts for 30% or the
mass of all known asteroids
•
Pallas was the next asteroid
discovered 1803
– It was discovered because it
changed position every night
•
Only two more, Juno and Vesta,
were found until mid 1800’s
Discovery of asteroids Part 2
• By mid 1800’s, improved telescopes were available
– Most asteroids are very small compared to Ceres
– New telescopes made it possible to see smaller asteroids
– Most are <1km across
• About 300 asteroids found lying between orbits of Mars and Jupiter
• This region is called the asteroid belt; the asteroids are called belt
asteroids
Discovery of asteroids Part 3
• Applied photographic technique in 1891 introduced by Max Wolf
• Take long exposures and look for tracks made by asteroids
• Wolf discovered 228 asteroids this way
Discovery of asteroids Part 4
• Improved digital image processing
• Don’t waste telescope time on long exposures
• Take multiple images and subtract
– LINEAR
– NEAT
LINEAR (Lincoln Near Earth
Asteroid Research)
• The GTS-2 telescope is a 1
meter folded prime focus
Cassegrain design identical to
that of the Ground-based
Electro-Optical Deep Space
Surveillance (GEODSS)
telescope used by the Air Force
for space surveillance. It is
located at the Experimental Test
Site on White Sands Missile
Base in NM
• Take several images of same
view each night & subtract to
detect new objects
NEAT (Near Earth Asteroid
Tracking)
• NEAT detects moving objects asteroids and comets - by
observing the same part of the
sky 3 times during an interval
of about 1 hour. The automatic
data analysis system searches
for moving objects by
comparing the 3 images.
• The NEAT system is now
mounted on the Maui Space
Surveillance Site (MSSS) 1.2-m
GEODSS telescope (same as
LINEAR).
Discovery of asteroids Part 5
• Estimate there are >1,000,000,000 asteroids in the solar system
Origin of asteroids
• Most likely the force of Jupiter on the planetesimals kept them from
coalescing into one object
• Less likely that they had been one object and were split into many
– If all of the asteroids in the asteroid belt were put together to form
a planet, it would be very small, smaller than Pluto.
But some may be fragments of
larger asteroids
• Some may break apart but not have enough speed to escape each
others gravitational attraction, so reassemble
• Some large fragments end up orbiting near each other or even in
contact
4 asteroids have been imaged at
close range
• The Galileo spacecraft (while
on its way to Jupiter) imaged
Gaspra and Ida.
• Gaspra and Ida have different
amounts of cratering, so may
have been broken apart from
their parent asteroids at
different times
Ida
Gaspra
4 asteroids have been imaged at
close range
• The NEAR (Near Earth Asteroid
Rendezvous) mission did a
flyby of Mathilde and orbited
Eros.
Eros
Mathilde
NEAR landed on Eros
• The touchdown speed of less
than 4 miles per hour was one
of the softest planetary landings
ever.
• Team members then
commanded craft's gamma-ray
spectrometer to gather data on
the elemental composition on
and just below the asteroid's
surface.
Eros landing site
Some asteroids have satellites
• At least 2 asteroids have moons
Dactyl
Ida and Dactyl
Asteroids outside the
asteroid belt
• Some have highly elliptical orbits that bring them inside the orbits of
some planets
– Apollo asteroids cross Earth’s orbit
• There are about 300 known Earth-crossing asteroids
– Amor asteroids cross Mars’ orbit
– Some asteroid orbits extend beyond Pluto’s orbit
Asteroids outside the
asteroid belt
• Some are located at stable Lagrange points
– There are ~460 asteroids in Jupiter’s Lagrange points, called
Trojan asteroids
– An asteroid has also been discovered at one of the Earth’s
Lagrange points
L4 and L5 are stable
L1, L2, and L3 are unstable
Comets
• Comets formed near Uranus and Neptune, where water was plentiful
and the temperature was low enough for ice to condense with roughly
equal amounts of rocky and metallic material into bodies that still
orbit the sun
• Gravitational forces from Uranus and Neptune flung the comets in
every direction
Kuiper belt
• Kuiper belt
– Centered on the ecliptic extending beyond the orbit of
Pluto.
– Largest comet is 1/5 size of Pluto
– ~200,000,000 comets
Oort cloud
• Oort cloud
– NOT centered on the ecliptic but is a spherical distribution around
the Sun extending about about 50,000AU. 1/5 of the distance to the
nearest star
– Can even have orbits perpendicular to the ecliptic
– ~Several billion comets
– A few pass through the inner solar system as Hale-Bopp and
Hyakutake
Comets
• Structure
– Because the Kuiper belt and the Oort clouds are far from the Sun,
comets are completely frozen.
– As a comet approaches the sun, some of the ice vaporizes forming
an atmosphere around the nucleus called the coma.
– The comet nucleus is only 10km across, but the coma can extend a
million km across
– The hydrogen envelope surrounds the nucleus
Comet tails
• Tail
– The tail develops from
coma gases that are pushed
outward by the solar wind
– This is why the solar wind
was initially predicted
– The existence of the solar
wind was verified by
Mariner 2
Comet tails
• Two types of tails
– Gas (or ion) tail
• Positively charged ions are pushed away from the Sun by the
solar wind
• Always points away from the Sun
– Dust Tail
• Formed when photons strike dust particles that have been freed
from the comet’s evaporating nucleus
• These particles are massive enough not to flow straight away
from the sun, so lie between the gas tail and the direction of the
comet’s motion
Comet tails
Hale-Bopp
Compositon of comets
• Stardust
– The goal of the Stardust
mission is to return both
particle samples from a
comet and interstellar dust.
By returning these samples
to Earth for analysis a great
deal is expected to be
learned about the
composition of the early
universe.
Comet orbits
• Long period comets
– Most move so fast that they leave the inner solar system after one
pass by the Sun and take millions of years to return
• Short period comets
– Some pass near a planet which changes their orbit, slows it, and
traps it in an orbit in the inner solar system.
– These comets may then have orbits of a few hundred years or less
(as Halley)
Comet lifetime
• Comets lose mass ( about 1/60 - 1/100 of its mass) every time they
pass the Sun
• After its ices have all evaporated, the remaining dust and rock spread
out in a loose collection the continues to circle the Sun along the
comet’s original orbit
– When the earth passes through this collection, we see a meteor
shower
– ~30 meteor showers can be seen each year
Comet lifetime
• A comet can be torn apart if it comes too close to a planet, like Comet
Shoemaker-Levy 9 which fragmented in the tidal force from Jupiter
Comet lifetime
• Comet LINEAR Breaks Up
Meteoroids, Meteors, Meteorites
• Meteoroids
– Rocky and metallic debris smaller than asteroids (10’s of meters
across to microscopic) scattered throughout the solar system
• Some are broken off from asteroids of from planets, but some
were never part of a larger body
• Meteors
– When a meteoroid is pulled by Earth’s gravity into Earth’s
atmosphere, air friction creates so much heat that the outer layer
vaporizes
• Common names are shooting stars, bolides, and fireballs
• Meteorites
– Meteors that reach the ground before completely vaporizing
Impact craters
• Any meteor that survives passage through the atmosphere may leave
an impact crater a comet back to Earth
– Barringer (or Meteor) Crater in Arizona which formed about
50,000 years ago
– Tunguska mystery
– Allende meteorite
– Alvarez discovery
95 Worlds and Counting
• Film
• Tour of the satellites of planets in our solar system
Vagabonds of the Solar System
•
On the morning of his departure he
put his planet in perfect order. He
carefully cleaned out his active
volcanoes. He possessed two active
volcanoes; and they were very
convenient for heating his breakfast
in the morning. He also had one
volcano that was extinct. But, as he
said, "One never knows!" So he
cleaned out the extinct volcano,
too. If they are well cleaned out,
volcanoes burn slowly and steadily,
without any eruptions. Volcanic
eruptions are like fires in a
chimney.
The Little Prince