Comets, Asteroids, and Meteors

Chapter 14: Solar System Debris
Asteroids, Comets, and Meteorites
Solar System Debris
• Debris:
– (1) scattered remains of something
broken or destroyed;
– (2) accumulation of fragments of rock.
• Solar system:
– 1 large object
• Sun
– several medium-sized objects
• planets and moons
– lots of debris
• asteroids, meteoroids, comets, dust
What is this debris?
• The classification of is a matter of minor controversy.
• Traditionally, the solar system has been divided into
planets (the big bodies orbiting the Sun),
their satellites (variously sized objects orbiting planets),
asteroids (small dense objects orbiting the Sun) and
comets (small icy objects with highly eccentric orbits).
• Unfortunately, the solar system has been found to be
more complicated than this would suggest:
– there are several small moons that are probably captured
– comets sometimes fizzle out and become indistinguishable from
– the Kuiper Belt objects and others like Chiron don't fit this
scheme well;
– Pluto/Charon system sometimes considered "double planet“
and like Kupier Belt objects.
How much debris is out there
and where is it?
• Most asteroids have been found orbiting the Sun
between Mars and Jupiter, but they also share orbits
with planets and cross into the inner solar system.
• Mass of all known asteroids < 1/10 mass of Moon.
• Most comets have been found to orbit the Sun in two
regions beyond the orbit of Neptune.
– Kupier Belt, 30-100 AU, never inside orbits of jovian planets
– Oort cloud, up to 50,000 AU from Sun
• Mass of all comets probably comparable to mass of
terrestrial planets.
Why Debris is Important?
• Early history of life on Earth pieced
together from fossil record.
• To study early history of solar system,
need cosmic fossils - materials that have
remained relatively unchanged since the
solar system was very young.
– planets: melted, battered by impacts,
tectonically active, and altered
• Look to smaller objects, asteroids,
and comets for clues.
Asteroids are believed to be left over from the beginning
of the solar system 4.6 billion years ago.
• Rocky and metallic objects too small to be considered planets.
• Range in size from Ceres (diameter of ~1000 km),
down to objects a few centimeters or less across.
• Name asteroids, meaning "star-like", derives from the fact that
they are more star-like in appearance than comets.
Asteroids: Discovery
•Too small be seen without a telescope.
•Ceres, largest of asteroids, orbiting Sun at 2.8 AU
was discovered first by Giovanni Piazzi in 1801.
–He was searching for the missing planet predicted to be
between Mars and Jupiter by Titius-Bode law.
–In next 6 years, three more objects found in region.
•Currently, more that 10,000 asteroids have
well-determined orbits.
–Each given a number for order of discovery & a name.
•Most orbits lie between those of Mars and Jupiter
Asteroids: Formation
• Believed to represent material left over from
formation of solar system.
• Although sometimes suggested that asteroids are remains of a
planet that was destroyed in a massive collision, it is more likely
that they represent material that never coalesced into a planet.
• Highest concentration of
asteroids in asteroid belt,
the region lying between orbits
of Mars and Jupiter.
• Likely that origin of the asteroid
belt is linked to gravitational
perturbation by Jupiter, which
kept these planetisimals from
coalescing into larger bodies.
Asteroids: Classification
Asteroids are classified into types according to their
(and hence their chemical composition)
Asteroids: Classes Compared
Gaspra: S-type asteroid
– Appear bright (reflectivity 15-20%)
– Predominately silicate materials
•C-type (e.g., Ceres and Pallas)
– Appear very dark (reflectivity 3-4%)
– Carbon–rich silicate materials
•composition thought to be similar to the Sun,
depleted in hydrogen, helium, and other volatiles.
•M-type (e.g., Psyche)
– Relatively bright (reflectivity 10-18%)
– Metals like iron and nickel
– Rare
Mathilde: C-type asteroid
Vesta: An Unusual Asteroid
• HST resolves features as
small as 50 miles across,
allowing astronomers to
map Vesta's geologically
diverse terrain.
• The surface is a complex
record of Vesta's four
billion-year history.
• Features include ancient
lava flows, and a gigantic
impact basin that is so deep,
it exposes the asteroid's
subsurface, or mantle.
Asteroids: Abundance & Location
Not all classes of asteroids are equally abundant and
different classes of asteroids found at different distances from Sun.
17% S-type dominate inner belt region
75% C-type dominate outer belt region
8% other types found in middle belt region
Known Asteroids
Asteroids: Orbits about the Sun
•Asteroids orbit the Sun in many regions at
different distances.
•They are often grouped by characteristics of
their orbits.
– Asteroid Belt: between Mars and Jupiter
– Trojan asteroids: co-orbital with Jupiter
– Apollo and Aten asteroids: Earth-crossing asteroids
– Amor asteroids: Mars-crossing asteroids
– Other asteroids
The Asteroid Belt
• All orbit Sun in west-to-east
direction (same as planets).
• Most orbits lie near plane of ecliptic.
• The asteroid belt defined as region containing
asteroids with semi-major axes in the range of
2.2-3.3 AU.
Asteroids in belt take 3.3-6 years to orbit Sun.
Contains 75% of known asteroids.
Spacing of asteroids in belt ~ several million km.
Many classified into families - groups with
similar orbital and physical characteristics .
Asteroids: Size and Location
• > 100,000 asteroids lie in the
asteroid belt.
• Asteroids differ from planets in
both their orbits and their size.
– generally move on quite eccentric
– few are >300 km in diameter, and
most are far smaller
(as small as 1/10 km across).
• Taken together, mass of
known asteroids amounts to
< 1/10 mass of Moon.
Asteroids: View from Space
•Galileo flew by main-belt asteroids.
• S-type
• 7 hour rotation period
• 16 x 11 x 10 km, irregular shape
• sparse crater count implies 200 million years old
• larger S-type
• more heavily cratered,
~1 billion years old
• satellite, Dactyl, 1.5 km diameter
– period = 24 hours;
– orbital distance = 100 km
– Ida’s density ~ 2.5 g/cm3
Mars Global Surveyor images of Phobos
Asteroids: Orbital Characteristics
The Asteroid Belt
– Most asteroids orbit in a zone between the orbits of
Mars and Jupiter called the Asteroid Belt
– Ranges 2 - 3.5 A.U. from the Sun.
– About 5,000 orbits have been calculated,
but 100,000 asteroids may exist.
– Orbits are elliptical but nearly circular and
near the plane of the ecliptic.
– Their orbits are very similar to the planets' orbits.
– The Kirkwood gaps are found in the orbits of belt
asteroids and are formed by Jupiter's strong
gravitational influence.
Orbits of other Asteroids
Asteroids: Orbital Characteristics
• Trojan Asteroids
– Found in the same orbit with Jupiter,
but are 60o ahead and behind the planet.
– They are stable positions in Jupiter's orbit where the
gravity of the Sun and Jupiter cancel.
– Such positions called Lagrange points.
• Apollo Asteroids
– Orbits cross Earth's orbit.
– About 50 known Apollo asteroids,
but may be as many as 1,000.
– All potential "Earth-colliders".
– Eros is an example. It is about 30 kilometers across.
Asteroids: Orbital Characteristics
• Other asteroid groups
– A few stray asteroids have been found that lie
completely outside of the asteroid belt.
– Chiron is the most famous example.
– Its orbit carries it between Saturn and Uranus.
– Chiron may actually be a dormant comet that has
lost most of its volatiles.
– When it is closest to the Sun, a very diffuse
atmosphere forms around it.
– If Chiron is a comet, it is the largest one known
with a diameter of about 180 kilometers.
• Chiron is unusual
– has a detectable coma, indicating that it is a cometary body,
– over 50,000 times the characteristic volume of a comet,
a size more commensurate with a large asteroid
– its curious orbit is unstable on time scales of a million years, indicating
that it hasn't been in its present orbit long.
Mt. Wilson Observatory - California
02 April 1995
Charles Morris, TIE telescope
• Chiron was the first of four bodies discovered so far
with similar orbits and properties.
• These bodies have been designated Centaurs,
after the race of half-man/half-horse beings from
Greek mythology, in recognition of their dual
comet/asteroid nature.
• It is believed that the Centaurs may be objects which
have escaped from the Kuiper belt.
• Distribution of
asteroid semi-major
axes shows some
prominent gaps
caused by resonances
with Jupiter's orbital
motion. These are
known as the
Kirkwood Gaps .
• Asteroid in resonance
with Jupiter receives
a strong gravitational
tug from planet each
time they are close
together. If asteroid's
period is in a welldefined ratio with
that of Jupiter’s, the
effects reinforce each
Kirkwood Gaps
Near-Earth and Trojan Asteroids
Trojan Asteroids
• Although most asteroids
orbit in the main belt,
2-3 A.U. from the Sun,
an additional class of
asteroids orbit at the
distance of Jupiter and
are called the Trojan
• Trojan asteroids are
locked into a 1:1 orbital
resonance with Jupiter.
• Several hundred such
asteroids are now known;
it is estimated that there
may be a thousand or
more altogether.
Trojan Asteriods: Lagrange Points
• There are exactly five
places in the solar system
where a small body can
orbit the Sun in synchrony
with Jupiter. These places
are known as the Lagrange
points of Jupiter's orbit.
• All five Lagrange points
revolve around the Sun at
the same rate as Jupiter.
Near Earth Asteroids (NEAs)
• NEAs are
asteroids that
closely approach
the Earth.
• Most are small,
diameter < 1 km
•Three groups:
• Atens: semi-major axes <1.0 AU; aphelion distance >0.983 AU
•Apollos:semi-major axes >1.0 AU; perihelion distance <1.017 AU
•Amors: perihelion distances between 1.017 and 1.3 AU and
only cross Mars’ orbit
Earth-Crossing Asteroids
• Certain groups of asteroids have
elliptical orbits that cross the orbit
of Earth and other inner planets.
• About 300 asteroids are known to
cross Earth's orbit.
• However, about 1500 unknown
NEO’s are estimated to exist.
• The good news is that none of the
known asteroids will strike the
• The bad news is that we have
discovered only a fraction of the
total number of Earth-crossing
asteroids, so there are many for
which we do not know the orbit.
Animation of Aten asteriod
Near Earth Asteroid Rendezvous
• NEAR spacecraft launched in 1996.
• Goals
–fly by Mathilde, C-type in main belt
• gravity measurements yield
density of 1.6 g/cm3
–match orbit with Eros, a C-type
asteroid and largest of Earthapproaching asteroids
Mathilde and Rhode Island
• map surface
• determine surface composition
and density
Close to Earth:Binary Asteroids
Binary asteroids -two rocky objects orbiting about one another
appear to be common in Earth-crossing
orbits. If one is ever found headed our
way, it could be double trouble.
Another near miss.
‘Falling Stars’
Meteoroids, Meteors, & Meteorites
• Meteoroids are simply smaller versions of asteroids.
– may be chunks that have been broken off asteroids by
• Meteors are streaks of light across the sky caused by a
meteoroid entering the Earth's upper atmosphere and
burning up in the process.
– Sometimes called "shooting" or "falling stars".
– Typically, 5 or 6 meteors are visible per hour across the sky
(sporadic meteors).
• Sometimes a portion of a large meteoroid may survive
its passage through the atmosphere and reach the
Earth's surface. This rock is called a meteorite.
– Meteorites provided astronomers with the first good estimate
of the age of the Solar System. Radiometric dating of
meteorites gives them an age of about 4.5 billion years.
• Meteorites are bits of the solar system that
have fallen to the Earth.
– most come from asteroids, including few are
believed to have come specifically from Vesta;
– a few probably come from comets
– a small number of meteorites have been shown to
be of lunar (23 finds) or Martian origin (22).
Types of Meteorites
• Iron
– Primarily iron and nickel; similar to type M asteroids
• Stony Iron
– Mixtures of iron and stony material like type S asteroids
• Chondrite
– by far the largest number of meteorites fall into this class;
– similar in composition to the mantles and crusts of the terrestrial planets
• Carbonaceous Chondrite
– similar to type C asteroids
• Achondrite
– similar to terrestrial basalts; the meteorites believed to have originated on
the Moon and Mars are achondrites
C. Chondrite
Martian Meteor
Meteor Showers
• At certain times of the year,
many more meteors are
observed to be radiating from
a particular point in space.
• These so-called meteor
showers are now known to be
associated with comet orbits.
• When the Earth crosses the
"dusty" trail of a comet, many
more meteors per hour can be
• There are several major
meteor showers each year.
Comet Debris
Meteor Shower Radiant
Leonids, 1998
Literature and Astronomy
• Meteor showers can be very impressive. Samuel Taylor
Coleridge's famous lines from The Rime of the Ancient
Mariner may have been inspired by the Leonid meteor
shower that he witnessed in 1797.
The upper air burst into life!
And a hundred fire-flags sheen,
To and fro they were hurried about!
And to and fro, and in and out,
The wan stars danced between.
And the coming wind did roar more loud,
the sails did sigh like sedge;
the rain poured down from one black cloud; The Moon
was at its edge .
Meteorite Hunting
Impact Sites
Meteor Craters
Barringer Crater
Chicxulub Crater
The End?
Predicted Annual Number of
Meteorite Falls
0.1 kg
1.0 kg
10 kg
106 km2
Land area
of Earth
Educated guesses about consequences of impacts of various sizes:
from 'The Impact Hazard', by Morrison, Chapman and Slovic,
published in Hazards due to Comets and Asteroids
Impactor Diameter
< 50
< 10
10 - 100
meteors in upper atmosphere;
most don't reach surface
irons make craters like Meteor Crater;
stones produce airbursts like Tunguska;
land impacts destroy area size of city
irons,stones hit ground;
comets produce airbursts;
land impacts destroy area size of large
area (New York, Tokyo)
100 - 1000
1000 - 10,000
land impacts destroy area size of small state;
ocean impact produces mild tsunamis
land impacts destroy area size of moderate
state (Virginia);
ocean impact makes big tsunamis
10,000 - 100,000
100,000 -1,000,000
land impact raises dust with global implication;
destroys area size of large state
(California, France)
Impact Energies
• Kinetic Energy : 1/2 mv2
• m = V
• 1 megaton = 4 x 1016 joules
• Comets: small bodies made out of dust and ices ("dirty snowballs").
• The term "comet" derives from the Greek aster kometes, which means "longhaired star"---a reference to the tail.
• Since the observations of Tycho Brahe, comets are known to be members of the
Solar System well beyond Earth's atmosphere.
• Most are on long elliptical orbits (perhaps parabolic in some cases) that take
them from the outer reaches of the Solar System to the vicinity of the Sun.
• When they come near the Sun they are heated and emit gases and dust that are
swept by the Solar Wind into the characteristic tail that always points away
from the Sun.
• Dirty snowballs
• Long term comets
– most in Oort cloud (up to 50,000 AU from Sun)
– normally orbit far from the Sun, very few enter planetary
region of solar system
– highly elongated orbits
– not confined to ecliptic, all orbital inclinations
– prograde and retrograde orbits
– roughly uniform distribution
• Short term comets (periods < 200 years)
– most originate in region beyond Neptune called Kupier belt
– approximately circular, prograde orbits 30-100 AU
– normally orbit outside jovian planets, occasionally kicked into
inner solar system
Parts of a Comet
Comets have several distinct parts when near the Sun and active:
• nucleus
– relatively solid and stable, mostly ice and gas with a small amount of dust
and other solids
• coma
– dense cloud of water, carbon dioxide and other neutral gases sublimed
from the nucleus
• hydrogen cloud
– huge (millions of km in diameter) but very sparse envelope of neutral
• dust tail
– up to 10 million km long composed of smoke-sized dust particles driven
off the nucleus by escaping gases;
– most prominent part of a comet to the unaided eye
• ion tail
– as much as several hundred million km long
– composed of plasma; laced with rays and streamers caused by
interactions with the solar wind.
Comet Tails
• Tails of bright comets can be
150 million kilometers (1 AU)
in length, making them the
"largest" objects in the Solar
• Many comets have two tails:
– gas tail (or ion tail)
composed of ions blown out
of the comet away from the
Sun by the solar wind, and
– dust tail composed of dust
particles liberated from the
nucleus as the ices are
Comet Orbits
• Comets interact gravitationally with the Sun and other objects.
• Their motion is also influenced to some degree by gases jetting out of them, so
their orbits are not completely determined by gravity.
• Most comets’ orbits appear to be elliptical, or in some cases parabolic.
• The most common comets are called short-period comets that have only mildly
elliptical orbits that carry them out to a region lying from Jupiter to beyond the
orbit of Neptune. These are normally seen only with telescopes.
• Comets visible to the naked eye are rare and are thought to come from a great
spherical cloud of cometary material surrounding the Solar System called the
Oort Cloud.
Comet Halley
• English astronomer Edmund Halley used Newton's new theory of gravitation to
determine the orbits of comets from their recorded positions in the sky as a
function of time.
• He found that the bright comets of 1531, 1607, and 1682 had the same orbits,
and concluded that these were different appearances of the same comet.
• He used his calculations to predict the return of this comet in 1758.
• If one traces back in the historical records for recordings of bright comets and
their positions in the sky, it can be concluded that Comet Halley has been
observed periodically as far back as 240 B.C.
Halley in 1910
Halley in 1986
Comet Shoemaker-Levy 9
In July of 1994, fragments of Comet Shoemaker-Levy 9 impacted the planet
Jupiter. The points of impact could be observed by the Galileo spacecraft.
Crater chain on surface of
Crater chain on surface of
Earth’s Moon
CONTOUR - Comet Nucleus Tour
– NASA Discovery mission.
– Planned launch July 1, 2002.
– Mission to visit two comets.
• Encke (3.2 year period inside orbit of Jupiter)
• Schwassmann-Wachmann 3
– Will intersect orbits when comets are near
Sun and most active.
Comets, Asteroids, and Meteors