An Introduction to Astronomy
Part VIII: Solar System Debris:
Minor Planets, Asteroids, Comets,
and Meteors
Lambert E. Murray, Ph.D.
Professor of Physics
Pluto – the Story of a Failed Planet
For many years, from 1930 until 2006, Pluto was
classified as a planet.
 Pluto is much smaller than the other planets,
closer in size to one of the larger asteroids.
However, most of the asteroids are found between
the orbit of Mars and Jupiter, not out beyond
Neptune. So, although Pluto’s orbit was a bit
different from the other planets, Pluto’s size was a
bit larger than most of the known asteroids, and
was in quite a different orbit.
 Thus, Pluto seemed, at first, more like a “planet”
than an asteroid, and so was initially classified as
such.

Discovery of Pluto
• Pluto was discovered quite by accident in 1930. The
astronomer Percival Lowell had commissioned a
search for an unknown Planet X based on calculated
deviations in Neptune’s orbit.
• Lowell’s assistant Clyde Tombaugh discovered Pluto
while doing very careful sky surveys and comparing
photographs of the same area taken at different times.
• Lowell’s search for Planet X continued fruitlessly. The
apparent discrepancies in the orbit of Neptune were
later found to be in error.
The Discovery of “Planet” Pluto
Pluto was discovered in 1930 when a dim, star-like object was observed that slowly
moved against the background stars. These two photographs were taken 1 day
apart.
Pluto’s Oribit
• When Pluto was first discovered, it was the most distant object that
was known to orbit the Sun. And it remained the most distant
object in the solar system during most of Pluto’s orbit.
• However, because Pluto’s orbit is highly eccentric, its orbit
sometimes falls inside of Neptune’s orbit. From Jan. 1979 until
Feb. 1999 it was closer to the sun than Neptune.
• In addition, the plane of Pluto’s orbit is more inclined to the
ecliptic than any of the Terrestrial or Jovian planets. It’s orbital
plane is inclined at about 17 degrees.
• Both of these observations always made Pluto a bit of a “strange”
planet. But initially, there were no other similar objects know in
our solar system, so it was classified as simply a “weird” planet.
• Not until several more, similar objects were found orbiting the Sun
at distances greater than Neptune did astronomers even consider
that Pluto was not a planet. In 2003, when Eris was discovered
with an orbit similar (although more elliptical and more inclined
with the ecliptic) to Pluto’s, and with a size somewhat larger than
Pluto’s, many Astronomers began to think that Pluto was actually a
member of a new class of objects which orbited the Sun, a Kuiper
Belt Object.
Orbit of Pluto
A nearly edge-on view of the ecliptic and Pluto’s orbit compared to it.
Discovery of Charon
Long ignored as just a
defect in the photographic
emulsion, the bump on the
upper left side of this
image of Pluto led
astronomer James Christy
to discover the moon,
Charon, in 1978.
Additional Data on Pluto and
Charon
• Pluto’s composition is unknown, but its average
density of ~ 2 gm/cm3 indicates that it is
probably about a 70/30 % mix of rock and ice.
• HST data gives fairly good values of Pluto’s size
and mass:
• radius 1160 km ( ~ 18% of earth)
• mass 1.27x1022 kg ( ~ 1/500 of earth)
• It’s orbital period is about 250 years and its
rotational period is 6 days 9 hrs. [It has been
known only since 1930.]
Pluto and Charon:
A Double Planet?
• Pluto’s moon Charon is so large compared to
Pluto and orbits so close that they almost
constitute a double-planet.
• Charon’s mass is about 1/8 Pluto’s
• Charon’s radius is over half Pluto’s
• Charon orbits only about 20,000 km from
Pluto’s center.
HST images showing surface brightness variations on Pluto’s surface.
Their cause is probably due to different surface materials.
Pluto’s moons Nix and Hydra
Observations by the Hubble Space Telescope in 2005 revealed two additional small
moons of Pluto, each about 5000 times dimmer than Pluto itself. Named Nix and
Hydra, they are between two and three times farther from Pluto than is Charon. (The
lines radiating from Pluto and Charon are artifacts of the exposure.)
Dwarf Planet Eris
Orbit of Eris compared to Pluto and the planets. Its orbit around the Sun ranges
from 38 to 98 AU, with orbital eccentricity, e = 0.44 and orbital inclination = 44o.
Asteroids
In addition to the larger planets, there are a large
number of smaller objects orbiting the Sun. These
rocky, or metalic bodies, called asteroids, are so
small that they are not forced into a spherical
shape, as are the planets. They are often called
“minor planets”.
 Most asteroids are found in the “asteroid belt”
between Mars and Jupiter (first predicted by the
Bode law).
 There is a separate class of asteroids associated
with Jupiter’s orbit, known as the “Trojan
asteroids”.
 The elliptical orbits of many asteroids is fairly
eccentric.

Ceres, the Largest Asteroid
The first asteroid to be discovered was Ceres, the largest
asteroid. It was discovered by Giuseppi Piazzi in 1801.
Sizes of Asteroids
 Ceres
is large enough that it is spherical in shape.
With a diameter of about 1,000 km (600 mi) it is
about ¼ the diameter of our Moon. As a result,
some asteroids are classified as minor planets.
 Ceres makes up about 30% of the mass of all
known asteroids.
 Of the other asteroids, the largest, Pallas and Vesta,
have diameters greater than 300 km; about 30 more
asteroids have diameters between 200 and 300 km;
about 100 are larger than 100 km; and all the rest
are under 100 km in diameter.
 When these spherical asteroids were first
discovered, there was considerable discussion as to
whether these should be classified a “planets”.
Ida and its Satellite Dactyl
The Orbits of Asteroids I
 The
orbits of some 5,000 asteroids have been
accurately determined. Approximate orbits are
known for thousands more.
 Astronomers estimate that 10,000 asteroids have
been captured on photographic surveys of the sky.
 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.
Orbits of Asteroids II
 Apollo
asteroids are some 50 asteroids with
diameters larger than 1 km that have eccentric
orbits that cross the Earth’s orbit.
 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. These
Kirkwood gaps are due to synchronous tugs from
Jupiter.
 Gaps in the asteroid belt also appear
corresponding to 2/5 and 3/5 of Jupiter’s orbital
period.
The Location
of the Trojan
Asteroids
Apollo Asteroids
Waves in
Saturn’s
Rings
Size of
Some
NearEarth
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.
 If all the asteroids were combined into one
object, they would only form a body about 1,500
km in diameter, much smaller than our Moon.

Moons or Captured Asteriods?
 The
moons of Mars and the outer moons of
Jupiter may be captured asteroids:
– Because of the proximity of the asteroids to
Mars’ and Jupiter’s orbits
– Because of the size of these moons
– Because the outer moons of Jupiter revolve
around that planet in a retrograde fashion.
Comets
Halley’s Comet
Halley’s comet is perhaps the most famous
periodic comet.
 Using Newton’s laws of gravity and his own
observations, Halley calculated the orbits for a
number of previously observed comets.
 He found that these prior comets were in fact the
same comet, and correctly predicted its next
return.
 The comet was then named in his honor.
 Halley’s comet has a period of about 76 years – it
varies slighly due to the gravitational influence of
the larger planets.
 About 100 comets have periods of less than 200
years.

Halley’s
Comet
March 1986
Comet Halley’s Orbit
Comet Orbits
 The
planes of revolution of comets are
not limited to the ecliptic but are
randomly oriented. Comets sweep past
the Sun from all directions.
 The head of a comet can be as large as a
million kilometers in diameter.
 The tail of a comet can be as long as 1
AU.
Structure of a Comet
 Head
of a comet consists of its coma and
nucleus.
– Coma is the part of a comet’s head made up
of diffuse gas and dust.
– Nucleus of a comet is the solid chunk of a
comet, located in the head.
 Tail
of a comet is the gas and/or dust
swept away from a comet’s head.
Structure of
a Comet
Vega-2 Image of the Nucleus of
Halley’s Comet
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.
 Many comets exhibit two tails:
– A straight tail consists of charged molecules
or ions.
– A curved tail is caused by dust in the coma
being pushed away by solar radiation
pressure.
The Comet’s
Tail Points
Away from the
Sun
The Two Tails of a Comet
The Ion and Dust Tails
Death of Comets
 Comets
“die”:
– through evaporation of all their volatile
materials, perhaps leaving chunks of rock
or rock debris to orbit the sun (the
probable source of meteor showers).
– by falling into the Sun.
Origin of Comets
 In
1950 Jan Oort proposed that a comet cloud
exists in a spherical shell between 10,000 and
100,000 AU from the planetary part of the
solar system. Billions of comet nuclei are
thought to exist in the Oort cloud. This would
be the source of long-period comets.
 The Kuiper belt is a band of comets thought
to exist closer to the solar system than the Oort
cloud. This would be the source of shortperiod comets.
Current Model of the Solar System
Long-Period
Comets
Captured by
Jupiter
Characteristics of Comets
Unlike the metallic asteroids, comets are icy
bodies that grow long gas and dust tails when they
are heated by the Sun (Whipple’s dirty snowballs).
 Their orbits are highly elliptical and may arise
when one of a large number of icy objects at the
outer edge of our solar system is gravitationally
perturbed.
 Notice that the asteroids and comets are smaller
bodies that exhibit the same spatial division as the
planets into rocky inner bodies and icy outer
bodies.

Meteors
What are Meteors?





The first confirmation of rocks falling from an
exploding meteor occurred in France in 1803.
Others have been observed since that time, some have
been large enough to cause severe damage, some have
actually hit people and their cars, some have been
large enough to create craters.
In fact, there is compelling evidence that an asteroid
some 10 km in diameter struck the Earth (near the
Yucatan peninsula) 65 million years ago and led to the
subsequent extinction of the dinosaurs.
However, most meteors are produced by meteoroids
with masses ranging from a few milligrams (grain of
sand) to a few grams (marble-size rock).
It is estimated that only 1 in 1 million meteoroids that
hit the atmosphere survives to reach the surface.
Terminology
A Meteoroid is an interplanetary chunk of
matter smaller than an asteroid.
 A Meteor is the phenomenon of a streak in
the sky caused by the burning of a rock or
dust particle (a meteoroid) as it falls into
our atmosphere.
 A Meteorite is an interplanetary chunk of
matter after it has hit a planet or moon.
 A Fireball is an extremely bright meteor.

Types of Meteorites
Meteorites are classified into 3 categories:
 1. Irons—iron meteorites that are made up of
80%–90% iron (with some nickel).
 2. Stones—stony meteorites that can contain
flakes of iron and nickel. These are also known
as Chondritic meteorites. These make up about
95% of all meteorites.
 3. Stony irons—meteorites that are half stone
and half iron.
Iron Meteorite
Stony or Chondritic
Meteorite
Source of Meteorites
It is thought that many small meteoroids are
debris from asteroid collisions.
 Many other meteors appear to come from
material evaporated from a comet’s nucleus (this
is the source of meteor showers).

A Meteor shower is the phenomenon of a large group of
meteors seeming to come from a particular area of the
celestial sphere. In reality it is the Earth actually passing
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.
Classifications of Solar
System Objects
Some of the current
definitions of the different
types of objects in the solar
system overlap. For example,
the largest asteroids are also
being classified as dwarf
planets; various transNeptunian objects (TNOs) are
asteroids or comets; some
comets are satellites of
Jupiter; some Kuiper belt
objects (KBOs) are satellites
of other KBOs. Furthermore,
TNOs exist in two groups:
Kuiper belt objects and Oort
comet cloud bodies.
End of Part VIII