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Asteroids
&
Comets
22 February 2005
AST 2010: Chapter 12
1
Debris of the Solar System
Asteroids are rocky or metallic
objects orbiting the Sun that
are smaller than a major
planet, but that show no
evidence of an atmosphere
and contain little volatile
(easily evaporated) material
Comets are icy bodies that
revolve around the Sun and
are smaller than a major
planet, but that contain frozen
water and other volatile
materials
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Discovery of Asteroids
Most asteroid orbits lie in the
asteroid belt, between Mars and
Jupiter
Too small to be visible without
a telescope
First discovered when
astronomers were hunting for
a planet between Mars and Jupiter
1st discovered in the 1801
Name: Ceres
Distance from the Sun: 2.8 AU
Discoverer: Giovanni Piazzi
Followed in subsequent years by the discovery of other
small planets in similar orbits
By 1890, more than 300 objects had been discovered
More than 20,000 asteroids now have well determined
orbits
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Asteroid Nomenclature
Given a number and a name
Names originally chosen
from Greek/Roman
goddesses; other female
names; all names go!
Asteroids 2410, and 4859
named after Morrison
and Fraknoi
Mathilde
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Gaspra
Ida
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Asteroid Census
Total number of asteroids in the solar system
very large
Must be estimated on the basis of systematic
sampling of the sky
Studies indicate there are 106 asteroids with
diameters greater than 1 km!
Largest: Ceres, with diameter of ~1000 km
Pallas and Vesta have diameter of ~500 km
15 more larger than 250 km across
100 times more objects 10-km across than
100-km across
Total mass of asteroids is less than the mass
of the Moon
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Asteroid Orbits
Revolve around the sun in west-to-east direction
Most lie in or near the ecliptic
Asteroid belt defined as region that contains all
asteroids with semi-major
axes of 2.2 to 3.3 AU
Periods: 3.3 to 6 years
75% of known asteroids
in the main belt
Not closely spaced
typically >million km
between them
Japanese astronomer
K. Hirayama found in
1917 that asteroids fall
into families
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Asteroid Families
The families are groups with similar
orbital characteristics
Each family may have resulted from a
breakup of a larger body, or from the
collision of two asteroids
Members of each family have similar speeds
Physical similarities between largest
asteroids of given families
Several dozen families are found
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Asteroid Physical Appearance
Majority: very dark
Do not reflect much light
Reflectivity ~3-4%
Some:
Sizable group
Typical reflectivity ~ 15-20% (similar to Moon)
Few:
Reflectivity ~60%
Understanding of the reasons for the above
difference provided by spectral analysis
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Asteroid Classification - 1
Dark asteroids
Believed to be primitive bodies
Chemically unchanged since beginning of solar
system
Composed of silicates with dark organic
carbon compounds
Ceres, Pallas, and most objects in outer
third of the belt
Most primitive asteroids part of class C
C stands for carbonaceous (carbon-rich)
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Asteroid Classification - 2
Class S
S stands for “stony” composition
No dark carbons
Higher reflectivity
Most asteroids of this type believed to be also primitive
Class M
M stands for “metal”
Identification difficult
Done by radar for the largest asteroids such as Psyche
Much less numerous
Suspected to originate from collision of a parent body
that had previously differentiated
Enough metal in 1-km M-type asteroid to supply the
world with iron for a long period of time
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Asteroid Classification - 3
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Trojan Asteroids
Located far beyond main belt
~5.2 AU, nearly same distance as Jupiter
Collectively called Trojans (from Homer’s Illiad)
Have stable orbits
because of Jupiter
Two points in Jupiter’s
orbit where asteroids
can stay indefinitely
2 points make equilateral
triangle with Jupiter and
the Sun
Since first discovery in 1906, several
hundreds have been found
Dark, primitive
appear faint, but some are nonetheless sizeable
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Asteroids in Outer Solar System
Many asteroids with orbits beyond Jupiter
Examples:
Chiron, just inside the orbit of Saturn, to almost the distance
of Uranus
Pholus, found in 1992, at 33 AU, red surface, of unknown
composition
Named after centaurs (half horse, half human)
because these objects have some attributes of comets
and asteroids
1988, on closest approach to the Sun, Chiron’s
brightness doubled, much like the comets, which
contain abundant volatile materials such as water ice,
or carbon monoxide ice
Chiron is however much bigger than comets
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Earth-Approaching Asteroids
1989 – a 200-m object passed within
800,000 km of the Earth
1994 – a 10-m object passed 105,000
km away
Some of these objects have collided with
the Earth in the past, and some are
likely to do so again in the future
Referred to as Near-Earth Objects
(NEOs)
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NEOs
640 NEOs larger than 1 km located by the end of
2002
Actual population more likely to be > millions
Unstable orbits
Fate:
Collide with our planet – and be destroyed
Be ejected from the solar system
Probability of impact: once every 100 million years
None of the known NEOs will end up crashing into the
Earth in the foreseeable future…
Larger impacts likely to generate environmental
catastrophes
A good argument for further investigation of NEOs
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NEO observation
5-km NEO Toutatis
approached the Earth at 3 million
km in 1992
less than 3 times the distance to the
Moon
Radar images show it is a double
object (two irregular lumps)
3- and 2-km objects
squashed together
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Comets
Observed since antiquity
Typical comets appear as rather
faint, diffuse spot of light
smaller than the Moon and many times
less brilliant
Small chunk of icy material that
develop an atmosphere as they
get closer to the Sun
As they get “very close” they may develop a faint,
nebulous tail extending far from the main body of the
comet
Appearance seemingly unpredictable
Typically remain visible for periods from a few days to
a few months
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Comet Orbits
Scientific study of comets dates back to
Newton who first recognized their orbits are
elongated ellipses
Edmund Halley (a contemporary of Newton) in
1705 calculated/published 24 cometary orbits
Noted that the orbits of bright comets seen in
1531, 1607, and 1682 were quite
similar — and could belong to the
same comet — returning to the
perihelion every 76 years
Predicted a return in 1758
When the comet did appear in 1758,
it was given the name Comet Halley
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Comet Halley
It has been observed/recorded on every
passage at intervals from 74 to 79 years since
239 B.C.
The period variations are caused by the jovian
planets
In 1910 the Earth was brushed by the comet’s
tail, causing much needless public concern
Its last appearance in our skies was in 1986
met by several spacecraft
It is predicted to return in 2061
Its nucleus approximately
16x8x8 km3
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Comet Census
Records exist for ~1000 comets
Comets are discovered at an average rate of 5
to 10 per year
Most visible only on photos made with large
telescopes
Every few years, a comet appears that is
bright enough to be seen with the naked eye
Recent flybys:
Comet Hyakutake, long tail, visible for about a
month, March (1996)
Comet Hale-Bopp (1997)
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Comet Components (1)
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 off of the nucleus
Hydrogen cloud: huge (millions of
km in diameter), but very sparse,
envelope of neutral hydrogen
Dust tail: up to 10 million km long composed of smoke-sized dust
particles driven off the nucleus by escaping gases
this is the most prominent part of a comet to the unaided eye
Ion tail: as much as several hundred million km long composed
of plasma and laced with rays and streamers caused by
interactions with the solar wind
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Comet Components (2)
ion tail
dust tail
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Nucleus and Coma of Comet
The nucleus is composed of
ancient ice, dust, and
gaseous core material
The nucleus has low gravity
cannot keep dust and gas from
escaping
The coma is the bright head
of the comet, as seen from
the Earth
The coma is a temporary
atmosphere of gas and dust
around the nucleus
The coma is 100,000's of
kilometers across
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Halley's nucleus
Halley's coma
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Ion Tail of Comet
Sun spews out charged particles, called the solar wind
The solar wind travels along solar magnetic field lines
extending radially outward from the Sun
Ultraviolet (UV) sunlight ionizes gases in the coma
These ions (charged particles) are pushed by solar wind
particles along magnetic field lines to form the ion tail millions
of kilometers long
The blue ion tail acts like a "solar" wind sock
The ion tail always points directly away from the Sun because
the ions move at very high speed
When the comet is moving away from the Sun, its ion
tail will be almost in front of it!
The blue color is mostly from the light emitted by
carbon-monoxide ions, but other types of ions also
contribute to the light
Since the gas is so diffuse, the observed spectrum is an
emission-line spectrum
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Dust Tail and Hydrogen Cloud of Comet
The dust tail forms when solar photons collide with the dust in
the coma
The ejected dust particles form a long, curved tail that lies slightly
farther our from the Sun than the nucleus' orbit
The dust tail has a yellow-white color from reflected sunlight
Both of the tails will stretch for millions of kilometers
The dust tail curves gently away from comet’s head because dust
particles are more massive than individual ions
They are accelerated more gently than the ions by the solar wind
and do not reach the same high speeds as ions
The hydrogen cloud forms when water vapor ejected in the jets
from the nucleus is dissociated by UV sunlight into oxygen and
hydrogen
The hydrogen clouds can be tens of millions of kilometers across
They are the largest things in the solar system!
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Origin and Evolution of Comets
Comets originate from very great distances
The aphelia of new comets are typically around
50,000 AU
This clustering of aphelia was first noted by Dutch astronomer
Jan Oort in 1950
He then proposed an idea for the origin of those comets,
which is still accepted by most astronomers today
Oort’s model of comet origin:
A star’s sphere of influence extends a little beyond 50,000 AU,
or 1 LY
Objects in orbit about the Sun at this distance can be easily
perturbed by passing stars
The new comets are some of these objects whose orbits have
been disturbed, bringing them much closer to the Sun
The reservoir of ancient icy objects from which such comets
are presumably derived is called the Oort comet cloud
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Oort Comet Cloud
Astronomers estimate that there may be about a
trillion (1012) comets in the Oort cloud
In addition, 10 times this number of comets could be
orbiting the Sun between the planets and the Oort
cloud
Such cometary objects remain undiscovered probably
because they are too faint to be seen directly and
because their stable orbit do not bring them closer to
the Sun
The total number of comets within the sphere of
influence of our Sun could therefore be on the order
of ten trillion (1013)!
Their total mass would be similar to that of 1000 Earths
Cometary material could thus be the most important
constituent of the solar system after the Sun itself
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The Kuiper Belt
Another possible source of comets lies just beyond the
orbit of Neptune
The existence of this region was first suggested by Gerard
Kuiper in 1951
The first object from this region, now called the Kuiper
belt, was discovered in 1992
The object is ~200 km across
Since then, several hundred more Kuiper-belt objects
(KBOs) have been found
It appears that these KBO are heavily influenced by
the gravity of Neptune
Many of the known KBOs have orbits like that of Pluto
Some astronomers have therefore suggested that Pluto can be
considered the largest member of the Kuiper belt
For this reason, KBOs are sometimes called plutinos
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Fate of Comets
Most comets probably spend nearly all their existence in the Oort
cloud or Kuiper belt
at a temperature near absolute zero
But once a comet enters the inner solar system, its life likely
changes dramatically!
If it survives the initial passage near the Sun, it will return towards
the cold aphelion
and may follow a fairly stable orbit for a “while”
It may impact the Sun
It may be completely vaporized as it flies by the Sun
It may interact with one or more planets with three possible fates:
It is destroyed after impacting a planet
It gets speeded up and ejected, leaving the solar system forever
It is perturbed into an orbit of shorter period
Each time a comet approaches the Sun, it loses part of its material
A few comets end their life catastrophically by breaking apart
Shoemaker-Levy 9 broke into ~20 pieces when it passed close to
Jupiter in July 1992
The fragments were then captured into a very elongated 2-year
orbit around Jupiter, before crashing into it in July 1994
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Comet Shoemaker-Levy 9
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