Since the beginning of our history, humans have looked up at
the sky and marveled at what they saw. The changing face of the
Moon, the brilliance of the Sun, the movement of the planets,
all of these and more awed our ancestors. In an attempt to
understand, they created stories and prescribed meanings to the
objects that filled the sky. Some also created elaborate models
to reproduce the motion of objects in the heavens.
Today, scientists understand more about these celestial bodies.
Some are unbelievably large, while others are smaller than a
particle of dust. In this book, we will be exploring “small
bodies” found in our solar system. Our solar system consists of
one star, the Sun, and all the objects currently held in orbit
by the gravity of our star. The solar system is just one part of
a much, much larger group of stars that form the Milky Way
Galaxy.
So what is a small body?
A small body is an object in space such as an asteroid or comet.
Interplanetary dust, Kuiper Belt Objects, material in the Oort Cloud,
planetary satellites and, yes, even Pluto and other dwarf planets can
be considered “small bodies.”
Oort Cloud
The Oort Cloud is a collection of small bodies at the
far reaches of our solar system (Tactile 1). These
objects are distributed in every direction and are not constrained to
the primary orbital plane on which we find most planets, comets and
asteroids. Countless icy bodies not confined to the ecliptic, or the
plane on which Earth orbits, are found at vast distances from the Sun.
The Oort Cloud is thought to be the birthplace of many long-period
comets who rarely, if ever, get pulled in towards the inner solar
system only to be flung back out towards the Oort Cloud due to the
gravity of the Sun. These icy bodies are part of the family of “small
bodies” in our solar system. As a whole, if the inner region of the
Oort Cloud (about 2000 Astronomical Units, or AU from the Sun) were
the size of a basketball (12” in diameter), the entire solar system
from the Sun out to the orbit of Neptune would be about the size of a
BB (3/16” in diameter)! The entire Oort Cloud extends from about 2000
AU to over 67,000 AU!
Note: An AU is a unit of distance and stands for an Astronomical
Unit, which is the average distance from the Earth to the Sun and is
about 150,000,000 km. Therefore, to say that Jupiter is found at 5
AUs means that it is about 5 times farther than the average distance
from the Earth to the Sun. In miles, an AU is about 93,000,000
mi.
Kuiper Belt
The region extending outwards from Neptune’s orbit and generally in
the same plane of the planets also contains many small bodies
including those of the short-period comets, or those comets that take
less than 200 years to make one orbit. This region is referred to as
the Kuiper Belt and extends roughly 30 to 50 AU;
although some suspected Kuiper Belt objects are found much further
than 50 AU.
The New Horizons spacecraft will fly by Kuiper belt object, 2014 MU69,
on December 31st 2018. Images from groundbased telescopes, Hubble
telescope and the NASA Startospheric Observatory for Infrared
Astronomy (SOFIA) indicate that MU 69 is an elongated object and may
have two lobes or a binary shape. Stay tuned!
What is a comet?
The Oort Cloud and Kuiper Belt seem to be the birthplace of comets,
therefore it is important to learn what these small bodies called
comets are. A comet is a ball of ice and gas mixed with dust and rock
that orbits the Sun. They date back to when the solar system was
formed. Comets are often called “dirty snowballs” because they consist
of a mixture of ice, rock and dust fragments.
A solid surface made of ice, rock and dust forms the nucleus of a
comet. Tactile 2 details the nucleus of a comet
discovered in 1969 called Comet 67P/Churyumov–Gerasimenko. The nucleus
has two distinct lobes suggesting it may be the result of the
collision of 2 comets in the past; again highlighting the dynamic
nature of small bodies! The “mountains” on the right edge of the
nucleus are 100’s of meters high. Note also the outgassing occurring
near the upper center of the core. As a comet is “pulled” into the
inner solar system by the gravity of the Sun, the solar radiation
pressure and increased temperature cause the material in the nucleus
to vaporize, releasing gas and dust formerly frozen in the nucleus.
When that happens, those on Earth are sometimes able to see the coma
and the tail as detailed in Tactile 3. The coma is
simply the gas and dust illuminated by the Sun directly surrounding
the nucleus. On this tactile, the nucleus is found as a raised area
towards the bottom of the coma and is labled to the left and probably
has similar structures to the one explored on Tactile 2. The nucleus
is, however, much much smaller than the coma or other features of a
comet. Because of the outward radiating pressure from the Sun called
solar wind, some of the material in the coma is “blown” away from the
comet forming the tail. Here is where an interesting twist enters the
story: The dust and gas are affected differently by the solar
radiation. The more dense and massive dust is less affected by the
solar radiation and retains more of a curved path due to the orbiting
nucleus. The less dense gas is easily carried away from the nucleus
and thus trails straight out away from the nucleus pointing directly
away from the Sun. This is illustrated in
Tactile 3 which shows the anatomy of a comet as it
nears our Sun.
What are asteroids?
Although the distinction between comets and asteroids is not always
easy to make, asteroids tend to be small rocky or metallic bodies that
orbit the Sun mostly between Mars and Jupiter in what is called the
Asteroid belt. Tactile 4 (not to scale by size or by
distance) illustrates the location of the asteroid belt in relation to
the other planets. Asteroids can be as big as 1000 km (600 miles) in
diameter down to the size of small pebbles. Although they orbit the
Sun, most are too small to be considered planets. The larger
asteroids, on the other hand, may be classified as dwarf planets. They
are not considered planets as they have not cleared the area around
them of surrounding debris.
Tactile 5 highlights the asteroid Vesta, which has a
diameter of 530 km. Vesta is unlike most asteroids in that it has
different layers in its interior. These are exposed in the three
“snowman” craters that can be found towards the top of the tactile and
to the left of the ridges and valleys. The asteroid Vesta dispels the
myth that all asteroids are uniform hunks of rock.
Asteroids are thought to be leftover material from the formation of
our Solar System. They are classified into three major types: C, S,
and X depending upon the abundance of rocky, metallic or clay like
material.
Debris from comets and asteroids can often enter the Earth’s
atmosphere and sometimes even collide with Earth. Smaller debris from
comets and asteroids are often referred to as meteoroids. The terms
“meteoroid,” “meteorite,” and “meteor” are often used interchangeably,
but scientists define these using three different terms. Meteoroids
are small parts of an asteroid or comet that are orbiting the Sun.
When a meteoroid enters Earth’s atmosphere it is called a meteor and
will usually vaporize and streak across the sky. Meteors are often
called “shooting stars” or “falling stars” but have nothing to do with
distant stars.
“Meteorites” are pieces of space debris that enter Earth’s atmosphere
and stay intact to hit the Earth. They are primarily pieces of
asteroids. They can be as small as a grain of rice or as large as a
boulder. Most meteors and meteorites are categorized as either iron
(mainly made of iron), stony (mainly made of minerals other than
iron), or stony-iron (composed of both).
Asteroids, comets and meteoroids routinely hit the Earth. However,
most of these are small and hardly noticeable. The larger the object,
the more dangerous it is to the Earth.
If an asteroid more than 400 meters (a quarter-mile) wide hit the
Earth, it could cause a global disaster such as the one that resulted
in the extinction of the dinosaurs. However, these are extremely rare.
Smaller asteroids or comets are still hazardous because, depending on
their speed, angle of impact, and size, they can cause injuries,
destroy cities, or cause deadly tsunamis. Scientists from NASA’s Near
Earth Object program continually scan and track the motion of many of
these objects, to proactively mitigate any potential collision issues
with Earth.
Aside from a potential threat, asteroids are also a possible source
for natural resources. Many of Earth’s metals and minerals are
seriously depleted. Mining asteroids could help with this problem in
the near future.
Unlike comets, most large asteroids are found in relatively stable
orbits around the Sun and do not intersect with Earth’s orbit. One of
these asteroids, Ceres (Tactile 6) is the largest
known asteroid and can be referred to as a dwarf planet. It also
sports curious features similar to those found on comets and also
hints at a place of origin in the Kuiper belt. Notice the bumps in a
crater at the center of Tactile 6. These bright spots
are referred to as “Spot 5” and were discovered by the Dawn spacecraft
in 2015. They are found in a crater about 80 km wide and are thought
to be brighter than the surrounding region as they contain a type of
salt that probably originated from salty water erupting onto the
surface and subsequent evaporation, hinting at recent geologic
activity. Material has also been observed, both directly and
indirectly, erupting onto the surface of Earth, Jupiter’s moon Io, and
Saturn’s moon Enceladus.
Dwarf Planets
As the name implies, these planets are small bodies and smaller than
the 8 known “major” planets in our solar system. From the Sun outward,
the “major” planets include Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus and Neptune and are labeled on
Tactile 4 which includes the location of the asteroid
belt (astr). Pluto, which was once classified as a major planet,
resides in the outermost region of our solar system but it does not
adequately fulfill all the requirements for maintaining major planet
status.
The International Astronomical Union is the only recognized body that
can officially name objects in space. They have determined that
planets must:
Orbit around the Sun
Have sufficient mass for it to take on a nearly round shape, and
Clear the neighborhood around its orbit
Many Dwarf Planets, on the other hand, are much smaller and fail to
meet the 3rd criteria of clearing their neighborhood around its orbit.
Pluto (Tactile 7), for example, orbits the Sun and
has sufficient mass for it to take on a nearly round shape, however,
it has not cleared its neighborhood around its orbit. In other words,
it is not massive enough to either capture other objects in its orbit,
or to fling them out of its orbit due to gravitational forces.
Therefore, Pluto and other objects meeting a similar fate are
considered Dwarf Planets. However, let’s be clear that the demotion of
Pluto to dwarf planet status has not altered Pluto itself or its
significance in our solar system or the fascinating science yet to be
discovered about this object. Tactile 7 is based off
recent data acquired from the New Horizons mission and highlights some
of the dynamic terrain found on its surface. Can you find the heart
shped feature located at the bottom of the tactile?
Other named dwarf planets include Ceres, Haumea, Makemake and Eris;
but many other yet to be discovered objects may meet the criteria of
being considered dwarf planets.
Satellites
Other small bodies in our solar system comprise what we call
satellites. Satellites are most often thought of as “moons” orbiting
around planets. However, the term can also refer to pieces of
asteroids or comets, which orbit around other small objects. Note that
as used here, the term ‘satellites’ refers to natural satellites and
does not refer to the man-made variety, or spacecraft!
Our solar system is filled with intriguing satellites, some of which
may even harbor oceans! Mercury and Venus have no satellites. The
Earth has one satellite, our Moon. The Moon orbits around the Earth
taking a little over 27 days to complete one revolution with respect
to the background stars. Mars has two satellites named after the Greek
characters Phobos and Deimos. Tactile 8 details the
satellites of Mars to scale with each other and to scale by distance
from Mars. Notice the large crater on the lower right of Phobos.
Notice also the odd shape of Deimos. What would it be like to have 2
moons orbting Earth?
The larger gas giant planets of Jupiter, Saturn, Uranus and Neptune
harbor a wide variety of satellites waiting to be investigated by
future scientists and explorers. Will you be one of them?
As a whole, small bodies in our solar system provide a window into
what the early solar system may have looked like and the materials
that formed it. Furthermore, the continued study of small bodies in
our solar system will benefit humans by providing better methods for
protecting our own planet and providing possible sources of resources
in the future.
Audio
We want to express our gratitude to Joe Lang, Operations Manager of
Connoisseur Media of Erie, Pennsylvania who donated his time and
talents for recording and editing the audio files for "Getting a Feel
for Lunar Craters," "Understanding Small Worlds," and "Mars Science
Laboratory" books.
