© John F. Graham, 1995
Photos courtesy NASA
From the Earth there are about 5000 celestial bodies visible to the naked eye
on a clear night at the top of some mountain far from city lights. We are well
acquainted with the obvious bodies the SUN and the Moon. Ancient humans were
perceptive enough to recognize that they could see a lot easier in the presence
of the Sun than the Moon and also they were warmed by the Sun while the Moon
seemed cold. Because of the warmth of the Sun, humans thought that it was a
perfect sphere of celestial fire created by the Gods. The Greeks went one step
further and named their god Apollo as the God of the Sun; Apollo brought heat
and light to Earth because of his countenance of golden hair, a tunic of golden
leopard skin, a chariot of beaten gold pulled by horses with golden manes and
flaming eyes, and his armament of a gold bow and gold arrows. Little did the
ancients know that their god was a ball of burning gas.
The Sun is a class G star composed of ionized hydrogen and helium gas located
150 million Km (93 million miles) from the Earth. Current calculations give its
age at about 5 billion years which is about half its life span. It is a second
generation star which means that it formed from the remains of some other star
which may have exploded between five and ten billion years ago.
The Sun has a mass of 1.98 x 1030 kilograms about 333,000 times larger than
the Earth's mass of 5.98 x 1024 kilograms. The Sun's radius is about 695,000 Km
compared to the Earth's radius of 6378 Km. These masses and radii really don't
make much sense to the ordinary human brain, so let's put them into terms we can
understand. If the Sun were an ordinary eight inch soccer ball, then the Earth
would be the size of a peppercorn. If you have enough patience and enough money
to buy 333,000 pepper corns to see if they fit into an eight inch soccer ball
you'd get an even greater appreciation of how large the Sun is. It is so large
that it contains 99.86% of the entire mass of the solar system. If one includes
all nine known planets, their moons, the asteroids, the meteorites, the comets,
and all the remaining dust in the local area it would be 0.14% of the entire
mass of the solar system.
Because the Sun is an enormous gaseous sphere it rotates faster around the
equator than it does at the poles, 27 days versus about 35 days. Its surface
temperature is about 5770º Kelvin (Kelvin scale equals the Fahrenheit scale per
degree with the Kelvin 0º starting at about - 463º Fahrenheit) and its
interior temperature is about 16 million degrees Kelvin. Its total radiated
energy is equivalent to 100 billion tons of TNT exploding per second or the same
as a significant portion of the Earth's entire nuclear arsenal exploding every
second. This power is obviously important to us because it supports all life on
the Earth. It causes seasonal changes, ocean current flows, and atmospheric
circulation. It also is responsible for photosynthesis for plant life from which
is derived all food and fossil fuels.
The Sun contains several major sections: the core along with the radiative
and the convective layers. Even though the nuclear burning occurs in the core,
the heat and light generated from this process take about 10 million years to
reach the Sun's surface. Once the photons depart the core they must travel
through the radiative layer to the convective zone where the temperatures go
from 8 million to 7000ºKelvin. After reaching the Sun's surface also known as
the photosphere, the photon travels through the chromosphere and it eventually
reaches the corona which extends from the Sun in the form of a solar wind and
finally the photon reaches the Earth millions of years after it was formed in
the Sun's core.
Beyond the Sun in constant, near circular orbits are the nine known planets
and other bodies which make up the matter in the solar system. These are divided
into three major categories: the terrestrial planets, the gas giants and the
The terrestrial planets include the inner four planets or Mercury, Venus,
Earth, and Mars. These bodies are characterized by their solid densities,
iron-nickel cores, solid mantles and crusts, and cratered surfaces. Only two of
these planets have moons, Earth and Mars.
The gas giants are Jupiter, Saturn, Uranus, and Neptune. These planets are
huge gas balls consisting largely of hydrogen, helium, and methane. Each of
these are characterized by a solid, rocky inner core surrounded by liquid or
metallic hydrogen, and topped by a gaseous hydrogen, helium, and methane
mixture. Each of these giant planets has a number of moons of varying sizes and
each has a turbulent churning atmosphere. Located in the equatorial plane of all
of these giants are rings of ice, small rocks, and dust. The most visible of
these are Saturn's gorgeous set of rings which make it the most beautiful body
in the solar system.
The minor planetesimals include Pluto and its moon Charon, the asteroids,
meteorites, comets, and dust. These are as the name implies minor bodies which
never accreted into anything larger than a minor moon of one of the gas giants.
Let's look at each of the planets, it's size and distance relative to the Sun,
and note some of the major discoveries made during the space age.
Mercury is the closest planet to the Sun. It can only be seen from Earth
under the most perfect conditions of viewing and then only at twilight or early
dawn. Because it was seen so rarely by the ancient astronomers it was named
after the swift messenger of the Roman Gods who had wings on his heels and his
helmet. This planet is located about ninety-five million Km (58.5 million miles)
from the Sun and moves so fast that one Mercury year is equivalent to 88 Earth
days. Scientists have also measured one Mercury day to be about 58.7 Earth days
Mercury's diameter is about 4880 Km or about 38% of the Earth's diameter of
12756 Km and it's about 1.4 times larger than the Earth's Moon. If the Sun were
an eight inch soccer ball, Mercury would be the size of a small pin hole in a
piece of paper. Mercury's mass is 5.6% of the Earth's mass of 5.9742 x 1024
kilograms, but is five times more massive than the Moon. This is because of the
density of its core. This means that Mercury's core of iron-nickel is larger
than the Earth's Moon. Even though it is almost the size of the Earth's Moon,
Mercury has a gravitational acceleration one third of Earth's while the Moon has
a value one sixth of Earth's gravity. Additionally, Mercury has no moon.
Surprisingly, Mercury was found to have a magnetic field; this is astonishing
because the core is thought to have solidified at least two billion years ago,
thus not allowing any convective currents for magnetism to flow through it for
the generation of a magnetic field.
Mercury's surface looks very much like the Moon with huge craters from the
hundreds of kilometers down to craters smaller than 100 meters. The most
distinctive feature on Mercury is the Caloris Basin, a crater with a diameter of
1300 Km which may have been formed from a gigantic impact occurring about the
time of Mercury's final formation.
There is no atmosphere on Mercury, although a thin layer of hydrogen, helium,
and sodium have been detected close to the planet. The best explanation for this
phenomenon is that hydrogen and helium particles from the solar wind have become
trapped in Mercury's magnetic field which is rather close to the planet.
Similarly, the sodium may have come from the planet itself, the remnants of
impacts many years ago which have become trapped in the planet's weak magnetic
As recently as the summer of 1994 the radio telescope at Arecibo, Puerto Rico
claims to have found possible ice caps on the Mercurian north and south poles.
This is really startling since the temperature on Mercury's surface is as high
as 430º Celsius ( 806ºFahrenheit) in the direct sunlight and -170º Celsius
(-274º Fahrenheit) in the shade.
This planet warrants further study and NASA has a robotic exploration of
Mercury in its plans for 2004. Perhaps then we will know whether or not Mercury
really has ice caps and get a good view of the planet as a whole system.
If the Earth had a twin it would be Venus. Both planets are nearly the same
in diameter (12,110 kilometers for Venus and 12,756 kilometers for Earth),
composition, mass, and density. If the Sun were an eight inch soccer ball, Venus
would be a peppercorn, just like the Earth. The ancients often pondered the
brilliant shimmering star which appeared early in the morning just before
sunrise or hung like a brilliant jewel a few hours after sunset. Because of this
beauty, Venus was named after the Roman Goddess of love and beauty.
With the invention of the telescope astronomers frequently looked at this
bright planet. They discovered that Venus had phases similar to the Moon and
showed evidence of a perpetual cloud cover. Imaginations abounded as scientists
and science fiction writers alike speculated that the planet was a jungle
teeming with plant and animal life known only in our own equatorial jungles.
This was soon discarded in the late 1950s when radio astronomers detected an
atmosphere of carbon dioxide and temperatures around 900º Fahrenheit. In this
atmosphere and at these temperatures, life as it exists on Earth would be
unlikely. The first space probes to Venus confirmed that the planet was lifeless
and would be a great candidate for one of Dante's circles of Hell in his Divine
Venus is located 108.2 million Km ( 66.8 million miles) from the Sun. It
rotates once around the Sun in 226.46 Earth days and has a day which is longer
than its year, 243 Earth days. The planet rotates extremely slowly on its axis
in a retrograde direction; this means that the Sun would rise in the west and
set in the east if a person were on a Venus with no clouds. Because of this slow
rotation the planet does not have the oblateness of the Earth; Venus is an
almost perfect sphere.
Venus has no oceans and it hides behind a perpetual cloud cover 15 Km thick;
it is so thick that only 1% of the sunlight which strikes Venus actually arrives
at its surface. Containing large amounts of sulfuric acid the clouds start about
45 Km altitude and move from east to west with a speed faster than three times
that of a hurricane on Earth or from 500 to 1000 Km per hour. Beneath the clouds
these winds subside substantially until they are non-existent on the planet's
Beneath the sulfuric acid clouds the atmosphere appears to contain 96.4%
carbon dioxide, 3.4% nitrogen, 0.02% sulfur dioxide, and 0.14% water vapor. The
clouds and the carbon dioxide have created an atmosphere so dense that it
crushes down on the planet 's surface with a pressure 97 times that on the
Earth's surface (14.7 lbs/in2). This pressure is equal to that found in the
Earth's oceans at a depth of 3000 feet. The various space probes have noted
lightning in the clouds especially in areas over volcanic regions which
indicates that there may still be frequent volcanic activity occurring on the
There is no moon orbiting Venus and even though Venus has a iron-nickel core
very much like Earth's there is no magnetic field on this planet; this lack of a
magnetic field may be due to the planet's slow rotation. Venus has a tortured
surface which is covered by at least 85% volcanic rock from lava flows. This
indicates volcanic activity and possible tectonic plate movement. There is also
abundant evidence of impact cratering over the entire planet. Additionally,
there are mountains on the planet which have been deformed strictly by geologic
activity. There is no erosion on the planet due to neither flowing water nor
Astronaut James Lovell upon returning from orbiting the Moon on Apollo 8
looked at the Earth and remarked that it hung in space like a beautiful jewel
and he wondered if a traveler from elsewhere would realize that there was life
on that fragile blue planet. Since we are so intimately familiar with the Earth,
most people think that humans have discovered all there is to know about our
home and that exploration should concentrate on the other planets of the solar
system. Nothing is further from the truth. We have made many discoveries about
our planet during the space age. In fact, every time we have discovered
something different on another planet of our solar system we have been able to
apply that discovery to our way of looking at the Earth and have learned more of
how our world works.
The Earth's orbit around the Sun is circular with a radius of 150 million Km
(93 million miles). This distance also describes an astronomical unit, a scale
used to define distances in the solar system. The Earth travels around the Sun
once every 365.25 days at an average speed of 30 Km per second. The Earth
rotates once every 24 hours on its axis bringing alternate periods of daylight
and darkness to the entire globe.
The Earth is a spheroid with a diameter of 12756 Km (7874 miles), a mass of
5.98 x 1024 kilograms, and a density of 5.5 grams per centimeter cubed. If the
Sun were an eight inch soccer ball the Earth would be the size of a peppercorn.
The interior of the Earth includes a core of liquid iron-nickel, a mantle and a
crust. On the crust are the Earth's land and oceans containing some evidence
that the Earth first formed as a solid coherent body about 4.7 billion years
The continents of the Earth drift on structures called tectonic plates. At
the boundaries of these plates are a number of volcanoes; earthquakes occur
along these boundaries with extreme regularity as the continental plates shift.
The formation of the Himalayan Mountains by the movement of the Indian landmass
into Asia provides proof of the tectonic plate movement. Each year the mountains
seem to grow taller as they are pushed higher by the pressure of India against
the Asian Landmass.
The Earth has a strong magnetic field with poles at the north in Canada and
the south on the shores of Antarctica. This magnetic field deflects the solar
wind particles preventing them from striking the ozone layer which protects the
Earth's surface from high electromagnetic energy. The Earth's magnetic field
also causes the Van Allen radiation belts, two toroidal regions t he first
ranging from 2000 to 5000 Km and the second from 13,000 to 19,000 Km. These
strong belts of trapped particles from the solar wind could become a serious
hazard to astronauts during time of increased solar activity.
The most abundant chemical on the Earth's surface is water. It is present in
liquid state in the oceans, rivers and lakes; in the gaseous state in the
atmosphere; and in solid state in the ice caps. It is present in every
biological entity on the planet and even combines with some of the rocks to form
hydrates. Every living thing on Earth needs water in order to survive.
Significant to life on this planet is the dense atmosphere consisting of 78%
nitrogen, 21% oxygen and one percent argon. Water vapor is also present varying
from zero to 4% of the atmosphere at given locations. The air pressure for the
Earth's surface is about 14.7 pounds per square inch equivalent to 760
millimeters of mercury. The atmosphere is divided into five distinct vertical
regions known as the troposphere, the stratosphere, the mesosphere, the
ionosphere, and the thermosphere. Most life lives in the troposphere while space
begins in the thermosphere.
Earth's atmosphere has been formed and dictated by biological activity taking
place at its surface. The most important of these cycles is known as the
oxygen/carbon dioxide cycle in which plants take carbon dioxide and water from
the air via photosynthesis and transform it into sugar and oxygen. Animals
inhale the oxygen and exhale carbon dioxide back to the plant life to repeat the
process. Most of this process has occurred by means of microscopic sea algae
living in the oceans and has given us our oxygen supply which we use today.
Earth's nearest neighbor, the Moon, is also the most explored terrestrial
body other than the Earth. Ancient humans used to speculate what the Moon was
and whether or not other beings lived there. With the invention of telescopes
and the scientific equipment of the 20th Century, the Moon was determined to be
a lifeless planet of various rocks formed into hills, valleys, mountains, and
thousands of craters.
The Moon has a diameter of 3476 Km (2146 miles) and its mass is 7.35 x 1022
kilograms. It has an acceleration due to gravity one sixth of Earth's, and
compared to an eight inch soccer ball Sun its size would be a pin head. The
Moon's average distance from Earth is about 389,000 Km (240,000 miles). Because
of this distance the Moon affects the tides on Earth. The Moon travels around
the Earth once every 28 days and it rotates once in the same time period. Due to
this rotation frequency, we only see on e side of the Moon. During its monthly
rotation around the Earth the Moon waxes and wanes in various phases from full
Moon to Half Moon to quarter Moon to new Moon and then back again.
The Moon has no magnetic field, but does have geological features called
highlands and Mare. The highlands are similar to mountains which have been worn
down by billions of years of bombardment by particles. The Mare are dark masses
of lava which came from molten seas formed when meteorites slammed deep into the
interior of the Moon covering its surface with this molten rock. Over the years,
the Mare in turn have also been bombarded by numerous meteorites which leave the
signatures of their visit spectacularly. This caused the other significant
features on the Moon, craters. There are thousands of these craters covering the
surface of the Moon. They range in size from the pinhead strikes of microscopic
meteorites to the giant craters such as Tycho and Copernicus.
For over 4 billion years the Moon waited in its orbit around the Earth until
1969 when humans began to explore it. Six human trips to the Moon returned 842
pounds of rock and soil samples to the Earth. These samples showed no life on
the Moon, no water in any form and no oxygen. In the first mission after Apollo,
the space probe Clementine showed there was a possibility of ice being formed in
the craters of the lunar poles. This discovery will be monumental because it
could pave the way to a permanent lunar base.
The fourth terrestrial planet from the Sun is Mars. Named for the Roman God
of War because of its red coloring, Mars is located 249 million Km (154 million
miles) from the Sun. When their telescopes were first trained on Mars,
astronomers noted two polar caps, which seemed to grow and shrink as the seasons
changed. Later, with more powerful telescopes, an Italian astronomer thought he
could see canals on the surface of the planet. An American millionaire, Percival
Lowell, was fascinated with the concept that there may be a dying civilization
on the planet that was bringing water from the polar caps to the equator to keep
the intelligent beings alive. Because of Mars, Lowell built an observatory in
his name to explore Mars by telescope from Earth. On July 14, 1965 the Mariner 4
spacecraft passed within 9800 Km of the planet and returned 21 pictures which
showed that not only were there no canals, but also there was a landscape which
resembled the Moon's.
Mars seems to be a frozen miniature Earth. It spins once on its axis every of
24.6 hours and revolves around the Sun in 687 Earth days. The mass of the planet
is 11% of the Earth's mass and compared to an eight inch soccer ball Sun, Mars
would be another pinhead. Mars diameter is 6760 Km giving it an average density
of 3.86 grams per centimeter cubed. The interior of Mars is similar to Earth's
with a large iron-nickel core surrounded by a mantle and a crust. Even though
Mars has a metal core and spin s on its axis it surprisingly has no magnetic
field. The only readily answer to the absence of the magnetic field is that
Mars' inner core has solidified similar to Mercury's and the Moon's. The lack of
a magnetic field subjects Mars to the full brunt o f the solar wind.
Although there were no canals found on Mars, craters were discovered in
abundance. Concentrated in the southern hemisphere of the planet these were not
the sharp craters found on the Moon, but were rather well-rounded which meant
that some sort of erosion was taking place by wind that caused planet-wide
sandstorms and blowing dust.
While Mars' southern hemisphere has all the craters, its northern hemisphere
has some fascinating geologic features. Among the most significant discoveries
in the northern hemisphere were the presence of volcanoes. Three large volcanoes
lie in a row near the Martian equator, but the largest volcano in the solar
system lies 1000 Km away. Olympus Mons is 500 - 600 Km across and towers 23 Km
(73,000 feet) above the Martian plains. The entire Martian volcanic region
sticks out like a large bulge away from the Martian surface. When Mars was
volcanically active from one to two billion years ago, the entire northern
hemisphere may have been covered with lava.
Another distinctive feature on the planet is a huge gash in the equatorial
region called Valles Marineris. This canyon easily dwarfs the Grand Canyon of
the Colorado and has a length of 2700 Km and a width of 200 Km and a depth of 6
Km. This valley could stretch across the United States. Because the canyon's
floor is sparsely cratered, Valles Marineris may have been formed fairly
recently in geologic time, possibly caused by volcanoes and the halt of tectonic
plate activity leaving the planet the dead, inactive world it is today.
As previously stated, Mars has two polar caps which vary seasonally. There is
still a great controversy about the composition of these ice caps. Some
spacecraft have indicated the caps are made of carbon dioxide while other probes
show that the ice caps are water ice mixed with dust blown from the surface.
These caps appear to be several Km thick and if melted would cover the planet
with an ocean a few meters deep. There is currently a theory that the entire
surface of Mars is covered with a permafrost containing 10 times greater amounts
of water than in the polar caps. Mars is like a frozen ice ball.
The atmosphere of Mars includes 96% thin carbon dioxide, 3% nitrogen, with
trace amounts of argon, oxygen, and water vapor. The atmospheric pressure is
0.8% of Earth's which means that life as we understand it could not exist. The
temperatures vary from -10ºC in the Summer to -130ºC in the winter. The
Martian sky has a pinkish cast caused by the red oxide dust which is always in
the atmosphere. Martian dust storms occur during the summer when Mars is at
perihelion (closest to the Sun) and last 100 days. During that time, the sand
storm covers the entire planet. The peak wind gusts are 120 km per hour while
the average wind speed is substantially lower than this.
Mars has two tiny moons, Phobos and Deimos, which are the size of New York
City. Phobos is the closer of the moons at 9300 Km while Deimos is located at
23,500 Km. Both moons travel around the planet in the same direction as the
planet rotates. Like Mars they are covered with craters; Phobos is the larger of
the two moons.
In the space between Mars and Jupiter located at about 2.8 astronomical units
(AU = 150 million km) are a number of chunks of rock orbiting the Sun which
never came together to form a planet. These are called asteroids. There are
probably about 500,000 pieces of these rocks at that location, the largest being
a 1000 km diameter moon-like object called Ceres, the smallest being dust the
size of sand grains. In the summer of 1994 the Jupiter-bound spacecraft Galileo
discovered an unusual phenomenon; a small asteroid, Ida, was found to have
another asteroid in orbit around it. Astronomers named this small orbiting
asteroid Dactyl. This discovery disproved the theory that there is a limit to
the size of a principal body in a two body orbiting system.
At first, the asteroids were speculated to have come from a planet that was
ripped apart by the tidal forces of Jupiter and the Sun, but now the popular
theory is that the planet never formed in the first place. If it had been a
planet, it would have bee n the smallest in the solar system. The combination of
Jupiter's gravity and the Sun's influence probably kept any planet from forming
at that point.
The fifth planet from the Sun, Jupiter, is also the largest planet in our
solar system. Named for the king of the Roman Gods, Jupiter would be the size of
a walnut in an eight inch soccer ball Sun solar system. Jupiter also contains
two thirds of the entire mass of all the planets. 318 Earths would equal
Jupiter is 5.2 AU from the Sun (778.3 million kilometers), travels around the
Sun once in 11.86 Earth years, and has a diameter of 143,200 Km. Jupiter's
composition resembles that of a small star. It has an atmosphere largely
consisting of hydrogen and helium, an interior pressure 100 million times that
of Earth's surface atmospheric pressure and a huge magnetic field which
stretches millions of miles into space. This field is so strong and so
far-reaching that it pours billions of kilowatts of energy into the Earth's
magnetic field every day.
Jupiter has its own ring system composed of three bands that are very dark in
visible light. These rings may be the result of impact debris. Sixteen moons
orbit Jupiter including the four large moons discovered by Galileo in 1610, Io,
Europa, Callisto, and Ganymede.
Jupiter's inner structure consists of a 30,000 km thick mantle made of liquid
hydrogen and helium. The liquid hydrogen is in the molecular phase known to us
on Earth as H2. In Jupiter's core the temperature becomes 11,000º C and the
hydrogen become s "metallic". In spite of the high temperatures this
core seems to composed of rock ice.
Jupiter's atmosphere is 81% hydrogen and 19% helium with slight traces of
ammonia, methane, water vapor, and several other hydrocarbon compounds. There is
a complex layer of clouds in the upper atmosphere which looks like a swirling
cauldron. Located in the southern hemisphere along the brightly colored bands is
a huge red spot which could hold three planet Earth's easily. This
"storm" has been going for over three hundred years since Galileo
first noted it with his observations. Rising plumes and spinning eddies suggest
there is a strong heat source in the planet's interior. This storm and other
clouds travel around the planet from east to west at speeds upward of 540
Km/hour. These clouds rotate as fast as the planet completing one revolution in
9 hours and fifty minutes.
As mentioned previously, Jupiter has a tremendously powerful magnetic field
containing 400 million times as much energy as Earth. It is probably produced by
convective currents generated through the liquid metallic core. This radiation
is so intense that humans could not approach Jupiter without storm shelters or
lead-lined protection because of the extensive radiation many times more
powerful than the Van Allen belts. At 42,800 km from Jupiter Pioneer 11 received
a radiation dose 100 times stronger than that required to kill a human. Even
electronic components aboard the spacecraft were damaged. Jupiter would not be a
hospitable planet for human exploration into its atmosphere; exploration would
have to be confined to its large moons.
The sixth planet from the Sun, Saturn, was the outermost edge of the solar
system until the age of the telescope. Named after the Roman God of agriculture
this second largest planet is distinguished from all the other planets by its
beautiful ring system making Saturn the "jewel of the solar system."
At 10 AU Saturn is twice as far as Jupiter from the Sun and has a orbital
period of 29.48 Earth years. It has a diameter of 116,400 Km which doesn't
include its nearly one thousand rings. Compared to our eight inch soccer ball
Sun, Saturn would be the size of a chestnut.
The planet rotates rapidly around its axis like Jupiter, making a complete
turn in 10.2 hours. Like Jupiter, Saturn is composed mainly of hydrogen and
helium, but there are significant structural differences which gives Saturn a
density of 0.71 g/cm3 which means, if we could find a big enough bathtub, Saturn
would float. In spite of Saturn's large size its acceleration due to gravity is
only 1.07 times that of Earth giving further credence to its light density.
Saturn's interior is much like Jupiter's with a mantle of liquid hydrogen and
helium over a slushy core of metallic hydrogen and helium which in turn
surrounds a rocky ice core very similar to Jupiter's. Similar to Jupiter, Saturn
has its own internal heat source which emits three times as much heat as it
receives from the Sun. Since Saturn is not as dense as Jupiter its heat source
is influenced by helium which sinks from the outer atmosphere to the planet's
Saturn's atmosphere is very similar to Jupiter's being composed mainly of
hydrogen and helium. Ammonia and methane are also present, but they are in
1/1000 proportions, not enough to be significant. Saturn's upper atmosphere
clouds reach speeds up to 40 0 m/sec, the fastest wind speeds in the solar
system, nearly four times faster than the winds on Jupiter. These winds may be
generated by condensing clouds of water and ammonia crystals. Saturn's
atmosphere is far less colorful than Jupiter's, probably because the clouds lie
so much deeper into the atmosphere than do the clouds on Jupiter. There is a
large storm akin to Jupiter's great red spot, but this storm is only 10,000 km
long and rotates at 100 m/sec. Because of Saturn's tornado winds, storms larger
than 1000 km cannot form and thus are extremely rare.
Saturn's ring system is the most strikingly beautiful feature about this
planet. Galileo saws these rings in 1610, describing them as a set of ears on
the planet. In 1655 Huygens reported the ring system for what it was; a thin,
flat disk coinciding with the planet's equatorial plane.
Saturn has eight major ring systems: the D system, the C ring, the B ring,
the A ring, the F ring, the G ring and the E ring. The Cassini division between
the A and B rings was noted to have several light rings in its composition so it
forms the eighth ring system. The rings are made of ice and rocks from the size
of boulders to dust particles. Nobody knows what scientific process created the
ring system. Theories abound from catastrophic tidal wave destruction to
cometary remnants and even debris ejected by its moons. The outer rings appear
to be shepherded by two moons gravitationally moving the rings into the outer F
ring and even braiding and clumping the ring particles.
Saturn has 18 known moons, the largest, strangest and most interesting being
Titan. This moon, which is slightly larger than Mercury has a largely nitrogen
atmosphere a possible model for Earth's original atmosphere. Large amounts of
carbon compounds abound in the atmosphere meaning that these type of materials
may have been frozen into the planet's ices as it was created. Astronomers and
space probes have not seen the surface of this moon, but the Cassini space probe
to be launched near the end of the century is scheduled to send a probe into
Titan's atmosphere to discover some of the shrouded planet's secrets.
In 1781 William Herschel noticed the movement of a faint star which had been
plotted in 1690. The movement he noticed indicated the star was not a star, but
rather a planet. He was persuaded to name the planet after the father of Saturn
and grandfather of Jupiter, Uranus. This planet is located at 19.2 AU from the
Sun, about twice as far as Saturn; it takes Uranus 84 years to circle the Sun.
Uranus has a diameter of 52,300 km which places it between the huge gas
giants and the large terrestrial planets. In our eight inch soccer ball Sun
solar system, Uranus would be the size of a coffee bean. The mass of the planet,
8.67 x 1025 kg or about 14.5 Earths, is readily determined by using the five
moons which circle it. The planet's density is sufficiently large enough to
indicate that its composition is not entirely hydrogen and helium like Saturn
and Jupiter, but rather it is a combination of heavier compounds such as water,
ammonia, and methane. These compounds give Uranus its pale blue-green color
noted through a telescope and as shown by the Voyager 2 flyby in January 1986.
Because there are no surface features or clouds it is difficult to measure
Uranus' rotation rate; it was finally determined to be 17 hours 14 minutes.
While trying to determine the planet's rotation rate, astronomers discovered
that the equator is perpendicular to the ecliptic plane ( the plane created by
the Sun's equator); the planet is turned over on its side. This is the only
planet with this characteristic; all of the other planets' equators lie
approximately parallel to the ecliptic plane. Uranus ' motion leads to extreme
seasonal variations at its polar regions; each pole has four seasons. The north
pole has 21 years of summer in perpetual sunlight; 21 years of winter in
constant darkness; 21 years of spring and 21 years of fall where there is a
combination of darkness and light. One would think that this would really affect
the planet's temperature, but at its distance from the Sun, Uranus gets an
energy input 360 times less than that of Earth. The temperature in the planet's
clouds remains at a constant -220ºC.
The wind blows westward on the planet at speeds between 100 and 600 km/hour,
but there are no significant markings or cloud as on Jupiter and Saturn;
however, like Jupiter and Saturn, Uranus has a ring system. There is a system of
11 dark rings around the planet. These thin rings orbit the planet at a distance
of 1.4 to 2.0 Uranian radii and are composed of bits of debris which make this
system extremely dark. The rings are located around the equator almost
perpendicular to the ecliptic. Uranus has 15 moons the largest of which are from
13 - 15% the size of the Earth's Moon. These moons have mostly frozen water,
craters and fractures with strange designs, not seen anywhere else in the solar
system. The moons are in the same plane as Uranus' equator.
One strange phenomenon on Uranus is the fact that its magnetic poles are
displaced 60º from the geographic poles. If one were to look at the magnetic
field without looking at the planet's normal rotation, he/she would think that
Uranus was like the other planets. This magnetic field has a similar strength to
that of Earth's.
In 1846 the German astronomer Johann Galle discovered Neptune. Because of the
irregularities in Uranus' orbit, mathematical astronomers predicted that there
was another planet beyond Uranus and so Neptune was discovered. Neptune is in
almost a perfect circular orbit about 30.1 AU from the Sun. It takes Neptune
158.5 Earth years to orbit once about the Sun. Neptune's mass( 1.03 x 1026 kg)
is almost the same as Uranus' which means in our eight inch soccer ball Sun
solar system, Neptune is also the size of a coffee bean. Neptune rotates once in
16 hours and three minutes and has an average temperature of -355ºF.
Like the other gas giants, Neptune has a ring system consisting of four
rings; two main or bright rings and two dark or diffuse rings. Scientists are
not sure whether Neptune's ring system is in the process of formation or if it
is being worn away.
Eight moons circle Neptune including Triton and Nereid. Six newly discovered
moons have highly irregular shapes with no signs of geologic evolution. Triton
is the only moon in the solar system which travels in the opposite direction to
its planet's rotation. This has led to a theory that Triton was captured rather
than accreted when the planet was formed. Triton is the coldest body in the
solar system with average temperatures of -400ºF. On Triton there appear to be
three types of volcanic activity: 20 miles wide fault line filled with viscous
icy material, multiple flooding phases in broad calderas with ice-like lava, and
nitrogen vents which may be the result of explosive discharges of liquid
nitrogen ice from the moon's interior.
Another discovery on Neptune was a magnetic field which is tilted 50º from
the planet's rotation axis. This is very similar to Uranus' both in strength as
well as location. Scientists do not know why.
Neptune's atmosphere is very dynamic with a great dark spot the same size as
Earth located in the southern hemisphere. This spot is similar to Jupiter's
great red spot in that both are located in the same area of the planet and both
are proportionally the same size compared to the size of their respective
planets. There is also a smaller dark spot located further south which could be
the upwelling of interior atmospheric gases. The atmospheric dynamics is very
surprising since the amount of sunlight which the planet receives and its own
internal heating is about 5% of that of Jupiter. There was also a small bright
feature rotating rapidly around the planet in a high-speed wind jet located
between the large and small spots. This little storm was nicknamed
"Scooter" by the imaging team at the Jet Propulsion Laboratory where
these discoveries took place. Other very high clouds are observed throughout the
surface of the planet. Neptune was the last planet explored up to this time by
Voyager 2 the spacecraft from Earth.
In 1995 the Hubble Space Telescope discovered another storm on Neptune's
northern hemisphere. This is the first time such a phenomenon was seen on the
gas giants. Saturn had several storms around its equator, but nothing in the
In 1930 astronomer Clyde Tombaugh was looking at a number of photographic
plates when he happened to notice a slight motion on plates taken during
successive nights. This new planet was named Pluto for the Roman God of the
underworld. Pluto is unique in that it is the coldest, the smallest and the
farthest planet from the Sun. Located at 6 billion kilometers from the Sun, 40
AU, Pluto has a diameter smaller than the Moon's, 2400 kilometers. Made largely
of rock and ice, the planet has an atmosphere one millionth of Earth's and it is
covered with exotic snows of methane, nitrogen, and carbon monoxide.
Pluto's orbit is very elliptical sometimes putting the planet inside
Neptune's orbit. It also is inclined 17º above the ecliptic plane which means
it behaves more like a comet or an asteroid rather than a planet. Many
scientists would like to reclassify Pluto as a planetesimal because of its small
size and strange orbit.
In 1978 astronomers discovered that Pluto had a moon half the size of the
planet. The moon was named Charon which makes the Pluto-Charon duo a nearly
binary planet system. Charon is covered by dirty water ice and doesn't reflect
as much light as Pluto. Due to a series of mutual eclipses in the 1980s
astronomers were able to map the surfaces of both Pluto and Charon. Pluto
appears to have polar ice caps and dark spots around its equator.
Because of its present position from the Sun, Pluto's atmosphere is
prominent, but as the planet travels farther from the Sun its atmosphere will
collapse in a huge planetary snowstorm and will remain collapsed until it is
again closer to the Sun. Before this collapse scientists would like to send a
probe to the strange, cold, planet. NASA has proposed the fast flyby of Pluto
before 2008 when its atmosphere is supposed to collapse, but this effort is
Beyond Pluto out as far as 100,000 AU lie the Oort clouds - breeding ground
for comets - the primordial snowballs. Comets frequently visit the inner solar
system and are though to be one of the origins of water on Earth and possibly
the other terrestrial planets. Life may have had its start from one impacting
comet while dinosaurs may have perished from another.
A comet consists of a nucleus, a coma, and a tail. A comet's nucleus is
basically a "dirty snowball" made of water ice, rock, dust and other
frozen gases. As the nucleus approaches closer to the Sun it heats up and the
coma is formed. The coma is the visible gas which surrounds the comet and
becomes similar to an atmosphere. A much larger hydrogen cloud also forms around
the entire comet. A dust tail is formed by the pressure of photons from the Sun
pushing dust particles from the coma into the opposite direction of the
traveling sunlight. This smooth, curved tail of dust particles reflects
sunlight. Plasma tails also form because of the solar wind particles interacting
with molecules and atoms from the coma. This tail is quite irregular and may or
may not appear on a comet. Some comets may have both tails, most have one or the
After the comet's trajectory starts to take it away from the Sun, the comet
begins to cool and the tails, the hydrogen cloud, and the coma scatter into
space leaving only the nucleus. If the comet has an elliptical orbit it will
again approach the Sun an d make a new coma, cloud, and tail. Each time a comet
approaches a planet, the nucleus' orbit may be changed. In 1993 Eugene and
Carolyn Shoemaker along with David Levy found a comet torn apart by Jupiter's
gravitational tidal forces. In 1994 comet Shoemaker-Levy's 21 pieces slammed
into Jupiter in a cosmic retribution for Jupiter's ripping it apart. In the
summer of 1995 the Hubble Space Telescope still records the remnants from this
These are basically the major bodies in the solar system. Is this solar
system typical around all stars? We don't know. The distances between stars
almost prohibit the discovery of planetary systems. Returning to our eight inch
soccer ball Sun solar system, the distance from the Sun to the Earth would be
about 26 meters. The distance from the Sun to Pluto would be about 1000 meters.
The distance to the Oort clouds would be about 1600 Km, one light year, and the
distance to the nearest star, Proxima Centauri would be 6500 Km or a little more
than four light years away, a light year being about six trillion miles. These
distances are indeed prohibitive at this time for exploration, but as one first
begins to walk we must explore a step at a time. But before a person takes
his/her first steps he/she must become familiar with the equipment required to
do the job: the feet. So, too, the space explorer has to begin with the very
basics before starting the journey into space. These basic space facts are
extremely important for the beginning of space travel. They are a knowledge of
orbital mechanics and rocket science.
Photos courtesy NASA
© John F. Graham, 1995
Photos courtesy NASA
When Sputnik was placed into orbit in 1957 it reawakened an entire physical
and mathematical science which had gone as far as it could go without space
travel, Celestial Mechanics. Celestial Mechanics is the study of the
motion of natural bodies in relation to a principal body. In the our case this
is the study of the Earth and the other planets in motion around the Sun.
With the advent of the space age in 1957 a new practical subset of Celestial
Mechanics was born, Orbital Mechanics. Orbital mechanics is the study of the
motion of human made objects as they travel around a principal body, in the
majority of flights the Earth. In the next chapters of the book we will discuss
the travel of astronauts and cosmonauts into near Earth space and journeys to
the Moon. What forces allowed these intrepid adventurers to travel in the vacuum
and microgravity of space? The answer to this question lies in the study of
To gain an elementary understanding of orbital mechanics one must first
understand the definition of an orbit. An orbit is the curved path a body
follows when the only force acting upon it is gravity. There are several key
words in this definition which one must grasp. The first is force, the second is
gravity, the third is the meaning of body, and the fourth is a curved path.
A force is an action which can change the motion or momentum of a body. Four
natural forces are currently in vogue. The strong nuclear force and its close
relative the weak nuclear force bind an atom's nucleus together. The third force
is the electromagnetic force which dictates electrical charges of electrons and
protons. These three forces all concern extremely small bodies on the scale of
atoms and molecules. The fourth force is gravity, an attraction force between
Gravity is a property of all matter. Matter is that of which everything is
composed. People have matter, trees have matter, and the Sun contains matter.
Matter makes up a body in the definition of an orbit. The amount of matter an
object contains is called the body's mass. The more mass a body contains, the
stronger is its gravitational field or attraction capabilities. One can actually
measure the gravitational force between two bodies such as two people, two
automobiles, or two buildings such as New York's World Trade Center's Twin
Towers. One can also measure this gravitational field between the Earth and the
Moon, the Earth and the Sun, and the Earth and a human made spacecraft.
An orbit is always the curved path around an object; this object is also
known as the central attracting body. The central attracting body for an
orbiting spacecraft is the Earth, the Moon, the Sun, or any other major massive
body. A body's gravitational field extends outward in all directions. This field
is strongest close to the body and gets progressively weaker as the distance
away from the body increases until this reaches infinity. In the real world, a
spacecraft can escape the influence of one primary body by reaching its radial
sphere of influence only to fall under the influence of a more massive body. For
example, a spacecraft can escape the earth's influence at about one million
kilometers, but the craft will at that point have the Sun as its central
The strength of the gravitational field depends only on the mass of an object
not its volume. If one could compress the Earth into the size of a grapefruit,
it would still have the same gravitational field as in its present size because
the amount of matter remains the same.
One can easily demonstrate an orbit by throwing a baseball or shooting a gun.
Once the ball leaves the hand or the bullet leaves the gun these objects are
primarily under the influence of the Earth's gravitational field. They are also
being acted upon by the drag associated with the Earth's atmosphere, but the
primary force is gravity. The person throwing the ball or shooting the gun only
sees a small part of the orbit. If the mass of the Earth were not in the
baseball's or bullet's path it would circle around the center of Earth's mass
and return to the thrower or the shooter. This small segment of the orbit that
occurs when the ball or the bullet strike the ground is called a trajectory.
There are four curved paths or trajectories which an orbit follows: a circle,
an ellipse, a parabola, and a hyperbola. These orbits are basically categorized
into two groups: the escape group containing the parabola and hyperbola; and the
captive group containing the circle and the ellipse. Spacecraft such as the
Voyager 2 fly escape orbits while the space shuttle flies a captive orbit.
The path of an orbit lies in a plane defined as a two dimensional flat
surface. This plane of an orbit must pass through the center of mass of the
primary or central attracting body. For most satellites this is defined as the
center of the Earth. Because the Earth is not a perfect sphere, with its mass
evenly distributed, a spacecraft has a very difficult if not impossible task of
flying a circular orbit. Even though a number of these orbits are very close to
being circular, they are still in imperfect circles called ellipses. The plane
of the elliptical orbit like the circular orbit still passes through the Earth's
center of mass.
An ellipse is a squashed circle. Instead of having a central point like the
center of a circle, the ellipse has two central points called foci. If the foci
get extremely close together the ellipse becomes circular; as the foci gets
further apart, the ellipse begins to resemble an egg and then a football. In
orbital mechanics, the center of the central attracting body is always at one
focus and the other focus is vacant.
The longest dimension of the ellipse is a straight line that passes through
both foci. This line is called the major axis, a very important dimension in
orbital mechanics because the major axis determines the size of an elliptical
orbit. The line perpendicular to the major axis and that bisects it is called
the minor axis. The minor axis as the name suggests has a minor role in orbital
The distance between the foci has no particular name. but it is very
important in determining the parameter determining the shape of the elliptical
orbit. This parameter is called the eccentricity. The eccentricity measures the
deviation of an ellipse from a circle. It is found by dividing the distance
between the foci by the length of the major axis. If the orbit is very close to
circular, the distance between the foci is zero. Zero divided by any length is
zero; therefore, the eccentricity of a circle is zero. When the distance of the
foci equals the major axis the curve becomes a parabola and when the distance
between the foci is greater than the major axis the curve becomes a hyperbola.
An easy method of determining an orbit's shape is to investigate its
eccentricity. If the eccentricity is equal to zero the curve is a circle; if the
eccentricity is between zero and one the curve is an ellipse; if the
eccentricity equals one the curve is a parabola; and, if the eccentricity is
greater than one the curve is a hyperbola.
Johannes Kepler (1571-1630) first discovered three laws which determined how
bodies orbit other bodies. The first law states that a planet travels around the
Sun in an elliptical orbit with the Sun at one focus; this is also known as the
law of ellipses. Kepler's second law states that a body sweeps out equal areas
in equal times; also called the law of areas. Kepler's third law states that the
square of a period of revolution is proportional to the cube of the orbit's
semi-major axis which is half a major axis. Kepler determined these laws after
studying accurate observations of the orbit of Mars for almost ten years.
Kepler told us how orbital motion acts, but he didn't say why. This was
determined by Sir Isaac Newton (1642-1727) and his four laws. Newton's first law
states that an object at rest remains at rest or an object in motion remains in
motion unless it is acted upon by some outside force; this is also known as the
law of inertia. Newton's second law states a force placed upon a body is
directly proportional to the product of the mass of the body and its change in
motion or momentum. This is called the law of acceleration. Newton's third law
states that for every action there occurs an equal and opposite reaction or the
law of reaction. These three laws are called Newton's Laws of Motion and
determine mechanically how a spacecraft's motion begins and changes. Newton's
fourth law pertaining to orbital mechanics is one of the most important because
it relates how the force of gravity works. The force of gravity is directly
proportion to the product of the masses of two bodies and is indirectly
proportional to the square of the distance between them. Simply stated, the more
massive or closer a body the greater its force of gravity.
Newton illustrated the use of his laws when he pictured a cannon mounted
parallel to the Earth's surface on top of a mountain above the atmosphere. The
cannon balls are fired from the cannon with increasingly larger charges. The
cannonballs fall to Earth with a larger and larger trajectory until just enough
powder is added to put the ball into a circular orbit. If more powder is placed
into the cannon the cannonball goes into an elliptical orbit and more powder
increases the orbit's eccentricity which elongates the ellipse. Finally, enough
powder is used to blow the cannonball away from Earth into an escape orbit and
the orbit becomes a parabola. If any more powder is used the orbit becomes
hyperbolic. Thus Newton was able to combine his laws of motion and gravity with
Kepler's Laws of orbits to give us today's orbital mechanics.
Only one speed will produce a circular orbit at a given altitude. This is
called the local circular speed. If the altitude is raised the local circular
speed will decrease. This speed gets progressively lower the greater the
distance from Earth because it is based upon the Earth's gravitational field
Another parameter which is measured in an orbit is its period or the time it
takes a spacecraft to complete one revolution. The period is directly relate to
the major axis. If the major axis' size increases meaning that the satellite's
altitude is higher, the period or time to complete one orbit increases. If a
satellite in a circular orbit around the equator has a period equal to about
twenty four hours it is said to be in a geosynchronous orbit because the orbital
speed is synchronized with the Earth's rotation. A satellite in such an orbit
would appear to remain stationary in the sky. Communications and weather
satellites use such orbits.
In an elliptical orbit around the Earth our home planet is at the position
known as the primary focus. The point on this orbit where the satellite is
closest to the Earth is called the perigee and the point where the satellite is
farthest is called the apogee. The line connecting the apogee and the perigee
through the middle of the Earth is the major axis of the ellipse. Another name
for the major axis is the line of apsides.
After a rocket's engines are shut down and it has placed a satellite into
orbit, a process known as orbital insertion, the spacecraft coasts around the
orbit influenced only by gravity. As the satellite goes from perigee to apogee
it is gaining altitude and as it goes from apogee to perigee it is losing
altitude. The mathematical portions of the orbit are determined from the center
of the Earth, not its surface.
A roller coaster is the best example to compare the satellite's motion in
orbit. A roller coaster is pulled to the top of the initial hill by a chain
powered by a motor; this is comparable to a rocket lifting a satellite from the
Earth's surface. Once a roller coaster has reached the top of the first hill
there is no more power for the rest of its trip. Likewise, because the rocket's
fuel has been depleted placing the satellite into orbit, there is no more rocket
power to change a satellite's velocity and its orbit.
To achieve orbit the rocket must carry the satellite to at least 80 miles
above the Earth's surface and give the satellite enough horizontal velocity,
17,000 miles per hour or 7.7 Km per second, to sustain its orbit. At first the
rocket rises vertically to escape the majority of the Earth's atmosphere and
then it pitches over to travel more and more horizontally to gain horizontal
velocity. By the time a rocket reaches orbital velocity it is traveling parallel
to the Earth's surface. The rocket continues to accelerate horizontally until it
reaches its correct velocity. At this time the rocket engines are shut down and
the spacecraft begins to coast. This point of the flight is called main engine
cutoff or MECO. The point at which MECO occurs is the orbital insertion point.
At the insertion point the engineers can arrange to have the spacecraft
either climbing, going toward apogee, or descending, traveling toward perigee,
or remaining horizontal. This angle of climb of descent is called the flight
path angle. If the spacecraft is horizontal with a flight path angle of zero
degrees and its orbital insertion speed exactly equals the local circular speed,
the spacecraft will remain in a near circular orbit. The orbit's size, major
axis, and the orbit's shape, eccentricity, are determined by two factors at
orbital insertion: the spacecraft speed and its flight path angle.
Using the roller coaster analogy, assume that the spacecraft arrived at
apogee when MECO is performed. At this point our roller coaster has completed
its initial climb via engine power and is at the very top of a high platform.
This is the point where the roller coaster begins its first plunge. Likewise,
the spacecraft will immediately begin to lose altitude as it descends toward
perigee. Just like the roller coaster gets faster as it goes downhill, the
spacecraft's speed will increase as it loses altitude and approaches perigee.
When the spacecraft passes perigee and begins to ascend to apogee, it will
slow down as it gains altitude just like the roller coaster does as it
approaches the next hill. For the satellite this speed up downhill and the
slowdown uphill will continue as long as the spacecraft stays in its elliptical
This speed and altitude change is best stated in terms of the law of the
Conservation of Energy. This law states that energy can be neither created no
destroyed; it can be changed from one form into another. There are three kinds
of energy associated with satellite flight. There is kinetic energy, the energy
of motion, potential energy, the energy of position, and total energy, the sum
of kinetic and potential energy.
Kinetic energy is based upon the mass and velocity squared of an object. The
greater the velocity of an object; the greater is its kinetic energy. Potential
energy is based upon the mass of an object, the acceleration due to gravity of
the principal attracting body, and the height or altitude the object is above
the attracting body. The higher the object is above the attracting body the
greater is the object's potential energy. The total energy, the sum of potential
and kinetic energy always remains constant.
As a satellite moves from apogee to perigee it loses altitude, therefore
decreasing potential energy, and increases velocity, thereby increasing kinetic
energy. The opposite happens when the spacecraft moves from perigee to apogee.
The increase in one kind of energy is exactly balanced by the loss of the other
kind of energy and the total energy remains constant. After MECO occurs there is
no other energy imparted to the spacecraft and the total energy of the satellite
will remain constant.
A circular orbit has no varying of kinetic energy because the velocity is
always the local circular speed and there is also no varying of potential energy
because the distance of the satellite from the center of the Earth remains the
Parabolic and hyperbolic orbits also obey the Law of Energy Conservation. As
a spacecraft continues on its way from Earth, its distance or potential energy,
keeps increasing while its velocity or kinetic energy, slows until the satellite
A study of the flight path angle is necessary to see how a spaceflight
relates practically to the law of Energy Conservation. The flight path angle is
the climb or descent angle of the satellite at various positions on its orbit.
To accurately describe the flightpath angle a coordinate system is needed.
Drawing a line from the center of the Earth through the center of the spacecraft
creates the satellite's local vertical. A line drawn perpendicular to the local
vertical through the middle of the spacecraft is called the local horizontal.
This coordinate system is called the LVLH (Local Vertical/Local Horizontal) and
moves around the orbit with the spacecraft.
The spacecraft's velocity is a vector tangent to the satellite's path with a
magnitude equal to the spacecraft's speed. The angle between the local
horizontal and the velocity vector is the flight path angle. When the velocity
vector is above the LH the flight path angle has a positive value and when the
velocity vector is below LH the flight path angle has a negative value. Just
like kinetic energy and potential energy the flight path angle changes as it
moves around an elliptical orbit.
At perigee, the velocity vector is exactly parallel to the LH and the flight
path angle is zero. As the spacecraft and the LVLH system move toward apogee,
the flight path angle increases to a maximum positive value and then decreases
to zero when the spacecraft reaches apogee. After apogee the flight path angle
becomes negative and decreases to a minimum value and then increases until it is
again zero at the orbit's perigee point.
To summarize, when the flight path angle is positive the spacecraft is
gaining altitude and decreasing its velocity and when the flight path angle is
negative the spacecraft is losing altitude and increasing its velocity. The
flight path angle is zero at apogee and perigee.
The flight path angle on a circular orbit is always zero because the LH is
always parallel to the satellite's velocity vector. The flight path angles on a
parabola and a hyperbola are always positive because the spacecraft's altitude
is always increasing away from the Earth.
If we know the flight path angle of a spacecraft on an elliptical orbit we
know the following facts:
(1) Given a positive flight path angle, the satellite is
(a) Moving toward apogee
(b) Losing speed
(2) Given a negative flight path angle, the satellite is
(a) Moving toward perigee
(b) Gaining speed
The orbit's size and the shape are determined by the velocity and the flight
path angle of the spacecraft at MECO. If the spacecraft has a zero flight path
angle and a local circular speed it will be in a circular orbit. If the
spacecraft has a zero flight path angle and insertion occurs at a greater than
circular speed the spacecraft will be in an elliptical orbit at perigee. If the
spacecraft is at a zero flight path angle and orbital insertion occurs at a less
than circular speed the spacecraft will be in an elliptical orbit at apogee. The
insertion speed will also determine the altitude on the opposite end of the
orbit. High speed at insertion equals high speed at a position opposite the
point of insertion and low speed equals low altitude at the position opposite
the point of insertion.
What happens if the spacecraft arrives at MECO at exactly local circular
speed, but with a positive flight path angle? The satellite will be moving
toward apogee on an elliptical orbit and be slowing down. If the satellite has
exactly local circular speed and a negative flight path angle it will be moving
toward perigee in an elliptical orbit and speeding up. For both of these cases
the orbital insertion point coincides with the respective ends of the minor
A satellite will be of no benefit to its users unless its position is known
in order to receive the spacecraft's information. To do this there are six items
known as orbital elements used by satellite trackers to determine a spacecraft's
orbit. These elements determine the size of an orbit, its shape, the orientation
of the orbit, and a time reference frame.
The size of the orbit is determined by the semi-major axis or half of the
major axis. This value is denoted as a. The eccentricity determines the orbit's
shape; it is designated by e.
An orbit's orientation is determined by measuring three angles. To find these
angles one has to note two planes which are flat two dimensional surfaces. The
first plane we've already discussed; it's called the orbital plane. The orbital
plane goes through the middle of the Earth and contains the satellite's orbit.
The second plane is the Earth's equatorial plane; it is a flat two dimensional
surface going through the middle of the Earth and contains the Earth's equator.
The first orientation element is called the inclination; it is designated by
i. The inclination is the angle between the orbital plane and the equatorial
plane. An orbit's inclination depends upon the spacecraft's launch site and the
direction in which its rocket is launched. Once in orbit, a satellite's
inclination determines how far its ground track will travel north and south of
The second orientation element is called the right ascension of the ascending
node. As a satellite travels in an orbit around the Earth, the points where the
orbit crosses the equator are called nodes. The point at which the spacecraft
crosses the equator going from south to north is called the ascending node. The
opposite point is called the descending node. The line of nodes connects these
two points through the center of the Earth. Right ascension is determined by a
line drawn from the Sun through the Earth to the constellation Aries at the
Vernal Equinox or the first day of Spring. This line is known as the First Point
of Aries and is a permanent reference point which never changes. It creates a
coordinate system located in the Earth's equatorial plane with the third axis
going from the center of the Earth through the north pole. The angle measured
from the first point of Aries to the ascending node in the equatorial plane is
called the right ascension of the ascending node and is designated by 1/2.
The third orientation element is called the argument of perigee. This angle
is measured from the ascending node in the orbital plane to the perigee point.
This element is designated by w (little omega).
The final orbital element is the time that the spacecraft passes the perigee
point of the orbit. If all six of these elements are known then the ground
stations can track these satellites and assist them in their missions.
Very often a satellite's mission dictates that it move from one orbit to
another. In order to do this the satellite must have some method to apply energy
to the spacecraft to accomplish this change. Most spacecraft have a variety of
maneuvering thrusters or engines. The space shuttle orbiter has two orbital
maneuvering engines (OMS) located in the rear of the spacecraft that fire only
along the spacecraft's longitudinal axis. It also contains three reaction
control systems (RCS) consisting of a number of small thrusters which can fire
along all three axes of the spacecraft's roll, pitch, and yaw. If any of these
engines are fired, the spacecraft experiences more kinetic energy which will
alter the satellite's orbit. In fact, this applied thrust will change every
point along the spacecraft's orbit except the point at which the thrust occurs.
This orbital change will occur only if the thrust is unbalanced, in other words,
it causes the spacecraft to move in a single direction and is not countered by
an opposing thruster to cause rotation around an axis.
Delta V,V, is the term used to described the
application of an unbalanced thrust that changes the speed or direction of a
spacecraft. These maneuvering thrusts are classified into four different
categories: posigrade, retrograde, radial and out-of-plane.
Posigrade thrust is any force that increases the velocity of the spacecraft.
As you recall if a satellite is in a near-circular orbit it is coasting at a
constant velocity, a nearly constant altitude, and its flight path angle is
zero. If thrusters are fired to increase the satellite's speed, the spacecraft
is now moving at greater than local circular speed and it can no longer be in a
circular orbit, but rather an elliptical one. This thruster firing is the same
as accomplishing an orbital insertion at zero flight path angle with speed
greater than local circular speed. The spacecraft will be at perigee on its way
toward apogee half an orbit away. Therefore, the larger the ÆV, the higher is
the apogee. A posigrade thrust will raise every point on the orbit except the
thrust point. This action will also lengthen the major axis which also lengthens
the orbit's period. Posigrade thrust will always increase an orbit's period.
Retrograde thrust is any force that decreases the velocity of a spacecraft.
Once again, if a satellite is in a circular orbit it has a constant altitude,
constant velocity, and zero flight path angle. If the thrusters are fired to
slow the spacecraft's speed, the results are the same as orbital insertion at a
zero flight path angle with lower than local circular speed. The thrust point
coincides with the new orbit's apogee and perigee is half an orbit away. The
greater the V, the lower the perigee. The
retrograde thrust lowers every point on the orbit except the thrust point; it
also shortens the major axis which means the orbit's period decreases.
Retrograde thrust always decreases an orbit's period.
Posigrade and retrograde thrusts are the most common types used in spacecraft
maneuvers. These thrusts are used frequently over the course of space missions
to rendezvous with another spacecraft for purposes of inspection, retrieval, or
repair. They were used almost exclusively during the Hubble Space Telescope
repair and in the rendezvous and docking of the space shuttle mission STS-71
with the Russian Space Station Mir.
A rendezvous, such as the space shuttle with the Hubble Space Telescope
(HST), begins with the shuttle in a lower orbit. To raise the shuttle to the HST
orbit two thrusts are required. If fuel is abundant, this maneuver is done by
two large thrusts in a high energy transfer. These thrusts must change both the
speed and direction of the shuttle and require copious amounts of energy which
equates directly to fuel. Fuel is at a premium on the shuttle and every effort
is made to conserve it. The high energy transfer is accomplished if time is
critical and fuel is no object.
A most fuel efficient method of orbital transfer used by the space shuttle is
called the Hohmann Transfer, named for the German engineer who developed it in
1925. This transfer uses two posigrade thrusts. The first V is designed to lengthen the spacecraft's major axis
until the orbit's apogee exactly coincides with the target's orbit. The second V is performed upon reaching the new apogee point.
This posigrade V is used to increase the
spacecraft's velocity up to the local circular speed. This procedure is called
orbital circularization. During a rendezvous an obvious problem is to time both V's so that the shuttle arrives at the orbital
circularization point the same time as the HST.
Similarly, the shuttle can be transferred to a lower orbit using a retrograde
V in a Hohmann Transfer orbit. This is how the
shuttle returns home. It executes a retrograde V
which shortens its major axis. The perigee point on this orbit correspond to a
position at 10 kilometers above the Earth. The increased kinetic energy from the
increased velocity created by the spacecraft going from apogee to perigee is
dissipated by the shuttle's drag increasing as it encounters the Earth's
atmosphere. Much of this drag converts the kinetic energy into heat energy which
is absorbed by the shuttle's thermal protection system (heat tiles). The shuttle
further reduces the kinetic energy by performing four large S-turns or roll
reversals which bleed off air speed thus reducing kinetic energy.
Another very important Hohmann Transfer is used to place satellites from the
shuttle into geosynchronous orbit (GEO). Following launch from Kennedy Space
Center, MECO occurs at 62 miles above the Earth at a speed greater than the
local circular speed. The shuttle coasts out to its designated orbit of about
185 miles and upon reaching that point accomplishes a V also known as an OMS-2 burn circularizes the
shuttle's orbit. The shuttle's payload bay doors are opened and the satellite is
deployed into the shuttle's orbit. Each satellite going to GEO has two rockets
on board to accomplish the required Vs. After
traveling for half an orbit, the satellite's first solid rocket fires to
accomplish a posigrade V. This solid rocket
motor, called a perigee kick motor (PKM), increases the satellite's orbit until
its apogee coincides with the GEO altitude of 22,300 miles or 35,862 kilometers.
The satellite discards its PKM and coasts to its new apogee position. After
arriving at apogee the spacecraft's second solid rocket motor, the apogee kick
motor (AKM), fires a posigrade V to increase the
satellite's speed to local circular velocity. The AKM usually stays with the new
GEO spacecraft after it has completed its V. GEO
satellite deployments must be timed with the Earth's rotation to arrive at GEO
altitude over the desired geographic location to take up its station.
A Hohmann transfer is also used to send spacecraft to other planets. A V must be performed to boost the spacecraft from Earth
orbit using the Sun as the new primary body to arrive at the target planet at
the spacecraft's aphelion point (Point in orbit farthest from the Sun which
corresponds to apogee, the farthest point in orbit from the Earth). An orbit
with the Sun as the primary attracting body is called heliocentric. Posigrade
thrusts would be required to send spacecraft out to planets farther from the Sun
than the Earth. These planets are called superior planet orbits. They include
the orbits of Mars, the asteroids, Jupiter, Saturn, Uranus, Neptune, and Pluto.
Retrograde thrusts, opposed to the Earth's orbit around the Sun. would be
required to reach the inferior planet orbits of Venus and Mercury.
Beside posigrade and retrograde thrusts there is also the capability to
perform radial thrusts. Since a radial thrust is one which is directed toward or
away from the central attracting body of the orbit there are two types of radial
Vs, an inward and an outward.
The space shuttle accomplishes an inward radial thrust maneuver by pointing
its nose directly at the center of the Earth and firing its OMS engines. Since
this V is perpendicular to the spacecraft's
velocity vector, it does not change the vehicle's speed, only its direction. By
performing the V in this direction, the
spacecraft's flight path angle is negative which means it's moving toward
perigee and increasing speed. Normally the spacecraft will reach the perigee
point within 90º of a inward radial thrust V.
The space shuttle performs a radially outward thrust by pointing its OMS
engines directly at the Earth and firing them. This V produces a positive flight path angle which means
that the spacecraft is now slowing down and will reach the apogee point of its
new orbit within 90º.
Since the speed of the spacecraft has not changed and the size of the orbit's
major axis has not changed because it is equal to the diameter of the old
circular orbit, the orbit's period remains the same. This is critical when the
shuttle is performing the final stages of a rendezvous when the shuttle has to
align its major axis with the target satellite's. Radial Vs are also mandatory for the spacecraft's final
approach to its docking target. Radial thrusts were used continuously when the
STS-71 docked with the Mir from 270 feet through docking on June 29, 1995.
The inclination of an orbit is the angle at which the orbital plane crosses
the equatorial plane. A satellite orbit over the equator has an inclination of
0º while that over the poles has one of 90º. Out-of-plane thrusts are designed
to change the orbital plane by changing the orbit's inclination. This is
accomplished by firing a rocket engine or thruster in a direction which is
perpendicular to the orbital plane. The details of this change vary depending
upon where the V takes place. If the shuttle's
nose is pointed toward the north, perpendicular to the orbital plane, the
resulting V will increase the orbital
inclination. If the shuttle's nose is pointed south, perpendicular to the
orbital plane, the resulting V will decrease the
orbital inclination. In order to keep the orbital parameters from changing,
these out-of-plane Vs are performed at an orbital
node. If such a V is not performed at the node
points, the line of nodes will rotate thus changing all of the orbit orientation
elements. Out-of-plane Vs are used to match the
shuttle's orbit with that of its target. Additionally, an out-of-plane change is
required for spacecraft being transferred to GEO because most of the initial
shuttle inclinations are 28.5º and it needs to change to 0º to be at the
correct position on the equator. If a GEO satellite has any inclination its
ground track will resemble that of a figure eight.
This has been a very minor presentation of a very complicated subject. There
are a number of outstanding textbooks explaining the very complicated items
discussed in a very understandable manner. Two of the best are Adventures in
Celestial Mechanics by Dr. Victor G. Szebhely, from the University of Texas, and
Spacecraft Mission Design by Charles D. Brown, from the American Institute of
Aeronautics and Astronautics in Washington, D.C.
© John F. Graham, 1995
Photos courtesy NASA
In order for a satellite to go into orbit it must accomplish two major tasks.
First, the satellite must rise above the atmosphere which surrounds the Earth's
surface. The atmosphere contains enough particles which slow the spacecraft
preventing it from orbiting the planet. A propulsion device must strain against
gravity to rise above the atmosphere. Second, the satellite must also be
provided with enough horizontal velocity above the atmosphere to at least equal
the local circular speed upon orbital injection otherwise it will reenter the
atmosphere and burn due to friction. Both of these jobs are done by rockets.
A simple rocket is usually a tall cylinder containing propellant. Propellant
always contains two items: fuel and oxidizer. Fuel is the item which burns to
provide rocket thrust. In a simple liquid rocket it is stored in its own
separate fuel supply tank. To support fuel combustion the rocket also contains a
source of oxygen needed after the spacecraft passes above the atmosphere and
cannot collect oxygen in any form. This oxygen is in the form of an oxidizer to
aid in combustion; it is stored in a container which resembles the fuel supply
The contents from fuel tank and the oxidizer tank flow from their respective
tanks via plumbing; valves, pipes and pumps; into an area called the combustion
chamber where the oxidizer joins with the fuel to burn. This combustion causes
pressure to build up within the chamber's walls; the resultant pressure, called
exhaust gas, is forced through a bell-shaped nozzle at the rocket's base. The
nozzle is tapered in the middle in an area called the throat to allow the
exhaust gas to build up even more pressure and to increase its flow rate out
into the wider portion of the nozzle. The gas goes into the wide nozzle portion
very fast and produces a force called thrust.
If the thrust is greater than the rocket's weight, the craft will lift off.
This principle is called the thrust-to-weight ratio which must be greater than
one or the vehicle will not lift off its pad. This thrust not only overcomes the
payload's mass, but also its gravitational attraction to Earth. Any additional
thrust above the thrust-to-weight ratio of one causes the rocket to accelerate.
The greater excess thrust means greater rocket acceleration in a unit known as
g's or numbers of times the norm al acceleration due to gravity at 9.8
meters/second2. In other words, one g equals 9.8 m/s2 , two g's equal an
acceleration of 19.6 m/s2, three g's equal 29.4 m/s2, etc. Weight is also
measured in g's because a natural part of weight is the acceleration due to
gravity. Therefore at one g a 100 pound woman weighs 100 pounds; at 2 g's she
weighs 200 pounds; 3 g's she weighs 300 pounds; etc. As more fuel and oxidizer
are used the rocket's weight (mass) decreases and its thrust to weight ratio
increases. To maintain the same thrust with a mass reduction, the spacecraft's
acceleration must increase in order to obey Newton's second law, F = ma.
The first rockets needed wings to guide or steer them into space. Engineers
soon found that these wings could be ripped off the rocket's body as it
approached the sound barrier. The wings were then downsized into small little
winglets called guide vanes . These guide vanes steered the rocket on to its
appropriate trajectory to gain altitude and to increase the vehicle's horizontal
velocity. As rockets matured, the engineers found that steering could be
accomplished more efficiently by including small jets instead of vanes around
the base of the craft. The original Atlas rocket employed this capability. As
the engineers grew cleverer they found that the same steering could be done by
moving the spacecraft engines and diverting the thrust into a different
direction. This was called gimbaling and is used exclusively to launch today's
At the top of the rocket is its business end, a hollow cone containing the
spacecraft's payload. The upper stage is shaped like a cone to minimize the
rocket's cross section which has to penetrate the atmosphere. This reduces the
amount of energy required to push the rocket through the atmosphere into space.
The nose cone protects the payload against aerodynamic wind blast which is very
prevalent when a vehicle speeds through the Earth's atmosphere.
As previously stated, rocket fuel is also called propellant. Propellant
includes not only a fuel which is actually burned, but also an oxidizer which
supplies oxygen for the combustion process. Propellant efficiencies are measured
by a term called specific impulse, Isp. This measurement determines how much
thrust a propellant produces; it is a similar gauge rating such as octane is for
gasoline. Isp is measured in seconds; it is the amount of time one pound of
propellant produces one pound of thrust. There are two classes of propellant
mixtures: liquid and solid.
Liquid propellants develop the most efficient thrusts for rocket power. There
are many liquid propellant combinations which are used for rocket flight such as
kerosene/liquid oxygen (LOX), alcohol/LOX, gasoline/LOX, and liquid hydrogen
(LH2)/LOX. Such a propellant with the highest Isp is LH2/LOX with a 400 second
rating for operation in the atmosphere and a 453 second rating in a vacuum.
Another liquid fuel with a medium efficiency is Aerozine 50 (kerosene) with
Nitro Tetroxide (N2O4) for an oxidizer. This propellant has an Isp of 254
seconds at sea level and a 302 second rating in a vacuum. These highly efficient
fuels and oxidizers still need an ignition spark to start the combustion
A special liquid propellant which does not need ignition to start combustion
is a fuel which ignites spontaneously when it comes into contact with its
oxidizer. This type of fuel is called hypergolic. A typical hypergolic
propellant combination is hydrazine (N2H4) and nitrogen tetroxide (N2O4). Most
spacecraft use a monopropellant for operation; the most popular of these fuels
is hydrazine which is easily stored and used on orbit for many years.
There are two important advantages of using a liquid fuel. The first is the
capability to throttle the thrust. A liquid engine can start, stop, restart, or
be reduced in thrust as the rocket flies. The second big advantage of liquid
fuel is its increased efficiency, Isp. LH2 has a typical Isp of about 496
seconds whereas the solid fuel has a typical Isp of about 250 seconds.
The disadvantages of using liquid fuel are included in three areas. The first
is the cryogenic nature of the fuel meaning it is difficult to store because of
the required cold temperatures. A second disadvantage is the handling of this
fuel by workers and the special precautions such as wearing heavy, insulated
gloves and eye protection must be taken. A third disadvantage is that liquid
fuel rocket engines are extremely complex with many moving parts including
pumps, valves, lines, and chambers. Every one of these parts must work perfectly
or the engine fails.
Solid propellant consists of a flammable putty or rubber which contains both
the fuel and the oxidizer within this mixture. For example, the mixture in the
space shuttle's solid rocket booster is a typical solid rocket fuel. The
ingredients include 16% atomized aluminum powder as the fuel, 70% ammonium per
chlorate as the oxidizer, a 12% polybutadiene acrylic acid acrylonitril as a
binding agent, 2% epoxy for curing and extremely small traces of iron oxide to
control the burn rates during flight.
The solid rocket fuel fills the inside of the rocket from its top to the
bottom. In the middle of the rocket is a shaped clearing that provides
combustion area and allows the mixture to burn evenly. This shape may be a
circle or a star depending upon the type of thrust desired for launch. The
rocket's ignition commences by shooting flames down its entire length to
initiate the combustion evenly throughout the grain, another term for solid
rocket propellant. As the fuel burns and its waste parts are ejected out the
nozzle the propellant area grows. As the propellant area grows there is more
propellant to burn which means that the rocket's thrust increases.
The advantages of the solid rocket boosters include easy handling. Once the
propellant is manufactured and shipped, the technicians need no extensive
protective clothing or procedures. The solid rocket fuel can be stored
indefinitely in its solid state with only random inspections to insure that its
seals are still functioning. In a solid rocket motor there are no moving parts
which can fail just by mechanical movement. Despite of the numerous advantages
of solid rocket propellant there are a number of disadvantages as well.
The biggest disadvantage of a solid rocket booster is that once it starts it
is not going to be stopped. Therefore, it has no control functions such as
throttles to control the burn. If thrust is to be either reduced or increased it
must be done in the design of the grain. For example, the space shuttle SRBs are
designed so that the burn reduces during transonic region passage also known as
the maximum dynamic pressure. A second disadvantage is the low Isp rating. Solid
propellant is just not efficient because it burns so quickly. A third
disadvantage is that the rocket emits solid particles not only polluting the
atmosphere, but once the vehicle gets into space, these particles also become
solid debris, a hazard for satellites and other launchers.
Which type of rocket propellant does one choose? It depends on the mission
and the type of energy required for it. If the engineer needs fast and
responsive energy, a solid would probably work best, but if the scientist needs
a slow, but steady launch capability then perhaps a liquid would be better. The
choice depends upon the mission.
There are also two types of rockets based upon their recovery: expendable
rockets and reusable rockets.
The only reusable rocket currently in the world's inventory is the space
shuttle. This system is really only a partially reusable rocket because the
orbiter, the space shuttle main engines and the solid rocket boosters return to
be used again, but the shuttle's largest component, the external tank, is thrown
away after use by letting it crash into either the Indian or Pacific Oceans.
Expendable launch vehicles abound around the world. The Russians use the
Soyuz, the Zenit, the Proton and the Energia. The European Space Agency launches
several variations of its highly successful Ariane. The Chinese have been highly
successful in developing their series of "Long March Launchers" based
upon their ICBM technology. The Japanese have launched their newest vehicle the
H-II with hopes of capturing much of the world's lucrative satellite
communications market. The Indians, the Israelis, and the Swedes have also
developed launchers for satellites. The United States has several expendable
launchers the most important of which are the Atlas, the Delta, the Titan, and
The Delta class rocket launches small to medium weight payloads with a
maximum mass of 8420 pounds. The Delta consists of a liquid propellant first and
second stage, a solid propellant third stage, and nine small solid strap-on
rockets attached to the first stage. These rockets have been noted for their
reliability and dependability since the first flight in 1960. McDonnell Douglas
Space Company manufactures the Delta and offers it for commercial use.
The Atlas class rocket offers the experience of a rocket which has evolved
since its first use in the late 1950s. The Atlas II, the latest evolution of
this rocket, can launch a medium size payload with a launch weight of 14100
pounds into Low Earth Orbit (LEO). Atlas II consists of two stages. The first
stage uses three liquid propellant engines that thrust at lift off. Two of these
engines drop off after three minutes of flight. The second stage is the Centaur
which employs two liquid propellant engines. The Atlas, formerly made by General
Dynamics Corporation, is now manufactured by Martin Marietta.
The Titan-4 flies heavy payloads with weights of 39,000 pounds These rockets
again date back to the early 1960s and served as ICBMs. The Titan-4 uses liquid
propellant first and second stages with two large solid rocket boosters for
first stage thrust augmentation. The Titan-4 uses the Centaur for its third
stage to place a payload into orbit with its highly efficient cryogenic
propellant. This Titan IV can place heavy payloads such as 35,000 pounds into
LEO and 10,000 pounds into geosynchronous orbit (GEO). Martin Marietta
manufactures the Titan-4 for commercial use as well. For heavier payloads the US
must rely on the space shuttle.
The Pegasus is the first orbital system designed for aircraft deployment
since the 1960s. Manufactured by Orbital Sciences Corporation, it was first
launched on April 5, 1990 from the right wing of a B-52; its principal
application is to place small payloads into LEO. Its three stages can place a
400 pound small satellite into orbit. This is a brand new spacecraft which
didn't have to rely upon old ICBM designs. It is normally a three stage solid
rocket configuration but can carry a fourth stage of hydrazine propellant. In
1994 a Lockheed L-1011, specifically designed for the Pegasus rocket, launched
its first satellite. The projected capability for this rocket is to be one
launch per month for the next three years. Recently, the Pegasus has undergone a
number of failures in its Pegasus XL model. Investigations by the U.S. Air
Force, NASA, and Orbital Sciences are underway to determine why the craft has
Rockets did not just appear for modern humanity to start using; they are the
result of years and years of evolution. The rocket had its true start about 3000
years ago in the deserts of China. Let's look at the history of piloted and
robotic space exploration.