Topical Outline - The Structure and Composition of
Gravity & Orbital Motion
Gravitation & Mechanics -
Law of Universal Gravitation
Inverse Square Relationship
Gravity Gradients & Tidal Forces
Gravity's strength is inversely proportional to the
square of the objects' distance from each other. For an object in orbit
about a planet, the parts of the object closer to the planet feel a slightly
stronger gravitational attraction than do parts on the other side of the
object. This is known as gravity gradient. It causes a slight torque to be
applied to any orbiting mass which has asymmetric mass distribution (for
example, is not spherical), until it assumes a stable attitude with the more
massive parts pointing toward the planet. An object whose mass is
distributed like a bowling pin would end up in an attitude with its more
massive end pointing toward the planet, if all other forces were equal.
Consider the case of a fairly massive body such as
our Moon in Earth orbit. The gravity gradient effect has caused the Moon,
whose mass is unevenly distributed, to assume a stable rotational rate which
keeps one face towards Earth at all times, like the bowling pin described
The Moon's gravitation acts upon the Earth's oceans
and atmosphere, causing two bulges to form. The bulge on the side of Earth
that faces the moon is caused by the proximity of the moon and its
relatively stronger gravitational pull on that side. The bulge on the
opposite side of Earth results from that side being attracted toward the
moon less strongly than is the central part of Earth. Earth's atmosphere and
crust are also affected to a smaller degree. Other factors, including
Earth's rotation and surface roughness, complicate the tidal effect. On
planets or satellites without oceans, the same forces apply, causing slight
deformations in the body. This mechanical stress can translate into heat, as
in the case of Jupiter's volcanic moon Io.
For Further Study
Select the "Links" section below for additional
references, including mathematical tutorials and example problems.
All objects with mass attract each other
Gravitation is the mutual attraction of all
masses in the universe. While its effect decreases in proportion to
distance squared, it nonetheless applies, to some extent, regardless of
the sizes of the masses or their distance apart.
The concepts associated with planetary motions
developed by Johannes Kepler (1571-1630) describe the positions and
motions of objects in our solar system. Isaac Newton (1643-1727) later
explained why Kepler's laws worked, by showing they depend on gravitation.
Albert Einstein (1879-1955) posed an explanation of how gravitation works
in his general theory of
In any solar system, planetary motions are orbits
gravitationally bound to a star. Since orbits are ellipses, a review of
Newton's 1st Law
Linear vs Circular Motion
Newton's Principles of Mechanics
Sir Isaac Newton realized that the force that makes apples fall to the
ground is the same force that makes the planets "fall" around the sun.
Newton had been asked to address the question of why planets move as they
do. He established that a force of attraction toward the sun becomes
weaker in proportion to the square of the distance from the sun.
Newton postulated that the shape of an orbit
should be an ellipse. Circular orbits are merely a special case of an
ellipse where the foci are coincident. Newton described his work in the
Mathematical Principles of Natural Philosophy (often called simply the
Principia), which he published in 1685. Newton gave his laws of motion as
- Every body continues in a state of rest, or of
uniform motion in a straight line, unless it is compelled to change that
state by forces impressed upon it.
- The change of motion (linear momentum) is
proportional to the force impressed and is made in the direction of the
straight line in which that force is impressed.
- To every action there is always an equal and
opposite reaction; or, the mutual actions of two bodies upon each other
are always equal, and act in opposite directions.
(Notice that Newton's laws describe the behavior
of inertia, they do not explain what the nature of inertia is. This is
still a valid question, as is the nature of mass.)
There are three ways to modify the momentum of a
body. The mass can be changed, the velocity can be changed (acceleration),
Force (F) equals change in velocity
(acceleration, A) times mass (M):
F = MA
Acceleration may be produced by applying a force
to a mass (such as a spacecraft). If applied in the same direction as an
object's velocity, the object's velocity increases in relation to an
unaccelerated observer. If acceleration is produced by applying a force in
the opposite direction from the object's original velocity, it will slow
down relative to an unaccelerated observer. If the acceleration is
produced by a force at some other angle to the velocity, the object will
be deflected. These cases are illustrated below.
The world standard of mass is the kilogram, whose definition is based on
the mass of a metal cylinder kept in France. Previously, the standard was
based upon the mass of one cubic centimeter of water being one gram, which
is approximately correct. The standard unit of force is the newton, which
is the force required to accelerate a 1-kg mass 1 m/sec2 (one
meter per second per second). A newton is equal to the force from the
weight of about 100 g of water in Earth's gravity. That's about half a
cup. A dyne is the force required to accelerate a 1-g mass 1 cm/s2.
Acceleration in Orbit
Newton's first law describes how, once in motion,
planets remain in motion. What it does not do is explain how the planets
are observed to move in nearly circular orbits rather than straight lines.
Enter the second law. To move in a curved path, a planet must have an
acceleration toward the center of the circle. This is called centripetal
acceleration and is supplied by the mutual gravitational attraction
between the sun and the planet.
Motion in a Circular Orbit
Rotation and Revolution
"Rotation" refers to an object's spinning motion
about its own axis. "Revolution" refers the object's orbital motion around
another object. For example, Earth rotates on its own axis,
producing the 24-hour day. Earth revolves about the Sun, producing
the 365-day year. A satellite revolves around a planet.
The Earth rotates on its axis relative to the sun
every 24.0 hours mean solar time, with an inclination of 23.45 degrees
from the plane of its orbit around the sun. Mean solar time represents an
average of the variations caused by Earth's non-circular orbit. Its
rotation relative to "fixed" stars (sidereal time) is 3 minutes 56.55
seconds shorter than the mean solar day, the equivalent of one solar day
Johannes Kepler's laws, as expressed by Newton, are:
- If two bodies interact gravitationally, each
will describe an orbit that is a conic section about the common mass of
the pair. If the bodies are permanently associated, their orbits will be
ellipses. If they are not permanently associated with each other, their
orbits will be hyperbolas (open curves).
- If two bodies revolve around each other under
the influence of a central force (whether or not in a closed elliptical
orbit), a line joining them sweeps out equal areas in the orbital plane
in equal intervals of time.
- If two bodies revolve mutually about each
other, the sum of their masses times the square of their period of
mutual revolution is in proportion to the cube of the semi-major axis of
the relative orbit of one about the other.
The major application of Kepler's first law is to
precisely describe the geometric shape of an orbit: an ellipse, unless
perturbed by other objects. Kepler's first law also informs us that if a
comet, or other object, is observed to have a hyperbolic path, it will
visit the sun only once, unless its encounter with a planet alters its
Kepler's second law addresses the
velocity of an object in orbit. Conforming to this law, a comet with a
highly elliptical orbit has a velocity at closest approach to the sun that
is many times its velocity when farthest from the sun. Even so, the area
of the orbital plane swept is still constant for any given period of time.
CLICK IMAGE TO START / STOP ANIMATION
Kepler's third law describes the
relationship between the masses of two objects mutually revolving around
each other and the determination of orbital parameters. Consider a small
star in orbit about a more massive one. Both stars actually revolve about
a common center of mass, which is called the barycenter. This is true no
matter what the size or mass of each of the objects involved. Measuring a
star's motion about its barycenter with a massive planet is one method
that has been used to discover planetary systems associated with distant
Obviously, these statements apply
to a two-dimensional picture of planetary motion, which is all that is
needed for describing orbits. A three-dimensional picture of motion would
include the path of the sun through space.
Orbiting a Real Planet
Isaac Newton's cannonball is really a pretty good
analogy. It makes it clear that to get a spacecraft into orbit, you need to
raise it up and accelerate it until it is going so fast that as it falls, it
falls completely around the planet.
In practical terms, you don't generally want to be
less than about 150 kilometers above surface of Earth. At that altitude, the
atmosphere is so thin that it doesn't present much frictional drag to slow
you down. You need your rocket to speed the spacecraft to the neighborhood
of 30,000 km/hr (about 19,000 mph). Once you've done that, your spacecraft
will continue falling around Earth. No more propulsion is necessary, except
for occasional minor adjustments. It can remain in orbit for months or years
before the presence of the thin upper atmosphere causes the orbit to
degrade. These same mechanical concepts (but different numbers for altitude
and speed) apply whether you're talking about orbiting Earth, Venus, Mars,
the Moon, the sun, or anything.
A Periapsis by Any Other Name...
Periapsis and apoapsis are generic terms. The
prefixes "peri-" and "ap-" are commonly applied to the Greek or Roman
names of the bodies which are being orbited. For example, look for
perigee and apogee at Earth, perijove and apojove
at Jupiter, periselene and apselene or perilune and
apolune in lunar orbit, perichron and apochron if
you're orbiting Saturn, and perihelion and aphelion if
you're orbiting the sun, and so on.
If you ride along with an orbiting spacecraft,
you feel as if you are falling, as in fact you are. The condition is
properly called free fall. You find yourself falling at the same
rate as the spacecraft, which would appear to be floating there
(falling) beside you, or around you if you're aboard the International
Space Station. You'd just never hit the ground.
Notice that an orbiting spacecraft has not
escaped Earth's gravity, which is very much present -- it is giving the
mass the centripetal acceleration it needs to stay in orbit. It just
happens to be balanced out by the speed that the rocket provided when it
placed the spacecraft in orbit. Yes, gravity is a little weaker
on orbit, simply because you're farther from Earth's center, but it's
mostly there. So terms like "weightless" and "micro gravity" have to be
taken with a grain of salt... gravity is still dominant, but some of its
familiar effects are not apparent on orbit.
For Further Study
Select the "Links" section below for additional
references, including mathematical tutorials and example problems.
Conservation of Orbital Energy
How Orbits Work
CLICK IMAGE TO
START / STOP ANIMATION
These drawings simplify the physics of orbital
mechanics, making it easy to grasp some of the basic concepts. We see Earth
with a ridiculously tall mountain rising from it. The mountain, as Isaac
Newton first described, has a cannon at its summit.
When the cannon is fired, the cannonball follows its ballistic arc, falling
as a result of Earth's gravity, and of course it hits Earth some distance
away from the mountain.
If we pack more gunpowder into the cannon, the next time it's fired, the
cannonball goes faster and farther away from the mountain, meanwhile falling
to Earth at the same rate as it did before. The result is that it has gone
halfway around the cartoon planet before it hits the ground. (You might
enjoy the more elaborate animation at
CLICK IMAGE TO
START / STOP ANIMATION
In order to make their point these cartoons ignore
lots of facts, of course, such as the impossibility of there being such
a high mountain on Earth, the drag exerted by the Earth's atmosphere on
the cannonball, and the energy a cannon can impart to a projectile...
not to mention how hard it would be for climbers to carry everything up
such a high mountain! Nevertheless the orbital mechanics they illustrate
(in the absence of details like atmosphere) are valid.
Packing still MORE gunpowder into the capable cannon, the cannonball goes
much faster, and so much farther that it just never has a chance to touch
down. All the while it would be falling to Earth at the same rate as it did
in the previous cartoons. This time it falls completely around Earth! We can
say it has achieved orbit.
|CLICK IMAGE TO
START / STOP ANIMATION
That cannonball would skim past the south pole, and
climb right back up to the same altitude from which it was fired, just like
the cartoon shows. Its orbit is an ellipse.
This is basically how a spacecraft achieves orbit.
It gets an initial boost from a rocket, and then simply falls for the rest
of its orbital life. Modern spacecraft are more capable than cannonballs,
and they have rocket thrusters that permit the occasional adjustment in
orbit, as described below. Apart from any such rocket engine burns, they're
just falling. Launched in 1958 and long silent, the
Vanguard-1 Satellite is still falling around Earth.
In the third cartoon, you'll see that part of the
orbit comes closer to Earth's surface that the rest of it does. This is
called the periapsis of the orbit. The mountain represents the
highest point in the orbit. That's called the apoapsis. The altitude
affects the time an orbit takes, called the orbit period. The period of the
space shuttle's orbit, at say 200 kilometers, is about 90 minutes.
Vanguard-1, by the way, has an orbital period of 134.2 minutes, with its
periapsis altitude of 654 km, and apoapsis altitude of 3969 km.
Units of Measure
the AU - Astronomical Unit
Light Years, Parsecs, Eons
Relative to Things Familiar
The Milky Way
Earth & the Other Planets
to the Sun
to the Moon
to the Other Planets
to the Nearest Star
to the Nearest Solar System
to the Center of the Milky Way
to the Nearest Galaxy
Our Solar Sytem
The Earth, Moon, and Planets
How Much Energy?
of the Universe
of Our Solar System
Sol - the Sun
Terra - the Earth
Luna - the Moon
Sources of Light & Energy
Heat Transfer in Space
Formation, Evolution, & Age
of the Universe
Galaxies and Stars
Our Solar System
The Earth & Moon
The structure and composition of the universe can be learned from studying stars
and galaxies and their evolution. As a basis for understanding this concept:
a. Students know galaxies are clusters of billions of stars and may have different
b. Students know that the Sun is one of many stars in the Milky Way
galaxy and that stars may differ in size, temperature, and color.
c. Students know how to use astronomical units and light years as measures of
distances between the Sun, stars, and Earth.
d. Students know that stars are the source of light for all bright objects in
outer space and that the Moon and planets shine by reflected sunlight, not by
their own light.
e. Students know the appearance, general composition, relative position and
size, and motion of objects in the
solar system, including planets, planetary satellites,
asteroids originally thought to be stars are now known to be distant galaxies.
Galaxies them-selves appear to form clusters that are separated by vast expanses
of empty space. As galaxies are discovered they are classified by their differing
sizes and shapes. The most common shapes are spiral, elliptical, and irregular.
Beautiful, full-color photo-graphs of astronomical objects are available on the
Internet, in library books, and in popular and professional journals. It may also
interest students to know that astronomers have inferred the existence of planets
orbiting some stars.
4.b The Sun is a star located on the rim of a typical spiral galaxy called the
Milky Way and orbits the galactic center. In similar spiral galaxies this galactic
center appears as a bulge of stars in the heart of the disk. The bright band of
stars cutting across the night sky is the edge of the Milky Way as seen from the
perspective of Earth, which lies within the disk of the galaxy. Stars vary greatly
in size, temperature, and color. For the most part those variations are related
to the stars’ life cycles. Light from the Sun and other stars indicates that the
Sun is a fairly typical star. It has a mass of about 2
kg and an energy output, or luminosity, of about 4
joules/sec. The surface temperature of the Sun is approximately 5,500 degrees
Celsius, and the radius of the Sun is about 700 million meters. The surface temperature
determines the yellow color of the light shining from the Sun. Red stars have
cooler surface temperatures, and blue stars have hotter surface temperatures.
To connect the surface temperature to the color of the Sun or of other stars,
teachers should obtain a “black-body” temperature spectrum chart, which is typically
found in high school and college textbooks.
4.c Distances between astronomical objects are enormous. Measurement units such
as centimeters, meters, and kilometers used in the laboratory or on field trips
are not useful for expressing those distances. Consequently, astronomers use other
units to describe large distances. The astronomical unit (AU) is defined to be
equal to the average distance from Earth to the Sun: 1 AU
meters. Distances between planets of the solar system are usually expressed in
AU. For distances between stars and galaxies, even that large unit of length is
not sufficient. Interstellar and intergalactic distances are expressed in terms
of how far light travels in one year, the light year (ly): 1 ly
meters, or approximately 6 trillion miles. The most distant objects observed in
the universe are estimated to be 10 to 15 billion light years from the solar system.
Teachers need to help students become familiar with AUs by expressing the distance
from the Sun to the planets in AUs instead of meters or miles. A good way to become
familiar with the relative distances of the planets from the Sun is to lay out
the solar system to scale on a length of cash register tape.
4.d The energy from the Sun and other stars, seen as visible light, is caused
by nuclear fusion reactions that occur deep inside the stars’ cores. By carefully
analyzing the spectrum of light from stars, scientists know that most stars are
composed primarily of hydrogen, a smaller amount of helium, and much smaller amounts
of all the other chemical elements. Most stars are born from the gravitational
compression and heating of hydrogen gas. A fusion reaction results when hydrogen
nuclei combine to form helium nuclei. This event releases energy and establishes
a balance between the inward pull of gravity and the outward pressure of the fusion
Ancient peoples observed that some objects in the night sky wandered about while
other objects maintained fixed positions in relation to one another (i.e., the
constellations). Those “wanderers” are the planets. Through careful observations
of the planets’ movements, scientists found that planets travel in nearly circular
(slightly elliptical) orbits about the Sun.
Planets (and the Moon) do not generate the light that makes them visible, a fact
that is demonstrated during eclipses of the Moon or by observation of the phases
of the Moon and planets when a portion is shaded from the direct light of the
Various types of exploratory missions have yielded much information about the
reflectivity, structure, and composition of the Moon and the planets. Those
missions have included spacecraft flying by and orbiting those bodies, the
soft landing of spacecraft fitted with instruments, and, of course, the visits
of astronauts to the Moon
Teachers should look for field trip opportunities for students to observe the
night sky from an astronomical observatory or with the aid of a local astronomical
society. A visit to a planetarium would be another way of observing the sky. If
feasible, teachers should have students observe the motion of Jupiter’s inner
moons as well as the phases of Venus. Using resources in the library-media center,
students can research related topics of interest.
solar system has been a topic of study from the beginning of history. For
nearly all that time, people have had to rely on long-range and indirect
measurements of its objects. For all of human history and pre-history,
observations were based on visible light. Then in the 20th century people
discovered how to use additional parts of the spectrum. Radio waves, received
here on Earth, have been used
since 1931 to investigate celestial objects. Starting with the emergence of
space flight in 1957,
instruments operating above Earth's obscuring atmosphere could take advantage
not only of light and radio, but virtually the whole spectrum (the
electromagnetic spectrum is the subject of a later
At last, with interplanetary travel, instruments can be carried to many solar
system objects, to measure their physical properties and dynamics directly and
at very close range. In the 21st century, knowledge of the solar system is
advancing at an unprecedented rate.
The solar system consists of an
average star we call the sun, the planets Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune, and Pluto. It includes the satellites of the planets,
numerous comets, asteroids, meteoroids, and the interplanetary medium, which
permeates interplanetary space. The sun is the richest source of
electromagnetic energy in the solar system. The sun's nearest known stellar
neighbor is a red dwarf star called
Centauri, at a distance of about 4.2 light years. (A light year is the
distance light travels in a year, at about 300,000 km per second.)
Motions Within the Solar System
The sun and planets each rotate on their axes. Because
formed from the same rotating disk, the planets, most of their satellites,
and the asteroids, all revolve around the sun in the same direction as it
rotates, and in nearly circular orbits. The planets orbit the sun in or near the
same plane, called the ecliptic (because it is where eclipses occur). Pluto is a
case in that its orbit is the most highly inclined (17 degrees) and the most
highly elliptical of all the planets. Because its orbit is so eccentric, Pluto
sometimes comes closer to the sun than does Neptune. It's interesting to note
that most planets rotate in or near the plane in which they orbit the sun, since
they formed, rotating, out of the same dust ring. Uranus must have suffered a
whopping collision, though, that set it rotating on its side.
Distances Within the Solar System
The most common unit of measurement for distances
within the solar system is the astronomical unit (AU). The AU is based on the
mean distance from the sun to Earth, roughly 150,000,000 km. JPL's Deep Space
Network refined the
value of the AU in the 1960s by obtaining radar echoes from Venus. This
measurement was important since spacecraft navigation depends on accurate
knowledge of the AU. Another way to indicate distances within the solar system
is terms of light time, which is the distance light travels in a unit of time.
Distances within the solar system, while vast compared to our travels on Earth's
surface, are comparatively small-scale in astronomical terms. For reference,
Proxima Centauri, the nearest star at about 4 light years away, is over 265,000
AU from the sun.
||~Earth-Moon Round Trip
||~Sun to Mercury
||Sun to Earth (1 AU)
||~1.5 x Sun-Jupiter Distance
||Voyager-1 (January, 2004)
||~Next closest star
Temperatures Within the Solar System
Select the above link for the Solar System Temperature Reference showing
examples and comparing temperatures of objects and conditions from absolute zero
through planet temperatures, to those of stars. The reference also includes
temperature conversion factors and links to a conversion engine.
Mass Distribution Within
the Solar System
In Cosmic Perspective
Our whole solar system, together
with all the local stars you can see on a clear dark night, orbits the center of
our home galaxy. This spiral disk we call the Milky Way includes some 200
billion stars, thousands of gigantic clouds of gas and dust, and enormous
quantities of mysterious dark matter
Interstellar space is the
term given to the space between stars in the galaxy. We are beginning to find
that many stars besides the sun harbor their own planets, called extrasolar
planets. As of January 2004 astronomers have detected about
100 planets orbiting other stars. They are all giant, Jupiter-like planets,
made mostly of gas, since current detection methods cannot reveal smaller
worlds. Their formation process is still unclear.
The image at right shows a galaxy
similar to the Milky Way, known as M100 (click the image for details).
The Milky Way has two small galaxies orbiting it nearby, which are visible from
the southern hemisphere. They are called the Large
Magellanic Cloud and the Small Magellanic Cloud. Our galaxy, one of billions
of galaxies known, is travelling through intergalactic space. On a cosmic
scale, all galaxies are generally receding from each other, although those
relatively close together may exhibit additional local motion toward or away
from each other
sun is a typical star. Its
spectral classification is "G2 V." G2 basically means it's a yellow-white
star, and the roman numeral V means it's a "main sequence" dwarf star (by far
the most common) as opposed to supergiant, or sub-dwarf, etc. You can view
current images of the sun as seen today by the suite of instruments aboard
SOHO (Solar & Heliospheric Observatory) as it views the sun from the L1
Lagrange point between the Earth and the sun.
The sun dominates the gravitational field of the solar
system; it contains about 99.85% of the solar system's mass. The planets, which
condensed out of the same disk of material that formed the sun, contain only
about 0.135% of the mass of the solar system. Satellites of the planets, comets,
asteroids, meteoroids, and the interplanetary medium constitute the remaining
0.015%. Even though the planets make up only a small portion of the solar
system's mass, they do retain the vast majority of the solar system's angular
momentum. This storehouse of momentum can be utilized by interplanetary
spacecraft on so-called "gravity-assist"
creates extreme pressures and temperatures within itself, sustaining a
fusing hydrogen nuclei and producing helium nuclei. This reaction yields
tremendous amounts of energy, causing the material of the sun to be plasma and
gas. These thermonuclear reactions began about 5 x 109 years ago in
the sun, and will probably continue for another 5 x 109 years. The
apparent surface of the sun has no clean physical boundary, as solid planets do,
although it appears as a sharp boundary when seen from the distance of Earth.
Click the SOHO solar image at right for more details about the image.
The sun rotates on its axis with a period of
approximately 25.4 days. This is the adopted value at 16° latitude. Because the
sun is a gaseous body, not all its material rotates together. (This
solar fact sheet describes how rotation varies with latitude). Solar matter
at very high latitudes takes over 30 days to complete a rotation. Our star's
output varies slightly over an
11-year cycle, during which the number of sunspots changes.
The sun's axis is tilted about 7.25 degrees to the axis
of the Earth's orbit, so we see a little more of the sun's northern polar region
each September and more of its southern region in March. As viewed from the
Earth's surface, the Sun subtends roughly half a degree of arc upon the sky (as
does the Moon, at this period in cosmic time.)
The sun has strong magnetic fields that are associated
with sunspots and coronal mass ejections, CMEs (also called solar flares). A
sunspot is a relatively cool area that appears dark against the hotter face of
the sun. Sunspots are formed when magnetic field lines just below the sun's
visible surface are twisted, and reach though the photosphere. CMEs are huge
magnetic bubbles of plasma that
erupt from the sun's corona and travel through space at high speed. View
this spectacular 5-Mbyte
SOHO movie of CMEs in August, 1999.
The solar magnetic field is not uniform, and it is very
dynamic. Solar magnetic field variations and dynamics are targets of major
interest in the exploration of the solar system
Our Bubble of Interplanetary Space
The "vacuum" of interplanetary
space includes copious amounts of energy radiated from the sun, some
interplanetary and interstellar dust (microscopic solid particles) and gas, and
the solar wind. The solar wind, discovered by
Eugene Parker in 1958, is a flow of lightweight ions and electrons (which
together comprise plasma) thrown from the sun. The solar wind inflates a bubble,
called the heliosphere, in the surrounding interstellar medium (ISM).
The solar wind has a visible effect on comet tails. It
flows outward from our star at about 400 km per second, measured in the vicinity
of Earth's orbit, and the
Ulysses spacecraft found that it approximately doubles its speed at high
Diagram courtesy Dr. Gary Zank, University of Delaware
The boundary at which the solar wind meets the ISM,
containing the collective "solar" wind from other local stars in our galaxy, is
called the heliopause. This is where the solar wind and the sun's
magnetic field stop. The boundary is theorized to be roughly teardrop-shaped,
because it gets "blown back" to form a heliotail, as the sun moves through the
ISM (toward the right in the diagram above). The sun's relative motion may also
create an advance bow shock, analogous to that of a moving boat. This is a
matter of debate and depends partly on the strength of the interstellar magnetic
But before it gets out to the heliopause, the solar
wind is thought to slow to subsonic speeds, creating a termination shock.
This appears at the perimeter of the green circle in the diagram. Its actual
shape, whether roughly spherical or teardrop, depends on magnetic field
strengths, as yet unknown.
In the diagram above, temperatures are theorized; none
have been actually measured beyond the termination shock. Note that even with
the high particle temperatures, their density is so low that massive objects
like spacecraft remain very cold (as long as they are shaded, or distant, from
The white lines in the diagram represent charged
particles, mostly hydrogen ions, in the interstellar wind. They are deflected
heliosphere's edge (the heliopause). The pink arrow shows how neutral
particles penetrate the heliopause. These are primarily hydrogen and helium
atoms, which are mostly not affected by magnetic fields, and there are also
heavier dust grains. These interstellar neutral particles make up a substantial
fraction of the material found within the heliosphere. The little black + in the
green area represents the location of Voyager 1 at 80 AU in January of 2001.
Voyager 1 is humanity's most distant object, and in 2004 the spacecraft is
evidence that it is probably beginning to encounter the termination shock
now, at a distance of just over 90 AU from the Sun.
The solar wind changes with the 11-year solar cycle,
and the interstellar medium is not homogeneous, so the shape and size of the
heliosphere probably fluctuate.
The solar magnetic field is the dominating magnetic
field within the heliosphere, except in the immediate environment of planets
which have their own magnetic fields. It can be measured by spacecraft
throughout the solar system, but not here on earth, where we are shielded by our
planet's own magnetic field.
The actual properties of the interstellar medium
(outside the heliosphere), including the strength and orientation of its
magnetic field, are important in determining the size and shape of the
heliopause. Measurements that the two Voyager spacecraft will make in the region
beyond the termination shock, and possibly beyond the heliopause, will provide
important inputs to models of the termination shock and heliopause. Even though
the Voyagers will sample these regions in discrete locations, this information
will result in more robust overall models.
For further information on this vast subject and its
many related topics, search the web for "heliosphere," "Alfven waves," "pickup
ions," and "local interstellar cloud."
The Terrestrial Planets
The planets Mercury, Venus, Earth, and Mars, are called terrestrial
because they have a compact, rocky surface like Earth's terra firma. The
terrestrial planets are the four innermost planets in the solar system. None of
the terrestrial planets have rings, although Earth does have belts of trapped
radiation, as discussed below. Only Earth has a substantial planetary magnetic
field. Mars and the Earth's Moon have localized regional magnetic fields at
different places across their surfaces, but no global field.
Of the terrestrial planets, Venus, Earth, and Mars have significant
atmospheres. The gases present in a planetary atmosphere are related to a
planet's size, mass, temperature, how the planet was formed, and whether life is
present. The temperature
of gases may cause their molecules or atoms to achieve velocities that escape
the planet's gravitational field. This contributes to Mercury's lack of a
permanent atmosphere, as does its proximity to the source of the relentless
The presence of life on Earth causes oxygen to be abundant in the atmosphere,
and in this Earth is unique in our solar system. Without life, most of the
oxygen would soon become part of the compounds on the planet's surface. Thus,
the discovery of oxygen's signature in the atmosphere of an extrasolar planet
would be significant.
lacks an atmosphere to speak of. Even though most of its surface is very hot,
there is strong evidence that water ice exists in locations near its north and
south poles which are kept permanently-shaded by crater walls. This evidence
comes from Earth-based radar observations of the innermost planet. The discovery
of permanently shaded ice at the poles of Earth's Moon strengthens arguments
that the indications of ice on Mercury may be real.
atmosphere of carbon dioxide is dense, hot, and permanently cloudy, making the
planet's surface invisible. Its best surface studies have come from landers and
imaging radar from orbiting spacecraft.
as of November 2001, is still the only place known to harbor life. And life has
flourished here since the planet was young. Our home planet is also unique in
having large oceans of surface water, an oxygen-rich atmosphere, and shifting
crustal sections floating on a hot mantle below, described by the theory of
Earth's Moon orbits the planet once every 27.3 days at an average
distance of about 384,400 km. The Moon's orbital distance is steadily increasing
at the very slow rate of 38 meters per millenium. Its distance at this point in
its history makes the Moon appear in the sky to be about the same size as the
Sun, subtending about half a degree.
atmosphere, also carbon dioxide, is much thinner than Earth's, but it sustains
wispy clouds of water vapor. Mars has polar caps of carbon dioxide ice and water
ice. The planet's surface shows strong evidence for extensive water coverage in
its distant past, as well as possible evidence for water flow in small springs
during recent times.
JPL's first spacecraft, Explorer 1, carried a single scientific
instrument devised and operated by James Van Allen and his team from the
University of Iowa. Early in 1958 the experiment discovered bands of rapidly
moving charged particles trapped by Earth's magnetic field in toroidal, or
doughnut-shaped regions surrounding the equator. The illustration below shows
these belts only in two dimensions, as if they were sliced into thin
The belts that carry Van Allen's name have two areas of maximum
density. The inner region, consisting largely of protons with an energy greater
than 30 million EV, is centered about 3,000 km above Earth's surface. The outer
belt is centered about 15,000 to 20,000 km up, and contains electrons with
energies in the hundreds of millions of EV. It also has a high flux of protons,
although of lower energies than those in the inner belt.
Flight within these belts can be dangerous to electronics and
to humans because of the destructive effects the particles have as they
penetrate microelectronic circuits or living cells. Most Earth-orbiting
spacecraft are operated high enough, or low enough, to avoid the belts. The
inner belt, however, has an annoying portion called the South Atlantic Anomaly (SAA)
which extends down into low-earth-orbital altitudes. The SAA can be expected to
cause problems with spacecraft that pass through it.
This table compares features of the terrestrial planets in
terms of the values for Earth. Light minutes are often used to express distances
within the region of the terrestrial planets, useful because they indicate the
time required for radio communication with spacecraft at their distances. If you
click on the planet's name at the top of the table, you'll see a complete set of
technical data for the planet, with a comparison to Earth.
Here is a more extensive table of planetary data.
|Mean distance from sun (AU)
|Light minutes from sun
|Mass (x Earth)
|Equatorial radius (x Earth)
|Orbit period (Earth years)
|Mean orbital velocity (km/s)
|Surface atmospheric pressure (bars)
|Global Magnetic field
Mean Distances of the Terrestrial Planets from Sun
Orbits are drawn approximately to scale.
The Jovian Planets
Jupiter, Saturn, Uranus, and Neptune are known as the Jovian
(Jupiter-like) planets, because they are all gigantic compared with Earth, and
they have a gaseous nature like Jupiter's -- mostly hydrogen, with some helium
and trace gases and ices. The Jovian planets are thus referred to as the "gas
giants" because gas is what they are mostly made of, although some or all of
them probably have small solid cores. All have significant planetary magnetic
fields, rings, and lots of satellites.
is more massive than all the other planets combined. It emits electromagnetic
energy from charged atomic particles spiraling through its strong magnetic
field. If this sizzling magnetosphere were visible to our eyes, Jupiter would
appear larger then the full Moon in Earth's sky. The trapped radiation belts
near Jupiter present a hazard to spacecraft as do Earth's Van Allen belts,
although the Jovian particle flux and distribution differ from Earth's. Bringing
a spacecraft close to Jupiter presents a hazard mostly from ionized particles.
Spacecraft intended to fly close to Jupiter must be designed with
radiation-hardened components and shielding. Spacecraft using Jupiter for
gravity assist may also be exposed to a harsh radiation dose. Instruments not
intended to operate at Jupiter must be protected by being powered off and by
having detectors covered.
the farthest planet easily visible to the unaided eye, is known for its
extensive, complex system of rings, which are very impressive even in a small
telescope. Using a small telescope one can also discern the planet's oblateness,
or flattening at the poles. Continued study of Saturn's ring system can yield
new understandings of orbital dynamics, applicable to any system of orbiting
bodies, from newly forming solar systems to galaxies.
which rotates on its side, and
are of similar size and color, although Neptune seems to have a more active
atmosphere despite its much greater distance from the sun. Both planets are
composed primarily of rock and various ices. Their extensive atmosphere, which
makes up about 15% the mass of each planet, is hydrogen with a little helium.
Neither of these planets were known to the ancients; Uranus was discovered in
1781, Neptune in 1846.
is neither a rocky terrestrial planet nor a Jovian gas giant. It is a Kuiper
Belt Object, composed of material left over after the formation of the other
Comets in the next section). Kuiper Belt Objects were never exposed to the
higher temperatures and solar radiation levels of the inner solar system. Pluto
has large quantities of nitrogen ice, and simple molecules of carbon, hydrogen
and oxygen. They remain on Pluto as a sample of the primordial material that set
the stage for the evolution of the solar system as it exists today, including
life. Its nitrogen atmosphere will precipitate out onto the surface as snow when
its orbit takes it much farther from the sun than it is today. Since it has not
yet been visited by a spacecraft, comparatively little is
known about this small, distant body. Objects that orbit the Sun beyond
Neptune's orbit are known as trans-Neptunian objects.
The gas giants have numerous satellites, many of which are
large, and seem as interesting as any planet. Small "new" satellites of the
Jovian planets are being discovered every few years.
Jupiter's Galilean satellites, so named because Galileo
Galilei discovered them in 1610, exhibit great diversity from each other. All
four can be easily seen in a small telescope or binoculars.
(pictured here) is the closest of these to Jupiter. Io is the most volcanically
active body in
the solar system, due to heat resulting from tidal forces (discussed further in
Chapter 3) which flex its crust. Powerful Earth-based telescopes can observe
volcanoes resurfacing Io continuously. Europa is covered with an extremely
smooth shell of water ice. There is probably an ocean of liquid water below the
shell, warmed by the same forces that heat Io's volcanoes. Ganymede has
mountains, valleys, craters, and cooled lava flows. Its ancient surface
resembles Earth's Moon, and it is also suspected of having a sub-surface ocean.
Callisto, the outermost Galilean moon, is pocked all over with impact craters,
indicating that its surface has changed little since the early days of its
Saturn's largest moon, enigmatic Titan, is larger than the planet Mercury.
Almost a terrestrial planet itself, Titan has a hazy nitrogen atmosphere denser
than Earth's. The
Huygens Probe executed a spectacurly successful mission in January 2005,
revealing rivers and lakebeds on the surface, and extensive details of its
atmosphere. Saturn also has many
satellites made largely of
TITAN, FALSE COLOR
IMAGED BY CASSINI.
CLICK FOR MORE INFO.
water ice. The "front," or leading, side of Saturn's icy satellite
is covered in dark material of some kind, and an equatorial mountain range
higher than Olympus Mons on Mars was recently discovered on this 1450-km
diameter moon. Icy Enceladus orbits within the densest part of Saturn's E Ring,
and may somehow be the source of that ring's fine ice-particle makeup.
All of Uranus's five largest moons have extremely different characteristics.
The surface of Miranda, the smallest of these, shows evidence of extensive
geologic activity. Umbriel's surface is dark, Titania and Ariel have trenches
and faults, and Oberon's impact craters show bright rays similar to those on
Neptune's largest moon Triton is partly covered with nitrogen ice and snow,
and has currently active nitrogen geysers that leave sooty deposits on the
IN FALSE COLOR
dust rings can be detected at close range in visible light and from Earth in
the infrared. They show up best when viewed from behind, in forward scattered
Neptune all have rings made up of myriad particles of ice ranging in size
from dust and sand to boulders. Each particle in a ring is an individual
satellite of the planet in its own right. Ring particles interact with each
other in complex ways, affected by gravity and electrical charge. They also
interact with the thin extended atmospheres of the planets.
magnificent ring system, as visible from Earth, spans about 280,000 km, yet its
thickness is only around 200 meters! The A-ring, measured at several points, was
found to be only ten meters thick.
When two satellites occupy orbits very close to each other within a ring
system, one orbiting farther from the planet than a ring, and the other one
orbiting closer to the planet than that ring, they confine particles between
their orbits into a narrow ring, by gravitationally interacting with the ring
particles. Thus these satellites are called shepherd moons.
Jovian Planetary Data (Approximate)
|Mean distance from sun (AU)
|Light hours from sun
|Mass (x Earth)
|Radius (x Earth)
|Rotation period (hours)
|Orbit period (Earth years)
|Mean orbital velocity (km/s)
|Known natural satellites (2004)
||Broken ring arcs
Orbits are drawn approximately to scale.
Pluto omitted to accommodate scale.
Mercury and Venus are referred to as inferior
planets, not because they are any less important, but because their orbits are
closer to the sun than is Earth's orbit. They always appear close to the sun in
Earth's morning or evening sky; their apparent angle from the sun is called
elongation. The outer planets, Mars, Jupiter, Saturn, Uranus, Neptune, and
Pluto are all known as superior planets because their orbits are farther
from the sun than the Earth's.
|Planet or Moon appears in crescent phase when nearly
between observer and sun.
Phases of Illumination
Inferior planets may pass between the Earth and the sun on part of their
orbits, so they can exhibit nearly the complete range of phases from the
earth's point of view... from the dark "new" phase, to slim "crescent" phase, to
the mostly lit "gibbous" phase (approximating the fully illuminated "full" phase
when approaching the other side of the sun). Our own Moon, of course, exhibits
all the phases. Superior planets, though, usually appear gibbous, and appear
full only when at opposition (see below), from our earthly point of view.
Viewed from superior planets, Earth goes through phases. Superior planets can
be seen as crescents only from the vantage point of a spacecraft that is beyond
Conjunction, Transit, Occultation, Opposition
When two bodies appear to pass closest together in the sky, they are said to
be in conjunction. When a planet passes closest to the sun as seen from
Earth and all three bodies are approximately in a straight line, the planet is
said to be in solar conjunction. The inferior planets Venus and Mercury can have
two kinds of conjunctions with the Sun: (1) An inferior conjunction, when
the planet passes approximately between Earth and Sun (if it passes exactly
between them, moving across the Sun's face as seen from Earth, it is said to be
in transit); and (2) A superior conjunction when Earth and the
other planet are on opposite sides of the Sun and all three bodies are again
nearly in a straight line. If a planet disappears behind the sun because the sun
is exactly between the planets, it is said to be in occultation.
Superior planets can have only superior conjunctions with the sun. At
superior conjunction the outer planet appears near its completely illuminated
The planet is said to be at opposition to the sun when both it and the
Earth are on the same side of the sun, all three in line. (The Moon, when full,
is in opposition to the sun; the Earth is then approximately between them.)
Opposition is a good time to observe an outer planet with Earth-based
instruments, because it is at its nearest point to the Earth and it is in its
Inferior planets can never be at opposition to the sun, from Earth's point of
Occultations, transits, conjunctions, and oppositions offer special
opportunities for scientific observations by spacecraft. Studies of the solar
corona and tests of general relativity can be done at superior conjunctions.
Superior conjunctions also present challenges communicating with a spacecraft
nearly behind the sun, which is overwhelmingly noisy at the same radio
frequencies as those used for communications. At opposition, such radio noise is
at a minimum, presenting ideal conditions for gravitational wave searches. These
special opportunities and challenges are further discussed in later chapters.
Chapter 5. Planetary Orbits
- Upon completion of this chapter you will be able to
describe in general terms the characteristics of various types of
planetary orbits. You will be able to describe the general concepts
and advantages of geosynchronous orbits, polar orbits, walking orbits,
sun-synchronous orbits, and some requirements for achieving them.
Orbital Parameters and Elements
The terms orbital period, periapsis, and apoapsis were introduced in
The direction a spacecraft or other body travels in orbit can be direct,
or prograde, in which the spacecraft moves in the same direction
as the planet rotates, or retrograde, going in a direction
opposite the planet's rotation.
True anomaly is a term used to describe the locations of
various points in an orbit. It is the angular distance of a point in an
orbit past the point of periapsis, measured in degrees. For example, a
spacecraft might cross a planet's equator at 10° true anomaly. Nodes
are points where an orbit crosses a plane. As an orbiting body crosses
the ecliptic plane going north, the node is referred to as the
ascending node; going south, it is the descending node.
To completely describe an orbit mathematically, six quantities must
be calculated. These quantities are called orbital elements, or
Keplerian elements, after
Johannes Kepler (1571-1630). They are:
- Semi-major axis and
- Eccentricity, which together are the basic measurements of
the size and shape of the orbit's ellipse (described in Chapter 3.
Recall an eccentricity of zero indicates a circular orbit).
- Inclination is the angular distance of the orbital plane
from the plane of the planet's equator (or from the ecliptic plane, if
you're talking about heliocentric orbits), stated in degrees. An
inclination of 0 degrees means the spacecraft orbits the planet at its
equator, and in the same direction as the planet rotates. An
inclination of 90 degrees indicates a polar orbit, in which the
spacecraft passes over the north and south poles of the planet. An
inclination of 180 degrees indicates a retrograde equatorial orbit.
- Argument of periapsis is the argument (angular distance) of
the periapsis from the ascending node.
- Time of periapsis passage and
- Celestial longitude of the ascending node are the remaining
The orbital period is of interest to operations, although it is not
one of the six Keplerian elements needed to define the orbit.
Generally, three astronomical or radiometric observations of an
object in an orbit are enough to pin down all of the above six Keplerian
elements. The following table gives a sense of the level of precision an
interplanetary mission commonly deals with. These elements are measured
during routine tracking by the Deep Space Network.
Elements of Magellan's Initial Venus Orbit
10 August 1990
||Argument of Periapsis:
||DOY 222 19:54 ERT
||Longitude of Ascending Node:
||( Orbit Period:
||3.26375 hr )
Types of Orbits
A geosychronous orbit (GEO) is a prograde, circular, low inclination
orbit about Earth having a period of 23 hours 56 minutes 4 seconds. A
spacecraft in geosynchronous orbit appears to remain above Earth at a
constant longitude, although it may seem to wander north and south.
CLICK IMAGE TO
START / STOP ANIMATION
To achieve a geostationary orbit, a geosychronous orbit is chosen
with an inclination of either zero, right on the equator, or else low
enough that the spacecraft can use propulsive means to constrain the
spacecraft's apparent position so it hangs motionless above a point on
Earth. (Any such maneuvering on orbit is a process called station
keeping.) The orbit can then be called geostationary. This orbit is
ideal for certain kinds of communication satellites or meteorological
To attain geosynchronous (and also geostationary) Earth
orbits, a spacecraft is first launched into an elliptical orbit with an
apoapsis altitude in the neighborhood of 37,000 km. This is called a
Geosynchronous Transfer Orbit (GTO). The spacecraft then
circularizes the orbit by turning parallel to the equator at apoapsis
and firing its rocket engine. That engine is usually called an apogee
motor. It is common to compare various
launch vehicles' capabilities according to the amount of mass they
can lift to GTO.
Polar orbits are 90 degree inclination orbits, useful for spacecraft
that carry out mapping or surveillance operations. Since the orbital
plane is nominally fixed in inertial space, the planet rotates below a
polar orbit, allowing the spacecraft low-altitude access to virtually
every point on the surface. The Magellan spacecraft used a nearly-polar
orbit at Venus. Each periapsis pass, a swath of mapping data was taken,
and the planet rotated so that swaths from consecutive orbits were
adjacent to each other. When the planet rotated once, all 360 degrees
longitude had been exposed to Magellan's surveillance.
To achieve a polar orbit at Earth requires more energy, thus more
propellant, than does a direct orbit of low inclination. To achieve the
latter, launch is normally accomplished near the equator, where the
rotational speed of the surface contributes a significant part of the
final speed required for orbit. A polar orbit will not be able to take
advantage of the "free ride" provided by Earth's rotation, and thus the
launch vehicle must provide all of the energy for attaining orbital
Planets are not perfectly spherical, and they do not have evenly
distributed surface mass. Also, they do not exist in a gravity "vacuum."
Other bodies such as the sun, or natural satellites, contribute their
gravitational influences to a spacecraft in orbit about a planet. It is
possible to choose the parameters of a spacecraft's orbit to take
advantage of some or all of these gravitational influences to induce
precession, which causes a useful motion of the orbital plane. The
result is called a walking orbit or a precessing orbit, since the
orbital plane moves slowly with respect to fixed inertial space.
A walking orbit whose parameters are chosen such that
the orbital plane precesses with nearly the same period as the planet's
solar orbit period is called a sun synchronous orbit. In such an orbit,
the spacecraft crosses periapsis at about the same local time every
orbit. This can be useful if instruments on board depend on a certain
angle of solar illumination on the surface. Mars Global Surveyor's orbit
is a 2-pm Mars Local Time sun-synchronous orbit, chosen to permit
well-placed shadows for best viewing.
It may not be possible to rely on use of the gravity
field alone to exactly maintain a desired synchronous timing, and
occasional propulsive maneuvers may be necessary to adjust the orbit.
This remarkable image of a Martian aquifer was
obtained by the Mars Global Surveyor spacecraft from its sun-synchronous
Martian orbit in January 2000. The view is to the north. Click the image
for more details.
Louis Lagrange (1736-1813) showed that three bodies can occupy positions
at the apexes of an equilateral triangle that rotates in its plane.
Consider a system with two large bodies being the Earth orbiting the sun
(or the Moon orbiting the Earth). The third body, such as a spacecraft
or an asteroid, might occupy any of five Lagrange points:
In line with the two large bodies are the L1, L2 and
L3 points. The leading apex of the triangle is L4; the trailing apex is
L5. These last two are also called Trojan points.
Select the "Links" section below for additional
references, including mathematical tutorials and example problems.
Solar System Temperature Reference
||Lowest achieved in a
Cosmic background microwave radiation
Liquid helium boils
||Solid hydrogen melts
||Liquid hydrogen boils
||Solid nitrogen melts
Neptune 1-bar level
Uranus 1-bar level
||Liquid nitrogen boils
||Liquid oxygen boils
Mercury surface, night
Saturn 1-bar level
Mars surface, night low
Jupiter 1-bar level
||Carbon dioxide freezes ("dry ice")
||Water ice melts
Mars surface, day high
||Standard room temperature
||Liquid water boils
Mercury surface, day
Uranus hydrogen "corona"
||Solid gold melts
Betelgeuse (red giant star) photosphere
||Sirius (blue-white star) photosphere
|Melting and boiling
points are shown to precision, for pressure of 1 atmosphere. Values for
stars, planet cloudtops, surfaces etc. are shown as round numbers rather
than precise conversions.
Basically all of space flight
involves the following concept, whether orbiting a planet or travelling
among the planets while orbiting the Sun.
As you watch the third cartoon's
animation, imagine that the cannon has been packed with still more
gunpowder, sending the cannonball out a little faster. With this extra
energy, the cannonball would miss Earth's surface at periapsis by a greater
Right. By applying more energy at
apoapsis, you have raised the periapsis altitude.
|A spacecraft's periapsis
altitude can be raised by increasing the spacecraft's energy at apoapsis.
This can be accomplished by firing on-board rocket thrusters when at
And of course, as seen in these
cartoons, the opposite is true: if you decrease energy when you're at
apoapsis, you'll lower the periapsis altitude. In the cartoon, that's
less gunpowder, where the middle graphic shows periapsis low enough to
impact the surface. In the next chapter you'll see how this key enables
flight from one planet to another.
Now suppose you increase speed when
you're at periapsis, by firing an onboard rocket. What would happen to the
cannonball in the third cartoon?
Just as you suspect, it will cause
the apoapsis altitude to increase. The cannonball would climb to a higher
altitude and clear that annoying mountain at apoapsis.
|A spacecraft's apoapsis
altitude can be raised by increasing the spacecraft's energy at
periapsis. This can be accomplished by firing on-board rocket
thrusters when at periapsis.
And its opposite is true, too:
decreasing energy at periapsis will lower the apoapsis altitude. Imagine
the cannonball skimming through the tops of some trees as it flys through
periapsis. This slowing effect would rob energy from the cannonball, and it
could not continue to climb to quite as high an apoapsis altitude as before.
In practice, you can remove
energy from a spacecraft's orbit at periapsis by firing the onboard rocket
thrusters there and using up more propellant, or by intentionally and
carefully dipping into the planet's atmosphere to use frictional drag. The
latter is called
aerobraking, a technique used at Venus and at Mars that conserves rocket
Chapter 6. Electromagnetic Phenomena
- Upon completion of this chapter you will be able to
describe in general terms characteristics of natural and artificial
emitters of radiation. You will be able to describe bands of the spectrum
from RF to gamma rays, and the particular usefulness radio frequencies
have for deep-space communication. You will be able to describe the basic
principles of spectroscopy, Doppler effect, reflection and refraction.
Electromagnetic radiation (radio waves, light, etc.) consists of
interacting, self-sustaining electric and magnetic fields that propagate
through empty space at 299,792 km per second (the
speed of light,
and slightly slower through air and other media. Thermonuclear reactions in
the cores of stars (including the sun) provide the energy that eventually
leaves stars, primarily in the form of electromagnetic radiation. These
waves cover a wide spectrum of frequencies. Sunshine is a familiar example
of electromagnetic radiation that is naturally emitted by the sun. Starlight
is the same thing from "suns" much farther away.
When a direct current (DC) of electricity, for example
from a flashlight battery, is applied to a wire or other conductor, the
current flow builds an electromagnetic field around the wire, propagating a
wave outward. When the current is removed the field collapses, again
propagating a wave. If the current is applied and removed repeatedly over a
period of time, or if the electrical current is made to alternate its
polarity with a uniform period of time, a series of waves is propagated at a
discrete frequency. This phenomenon is the basis of electromagnetic
Electromagnetic radiation normally propagates in
straight lines at the speed of light and does not require a medium for
transmission. It slows as it passes through a medium such as air, water,
Electromagnetic energy decreases as if it were
dispersed over the area on an expanding sphere, expressed as 4πR2
where radius R is the distance the energy has travelled. The amount of
energy received at a point on that sphere diminishes as 1/R2.
This relationship is known as the inverse-square law of
(electromagnetic) propagation. It accounts for loss of signal strength over
space, called space loss.
The inverse-square law is significant to the
exploration of the universe, because it means that the concentration of
electromagnetic radiation decreases very rapidly with increasing distance
from the emitter. Whether the emitter is a distant
spacecraft with a low-power transmitter or an extremely powerful star, it
will deliver only a small amount of electromagnetic energy to a detector on
Earth because of the very great distances and the small area that Earth
subtends on the huge imaginary sphere.
Chapter 6. Electromagnetic Phenomena CONTINUED
Light is electromagnetic radiation (or
electromagnetic force) at frequencies that can be sensed by the human
eye. The whole electromagnetic
spectrum, though, has a much broader range of frequencies than the human
eye can detect, including, in order of increasing frequency: audio
frequency (AF), radio frequency (RF), infrared (meaning
"below red," IR), visible light, ultraviolet (meaning "above
violet," UV), X-rays, and finally gamma rays. These
designations describe only different frequencies of the same phenomenon:
All electromagnetic waves propagate at the speed of light. The
wavelength of a single oscillation of electromagnetic radiation means
the distance the wave will propagate in vacuo during the time
required for one oscillation.
The strength, or "loudness" or intensity of the wave is known as its
amplitude. For wavelengths up through radio frequencies, this quantity
is commonly expressed as a power ratio in
Frequency is expressed in Hertz (Hz), which represents cycles per second.
There is a simple relationship
between the frequency of oscillation and wavelength of electromagnetic
energy. Wavelength, represented by the Greek lower case lambda (λ), is equal
to the speed of light (c) divided by frequency (f).
Electromagnetic energy of all frequencies or
energies can be viewed in physics as if it were waves, as described above,
and also as particles, known as photons. It is generally common to
speak of waves when talking about lower frequencies and longer wavelengths,
such as radio waves. Reference to photons is common for physicists talking
about light and electromagnetic force of higher frequencies (or energies).
Waves are described in terms of frequency, wavelength, and amplitude.
Photons, seen as particle carriers of the electromagnetic force, are
described in terms of energy level using the
electron Volt (eV).
Throughout this document the preferred treatment will be waves, which is
arguably a more informative approach.
Natural and Artificial Emitters
Radio Image of Jupiter
Click image for details.
Deep space communication antennas and receivers are capable of detecting
many different kinds of natural emitters of electromagnetic radiation,
including the stars, the sun, molecular clouds, and gas giant planets such
as Jupiter. These sources do not emit at truly random frequencies, but
without sophisticated scientific investigation and research, their signals
appear as noise -- that is, signals of pseudo-random frequencies and
Radio Astronomy is the scientific discipline which investigates natural
emitters by acquiring and studying their electromagnetic radiation. The Deep
Space Network participates in radio astronomy experiments.
Deep space vehicles are equipped with radio transmitters ("artificial
emitters") and receivers for sending and receiving signals (electromagnetic
radiation) to and from Earth-based tracking stations. These signals utilize
pre-established discrete frequencies. On the other hand, various natural and
human-made emitters combine to create a background of electromagnetic noise
from which the spacecraft signals must be detected. The ratio of the signal
level to the noise level is known as the signal-to-noise ratio (SNR).
SNR is commonly expressed in decibels.
We know from Einstein's special theory of relativity that mass, time, and
length are variable and the speed of light is constant. And from general
relativity, we know that gravitation and acceleration are equivalent, that
light bends in the presence of mass, and that an accelerating mass radiates
gravitational waves at the speed of light.
Spacecraft operate at very high velocities compared
to velocities we are familiar with in transportation and ballistics here on
our planet. Since spacecraft velocities do not approach a significant
fraction of the speed of light, Newtonian physics serves well for operating
and navigating throughout the solar system. Nevertheless, accuracies are
routinely enhanced by accounting for tiny relativistic effects. Once we
begin to travel between the stars, velocities may be large enough fractions
of light speed that Einsteinian physics will be indespensible for
For now, spacecraft do sometimes carry out
experiments to test special relativity effects on
moving clocks, and experiments to test general relativity effects such
space-time warp caused by the sun,
equivalence of acceleration and gravitation (more precisely the
equivalence between inertial mass and gravitational mass) and the search for
direct evidence of
gravitational waves. As of July 2004 there has been no test by which an
observer could tell
acceleration from gravitation, nor has gravitational radiation been
directly observed. Some of these subjects are explored in Chapter 8.