Liftoff to Learning: Tethered Satellites
|Video Title: Tethered Satellites
Part 1 - Tethered Satellite
Forces and Motion
Part 2 - Electrical Circuits
in Space: The Electrodynamics of the Tethered Satellite
Video Length: Part 1 - 21:11
Part 2 - 18:50
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Part 1 describes the tethered satellite concept and shows how the
satellite is deployed and extended in space. The mathematics describing
the forces acting on the tethered satellite/Space Shuttle orbiter
system is presented.
Part 2 demonstrates how the tethered satellite and the Space Shuttle
orbiter interact with Earth's magnetic field to produce an electric
current. Future applications of the tethered satellite/Space Shuttle
orbiter system as a motor are described.
Subjects: Part 1 - Force, motion, and gravity
Part 2 - Electricity and magnetism
- Position and motion of objects
- Properties of objects and materials
Unifying Concepts and Processes
- Change, constancy, and measurement
- Evidence, models, and exploration
Science Process Skills:
Computation and Estimation
Table of Contents
In 1992, and again in 1996, NASA and the Italian Space Agency (ASI) tested
a concept for a tethered satellite. Deployed from the payload bays of
the Space Shuttles Atlantis (STS-46) and Columbia (STS-75),
the spherical 1.6-meter diameter satellite was reeled out into space.
Its tether, a 21-kilometer-long leash, consisted of a core of Nomex wrapped
with copper wire and covered with a protective sheath of Kevlar and additional
Nomex. For deployment, a 12-meter boom elevated the satellite out of the
payload bay so the satellite would not hit the orbiter. Small thrusters
on the satellite gave it its initial push into space. Once moving, forces
on the orbiter-tethered satellite system kept the satellite and orbiter
moving apart from each other.
Tethered Satellite Mission Objectives
The primary scientific and engineering objectives of tethered satellite
missions were to deploy, stabilize and retrieve a tethered satellite in
space and operate it as an electrically conducting system within Earth's
magnetic field. Manipulating a satellite on a tether from the orbiter
turned out to be a unique engineering challenge. Because gravity, centrifugal
acceleration and atmospheric drag vary with altitude, each of the two
bodies in a tethered system, one orbiting above the other, is subject
to different magnitudes of influence.
On both missions significant technical problems occurred. On STS-46, the
tethered satellite jammed twice on the reel while the satellite was 179
and, later, 256 meters away from the orbiter. The satellite could not
be deployed any further. On STS-75, the satellite was extended 19.7 kilometers
when the tether broke near the boom. The satellite and tether drifted
away safely from the orbiter and were not retrievable. Up to the time
of the severing of the tether, the orbiter-tethered satellite system had
been generating 3,500 volts and up to 0.5 amps of current.
Later investigation of the problems that occurred on the two missions
determined that the tether on the first flight was snagged by the mechanism
that unreeled it from the orbiter. The break of the tether on the second
mission occurred because of either a flaw in the insulation or the penetration
of a foreign object into the insulation causing an electrical arc from
the tether to a nearby ground. The arcing severed the tether.
In spite of these problems, each major objective was met. The flights
demonstrated that the tethered satellite would deploy, and the forces
acting on the satellite and the orbiter worked to keep the satellite extended
from the orbiter without additional thruster firing. Both missions also
demonstrated that tethered satellites will generate an electric current
as they pass through Earth's magnetic field.
Part 1 of this video concentrates on the forces and motions acting on
the tethered satellite and Space Shuttle orbiter. Orbital scenes were
taken on the STS-46 mission which launched on July 31, 1992. Part 2 of
this video concentrates on the electrodynamics of the tether in space.
|Mathematical Equations Illustrated
in the Video
To explain the dynamics of the tethered satellite system, the following
equations appear in Part 1:
Newton's Second Law of Motion
Force equals mass times acceleration.
Universal Law of Gravitation
For two masses, the attractive force of gravity between them is proportional
to the product of their masses and inversely proportional to the square
of the distance between their centers of mass.
During the video, students are challenged to verify the relationship between
the distance the tethered satellite moves during deployment and the distance
the Space Shuttle moves. The following data were provided:
Mass of the Space Shuttle -100,000 kg
Mass of the tethered satellite - 500 kg
Distance the Space Shuttle moves - 100 m
Distance the tethered satellite moves - 20,000 m
500 kg x 20,000 m = 100,000 kg x 100 m
To explain the electrical nature of the tethered satellite system, the
following equations appear in Part 2:
i = v/r
This law states that the amount of current (i) in an electrical circuit
is directly proportional to the voltage (v) of the circuit and inversely
proportional to the resistance (r) of the circuit. Two variations of the
equation above are used in the video. The equations and the values used
in the video are given below.
An electron passing through a magnetic field experiences a force that
is perpendicular to both the direction of motion and Earth's magnetic
BxVxL = Voltage
Amperage - This is the measure of the amount of electrical current
flowing through a circuit. The unit of measurement is the ampere (one
coulomb of charge per second).
Angular momentum - The product of an object's rotational velocity
and its rotational inertia about an axis.
Center of Mass - A single point about which the mass of an
object is considered to be concentrated.
Circuit - A complete pathway along which an electrical current
Conductor - A material through which an electrical current
Conservation of Angular Momentum - As long as no external torques
are exerted, the angular momentum of an object remains constant.
Conservation of Energy - Energy cannot be created or destroyed;
it may be transformed from one form into another, but the total
amount of energy never changes.
Coriolis Effect - The deflection of a moving object into
a curved path due to Earth's rotation.
Current - A flow of an electric charge between two points
in which there is a difference in potential.
Equilibrium - The state of an object when not acted upon
by a net force or net torque.
Force - a push or pull that causes an object to accelerate.
Gravity - The attraction of objects to one another due to
Gravity Gradient Force - Differences in the force of gravity
felt in various parts of a system due to varying distances from
the center of the Earth.
Inertia - A property of matter causing it to resist changes
Inverse Square Force (law) - A law relating the intensity
of an effect to the inverse square of the distance from the cause.
Ion - An atom that has an electrical charge due to the loss
or gain of electrons.
Ionosphere - The upper region of Earth's atmosphere extending
from about 85 to 1,000 kilometers. This region is also called the
Lorenz Force - Electrons moving through a magnetic field
experience a force that is perpendicular to both the direction of
motion and the magnetic force lines.
Magnetic Field - The force field that surrounds every magnet
and electrical current-carrying conductor.
Newton - A unit of force: 1 Newton accelerates a mass of 1 kilogram
1 meter per second per second.
Ohm - The unit of measurement for electrical resistance in a
circuit. The resistance of a device that draws a current of one
ampere when one volt is impressed across the circuit.
Potential - The electric potential energy at a point within
an electrical circuit.
Rendezvous - In spaceflight, the close approach of two spacecraft
traveling in the same orbit.
Resistance - A property of a conductor that causes it to resist
the flow of electricity. The unit of measurement is the ohm.
Resonance - Phenomenon where energy is transferred to an
object at its natural vibration frequency by a second object or
wave vibrating at that same frequency.
Restoring Force - A force that returns equilibrium to a system.
Tesla - The unit of measure for a magnetic field (Webber/m2).
Tethered Satellite - A satellite attached to another space vehicle
by means of some sort of cord.
Torque - A product of force and lever-arm distance, which
tends to produce rotation in an object.
Voltage - The measure of the electric potential difference
in a circuit. The electric potential at which one coulomb of charge
would have one joule of potential energy.
The following hands-on activities demonstrate some of the concepts presented
in these two programs.
Conserving Angular Momentum
Rotating stool or rotating platform
Two exercise hand weights (1 to 2 kg each)
The Space Shuttle orbiter/tethered satellite system operates under the
law of the conservation of angular momentum as it orbits the Earth. Angular
momentum is a product of the rotational inertia of an object and its rotational
speed. The system can be compared to a spinning ice skater. When the skater
tucks his or her arms in tightly, rotational speed increases while rotational
inertia decreases. Discounting frictional effects, the skater's angular
momentum is conserved. When the skater's arms are extended, rotational
speed decreases while rotational inertia increases. Again, angular momentum
is conserved. Like a skater extending arms, when the tethered satellite
is extended above the orbiter, its distribution of mass is changed. The
rotational inertia of the system is conserved by decreasing its rotational
speed while increasing its rotational inertia. The reduction of rotation
speed actually lowers the orbiter in its orbit. However, when the tethered
satellite is retrieved, rotational speed increases as rotational inertia
decreases. Because angular momentum is again conserved, the orbiter actually
raises its altitude. The following activity permits students to experience
the conservation of angular momentum.
- Place the rotating stool or platform in the middle of a clear
area at least 2-3 m across.
- Have a student sit on the stool or stand on the platform.
- Give the student the two hand weights and ask the student to
extend his or her arms.
- Gently start the student spinning while standing nearby to help
the student maintain balance.
- On command, the student should move the weights to his or her
chest. What happens to the student's rotation rate? Is the student
- Once the student becomes accustomed to balancing on the stool
or platform, the rotation rate can be increased slightly to dramatize
Discussion and Extensions
|Besides figure skaters, can you think of other examples
of conservation of angular momentum?
How could a tethered satellite be used to alter spacecraft orbits
without the expenditure of rocket propellant?
Additional information on the conservation of angular momentum can
be found in any physics textbook.
Moment of Force Apparatus or the items listed below:
10 large metal washers
Small triangular block or other object to serve as a fulcrum
When a Space Shuttle orbiter deploys a tethered satellite, the center of
mass of the two bodies remains in a constant orbit. What changes is the
respective distance of the two bodies from that center of mass. Because
it contains far less mass than the orbiter, the tethered satellite travels
a great distance in one direction while the orbiter moves a short distance
the opposite way. The orbiter has a mass of about 100,000 kg and the tethered
satellite has a mass of 500 kilograms. When the tethered satellite is deployed
to a distance of 20 kilometers, the orbiter moves about 100 meters in the
opposite direction. The center of mass of these two bodies remains constant.
(Follow the instructions that come with the Moment of Force Apparatus or
use these instructions with the alternative apparatus)
|Place the meter stick on the fulcrum so that it balances.
Divide the washers into two equal piles and place them on opposite
sides of the fulcrum. Adjust the positions of the piles to bring the
stick into balance by moving them closer or farther from the fulcrum.
How far is each pile from the fulcrum?
Divide the washers into two piles of two and eight washers respectively.
Place them on the meter stick and adjust their positions until the
stick is in balance. How far is each pile from the fulcrum?
Is there a mathematical relationship between the masses of the two
piles and their distances from the fulcrum? How does this activity
relate to the deployment of tethered satellites?
Discussion and Extensions
||How far would a Space Shuttle orbiter move if a tethered
satellite with a mass of 2000 kilograms were deployed to a distance
of 10 kilometers?
The oscillations that can occur with a tethered satellite system can
be reproduced with a ball attached to an elastic cord. The tether
may compress and stretch, causing the satellite to bounce up and down
(longitudinal oscillation). The satellite and tether may move in a
circular (skip rope) fashion. The satellite can remain fairly still,
but the tether can oscillate (transverse).
Each type of oscillation occurs with a particular frequency, which
in turn depends upon the length and tension of the tether. When frequencies
are different, the motions do not interact. However, at some tether
lengths, the frequencies of two or more oscillations can become very
close. Energy can be transferred from one type of oscillation to another
in a phenomenon known as resonance.
Many different factors, such as control motions of the Space Shuttle,
can trigger oscillations. One of the tethered satellite experiments
is to use the interaction of the tether with Earth's magnetic field
to generate an electric current. When a current is produced, another
magnetic field is created. The two fields may interact, (if the current
is pulsed at the natural frequency) causing the tether to skip rope.
|Attach an elastic cord to a solid ball.
Suspend the ball by holding the opposite end of the elastic. Create
the following oscillations:
Longitudinal (bounce ball up and down by raising and lowering your
Transverse (with the ball hanging still, move your hand from side
Skip rope (with the ball hanging still, move your hand in a circle)
Discussion and Extensions
|Why is it important for scientists to study possible
oscillations of tethered satellite systems?
Could there be other kinds of oscillations that might affect the tethered
How can oscillations affect structures on Earth?
Sheet of white paper or overhead projector transparency paper
Plastic sandwich bag
Overhead projector (optional)
The Space Shuttle orbiter/tethered satellite system makes use of Earth's
magnetic field and its electrically charged ionosphere to produce
a current through the tether. The way the current is produced will
be discussed in the next activity.
All magnetic objects produce invisible lines of force that extend
between the poles of the object. This phenomenon is visualized with
iron filings sprinkled around a bar magnet. In very simple terms,
Earth can be thought of as a dipole (2 pole) magnet. Magnetic force
lines radiate between Earth's north and south magnetic poles. Electrically
charged particles become trapped on those field lines just as iron
filings become trapped on the force lines of the bar magnet. The particles
are able to move along the force lines, and when they contact thin
gases near Earth's polar regions, trigger auroral displays. Unlike
the symmetrical field of the bar magnet, Earth's magnetic field is
asymmetrical. On the sunlit side of Earth, the pressure of the solar
wind (streams of electrically charged particles ejected from the Sun)
compresses the magnetic force lines, while on the far side, the lines
are stretched out.
- Lay a bar magnet on a table top and cover it with a sheet of
- Carefully sprinkle iron filings on the paper to delineate the
magnetic force lines.
- Make a sketch of the patterns of the magnetic filings.
- Optional. Lay the magnet on the stage of an overhead projector
and cover the magnet with a sheet of transparency paper. Sprinkle
the filings over the magnet and project the patterns on the screen.
- Return the iron filings to the storage container by shaping
the paper into a funnel. Place a bar magnet inside a sandwich
bag and sweep the area around the magnet for filings that escaped.
Turn the bag inside out and pull away the magnet.
Discussion and Extensions
- What creates the magnetic field of the Earth?
- Do other bodies in our solar system have magnetic fields?
- Sprinkle iron filings around other kinds of magnets, such as
ring magnets, to observe their magnetic fields.
- Make a permanent magnetic field indicator by placing iron filings
between two transparency sheets or sheets of scrap laminating
film and hot gluing the edges of the sheets together.
As the Space Shuttle orbiter/tethered satellite system orbits the
Earth, the tether rapidly cuts across the Earth's magnetic force lines.
The interaction creates an electric current that travels through the
conductor of the tether. The effect is analogous to the way power
is generated by an automobile alternator. Free electrons in the thin
ionosphere where the Space Shuttle operates are attracted to the satellite.
The electrons travel along the tether to the orbiter. However, in
order for the current (a flow of charged particles) to be produced,
a complete circuit must be formed. This is accomplished by using an
electron generator on the orbiter to return charged particles back
into the ionosphere.
|Connect the wire to the terminals of the volt/ohm meter.
Set the meter to a low voltage range.
Quickly move the wire through the magnet's magnetic field. Observe
the meter's display to see if a current is produced.
Move the wire at different speeds through the magnetic field and observe
the amount of voltage produced
Discussion and Extensions
|What is the relationship between the speed of the moving
wire and the voltage produced?
How do traditional electric generators work?
Commander STS-46: Loren J. Shriver (Col., USAF).
Commander STS-75: Pilot STS-46 Andrew M. Allen (Lt. Colonel,
Mission Specialist STS-46 and STS-75: Claude Nicollier (ESA).
Mission Specialist STS-46: Marsha S. Ivins.
Mission Specialist STS-46 and STS-75: Jeffrey A. Hoffman,
Mission Specialist STS-46: Payload Commander STS-75: Franklin R. Chang-Diaz,
Payload Specialist STS-46 (ASI): Franco Malerba, Ph.D.
Pilot STS-75: Scott J. Horowitz (LTC,
Mission Specialist STS-75: Maurizio Cheli (ESA)
Payload Specialist STS-75: Umberto Guidoni (ASI)
To obtain biographic information, click on highlighted names