Topic outline

  • General

    Satellites-  Understanding How They Work  !


    Written By:  Frances Dellutri, Middle School/Intermediate Level EIS Education Team, April 2016

    EIS Topic: Atmosphere, Centripetal Force, Computers, Mathematics, Micro-gravity, Free-fall, Orbital Mechanics, Physics, Satellites, Spacecraft, Weightlessness

    Grade (Age) Level:  Grades 5-8 (Ages 10-13)

    Key Topics Associated with Standards: Collecting, Analyzing and Interpreting Data;  Gravitational Interactions; Forces and Motion, Relationship between Energy and Forces

    This project will acquaint you and students with what satellites are, the forces that act on satellites, types of orbits satellites can be found circling Earth, and  how to track satellites from the ground.The NSS Enterprise orbiter will be tracked upon launch in 2020 carrying 100+ student experiments!  Learn about tracking satellites by using the Trek-A-Sat Activity.


    1. What do you know about gravity?
    2. Introduction to forces and gravitational pull here on Earth! 
    3. Understanding Centripetal Force
    4. What is Weightlessness all about, anyway!
    5. Satellites
    6. Understanding satellite orbits.
    7. Exit slip - Let's find out what you learned.

  • 1. What do you know about gravity?

    Before we begin on the Trek-A-Sat Projects, please answer the following questions to assess what you learn with this topic!  This information is just for you and the teachers to follow your progress and determine how to improve the projects.  Thank you!

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  • 2. Introduction to forces and gravitational pull here on Earth!

    re the pendulum hangs is the 'pivot point,' and the distance of the pendulum from the pivot point is the 'length' of the pendulum.Here is a simple pendulum.  The mass is called a 'bob,' the rigid support where the pendulum hangs is the 'pivot point,' and the distance of the pendulum from the pivot point is the 'length' of the pendulum.

    Take a simple pendulum - you have been in a swing many times- and let's look at how the pendulum works.  There are forces responding to it, even if it doesn't move!

    The mass of the pendulum is known as the 'bob,' the point at which the pendulum is hung is the 'pivot point,' and the distance from the pivot point to the bob is the 'length' of the pendulum.  There are two dominant forces that act on a pendulum bob: 1. a downward force that is gravity (Earth's attraction pulling the bob down), and 2. the tension force acting upwards from the the bob to the pivot point and that the string pulling on the bob to hold it to the pivot point.

    In this investigation, we will ignore air resistance as an influence on the bob's motion because it is relatively weak compared to the other two forces.

    Let's get started with working a pendulum interactive. You can change the mass of the bob, and the length of the pendulum as well as comparing how your pendulum will move on Earth, Jupiter, the moon and 0 gravity! You can see that even Planet X has gravity. (Scientists believe there may be another planet or lots of asteroids way out beyond Pluto in the 'Kuiper Belt' that will account for the strange effects they see in planet orbits!  So you can now understand that gravity goes very far from a star and that objects revolving around stars have a gravitational force as well.) If you wish to take data on how long the period takes (the time for the bob to make one swing from it's original position and returning to that position), that will reinforce the lesson.

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  • 3. Understanding Centripetal Force. (Contributed from Physics Classroom.)

    Just as you learned that there are always a gravitational pull and the upward tension pull toward the pivot point on a pendulum, this 'tension pull' can also be compared to a pull of an object toward the center of another object it is moving around, or orbiting - this is a gravitational pull.

    When an object is moving around another, the object orbiting is also moving forward so there is a 'gravitational vector' and an 'acceleration vector." The acceleration vector keeps the object rotating. The speed or momentum of the object in its circular path keeps the object from allowing gravity to pull the object out of orbit.  If the circular momentum of the orbiting object decreases by enough, the object will fall out of orbit and  gravity will pull the object out of orbit toward the object it is orbiting.

    Here are some interactive games to try your skill at keeping an object moving around another(in a circular motion) just as its gravitational and acceleration vectors are changing!

    image:race track


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  • 4. What is Weighlessness all about, anyway! (Source: Physics Classroom.)

    Weightlessness in Orbit    

    Astronauts who are orbiting the Earth often experience sensations of weightlessness. These sensations experienced by orbiting astronauts are the same sensations experienced by anyone who has been temporarily suspended above the seat on an amusement park ride. Not only are the sensations the same (for astronauts and roller coaster riders), but the causes of those sensations of weightlessness are also the same. Unfortunately however, many people have difficulty understanding the causes of weightlessness.

    Take a look at the two categories of forces, then try the elevator interactive to help you understand about weightlessness.

    Contact versus Non-Contact Forces

    Before understanding weightlessness, we will have to review two categories of forces - contact forces and action-at-a-distance forces. As you sit in a chair, you experience two forces - the force of the Earth's gravitational field pulling you downward toward the Earth and the force of the chair pushing you upward. The upward chair force is sometimes referred to as a normal force and results from the contact between the chair top and your bottom end. This normal force is categorized as a contact force. Contact forces can only result from the actual touching of the two interacting objects - in this case, the chair and you. The force of gravity acting upon your body is not a contact force; it is often categorized as an action-at-a-distance force. The force of gravity is the result of your center of mass and the Earth's center of mass exerting a mutual pull on each other; this force would even exist if you were not in contact with the Earth. The force of gravity does not require that the two interacting objects (your body and the Earth) make physical contact; it can act over a distance through space. Since the force of gravity is not a contact force, it cannot be felt through contact. You can never feel the force of gravity pulling upon your body in the same way that you would feel a contact force. If you slide across the asphalt tennis court (not recommended), you would feel the force of friction (a contact force). If you are pushed by a bully in the hallway, you would feel the applied force (a contact force). If you swung from a rope in gym class, you would feel the tension force (a contact force). If you sit in your chair, you feel the normal force (a contact force). But if you are jumping on a trampoline, even while moving through the air, you do not feel the Earth pulling upon you with a force of gravity (an action-at-a-distance force). The force of gravity can never be felt. Yet those forces that result from contact can be felt. And in the case of sitting in your chair, you can feel the chair force; and it is this force that provides you with a sensation of weight. Since the upward normal force would equal the downward force of gravity when at rest, the strength of this normal force gives one a measure of the amount of gravitational pull. If there were no upward normal force acting upon your body, you would not have any sensation of your weight. Without the contact force (the normal force), there is no means of feeling the non-contact force (the force of gravity).

    Meaning and Cause of Weightlessness

    Weightlessness is simply a sensation experienced by an individual when there are no external objects touching one's body and exerting a push or pull upon it. Weightless sensations exist when all contact forces are removed. These sensations are common to any situation in which you are momentarily (or perpetually) in a state of free fall. When in free fall, the only force acting upon your body is the force of gravity - a non-contact force. Since the force of gravity cannot be felt without any other opposing forces, you would have no sensation of it. You would feel weightless when in a state of free fall.

    These feelings of weightlessness are common at amusement parks for riders of roller coasters and other rides in which riders are momentarily airborne and lifted out of their seats. Suppose that you were lifted in your chair to the top of a very high tower and then your chair was suddenly dropped. As you and your chair fall towards the ground, you both accelerate at the same rate - g. Since the chair is unstable, falling at the same rate as you, it is unable to push upon you. Normal forces only result from contact with stable, supporting surfaces. The force of gravity is the only force acting upon your body. There are no external objects touching your body and exerting a force. As such, you would experience a weightless sensation. You would weigh as much as you always do (or as little) yet you would not have any sensation of this weight.

    Weightlessness is only a sensation; it is not a reality corresponding to an individual who has lost weight. As you are free falling on a roller coaster ride (or other amusement park ride), you have not momentarily lost your weight. Weightlessness has very little to do with weight and mostly to do with the presence or absence of contact forces. If by "weight" we are referring to the force of gravitational attraction to the Earth, a free-falling person has not "lost their weight;" they are still experiencing the Earth's gravitational attraction. Unfortunately, the confusion of a person's actual weight with one's feeling of weight is the source of many misconceptions.


    Scale Readings and Weight

    Technically speaking, a scale does not measure one's weight. While we use a scale to measure one's weight, the scale reading is actually a measure of the upward force applied by the scale to balance the downward force of gravity acting upon an object. When an object is in a state of equilibrium (either at rest or in motion at constant speed), these two forces are balanced. The upward force of the scale upon the person equals the downward pull of gravity (also known as weight). And in this instance, the scale reading (that is a measure of the upward force) equals the weight of the person. However, if you stand on the scale and bounce up and down, the scale reading undergoes a rapid change. As you undergo this bouncing motion, your body is accelerating. During the acceleration periods, the upward force of the scale is changing. And as such, the scale reading is changing. Is your weight changing? Absolutely not! You weigh as much (or as little) as you always do. The scale reading is changing, but remember: the SCALE DOES NOT MEASURE YOUR WEIGHT. The scale is only measuring the external contact force that is being applied to your body.

    Now consider Otis L. Evaderz who is conducting one of his famous elevator experiments. He stands on a bathroom scale and rides an elevator up and down. As he is accelerating upward and downward, the scale reading is different than when he is at rest and traveling at constant speed. When he is accelerating, the upward and downward forces are not equal. But when he is at rest or moving at constant speed, the opposing forces balance each other. Knowing that the scale reading is a measure of the upward normal force of the scale upon his body, its value could be predicted for various stages of motion. For instance, the value of the normal force (Fnorm) on Otis's 80-kg body could be predicted if the acceleration is known. This prediction can be made by simply applying Newton's second law as discussed in Unit 2. As an illustration of the use of Newton's second law to determine the varying contact forces on an elevator ride, consider the following diagram. In the diagram, Otis's 80-kg is traveling with constant speed (A), accelerating upward (B), accelerating downward (C), and free falling (D) after the elevator cable snaps.

    In each of these cases, the upward contact force (Fnorm) can be determined using a free-body diagram and Newton's second law. The interaction of the two forces - the upward normal force and the downward force of gravity - can be thought of as a tug-of-war. The net force acting upon the person indicates who wins the tug-of-war (the up force or the down force) and by how much. A net force of 100-N, up indicates that the upward force "wins" by an amount equal to 100 N. The gravitational force acting upon the rider is found using the equation Fgrav = m*g.

    Stage A

    Stage B

    Stage C

    Stage D

    Fnet = m*a

    Fnet = 0 N

    Fnet = m*a

    Fnet = 400 N, up

    Fnet = m*a

    Fnet = 400 N, down

    Fnet = m*a

    Fnet = 784 N, down

    Fnorm equals Fgrav

    Fnorm = 784 N

    Fnorm > Fgrav by 400 N

    Fnorm = 1184 N

    Fnorm < Fgrav by 400 N

    Fnorm = 384 N

    Fnorm < Fgrav by 784 N

    Fnorm = 0 N

    The normal force is greater than the force of gravity when there is an upward acceleration (B), less than the force of gravity when there is a downward acceleration (C and D), and equal to the force of gravity when there is no acceleration (A). Since it is the normal force that provides a sensation of one's weight, the elevator rider would feel his normal weight in case A, more than his normal weight in case B, and less than his normal weight in case C. In case D, the elevator rider would feel absolutely weightless; without an external contact force, he would have no sensation of his weight. In all four cases, the elevator rider weighs the same amount - 784 N. Yet the rider's sensation of his weight is fluctuating throughout the elevator ride.


    Weightlessness in Orbit

    Earth-orbiting astronauts are weightless for the same reasons that riders of a free-falling amusement park ride or a free-falling elevator are weightless. They are weightless because there is no external contact force pushing or pulling upon their body. In each case, gravity is the only force acting upon their body. Being an action-at-a-distance force, it cannot be felt and therefore would not provide any sensation of their weight. But for certain, the orbiting astronauts weigh something; that is, there is a force of gravity acting upon their body. In fact, if it were not for the force of gravity, the astronauts would not be orbiting in circular motion. It is the force of gravity that supplies the centripetal force requirement to allow the inward acceleration that is characteristic of circular motion. The force of gravity is the only force acting upon their body. The astronauts are in free-fall. Like the falling amusement park rider and the falling elevator rider, the astronauts and their surroundings are falling towards the Earth under the sole influence of gravity. The astronauts and all their surroundings - the space station with its contents - are falling towards the Earth without colliding into it. Their tangential velocity allows them to remain in orbital motion while the force of gravity pulls them inward.

    Many students believe that orbiting astronauts are weightless because they do not experience a force of gravity. So to presume that the absence of gravity is the cause of the weightlessness experienced by orbiting astronauts would be in violation of circular motion principles. If a person believes that the absence of gravity is the cause of their weightlessness, then that person is hard-pressed to come up with a reason for why the astronauts are orbiting in the first place. The fact is that there must be a force of gravity in order for there to be an orbit.

    One might respond to this discussion by adhering to a second misconception: the astronauts are weightless because the force of gravity is reduced in space. The reasoning goes as follows: "with less gravity, there would be less weight and thus they would feel less than their normal weight." While this is partly true, it does not explain their sense of weightlessness. The force of gravity acting upon an astronaut on the space station is certainly less than on Earth's surface. But how much less? Is it small enough to account for a significant reduction in weight? Absolutely not! If the space station orbits at an altitude of approximately 400 km above the Earth's surface, then the value of g at that location will be reduced from 9.8 m/s/s (at Earth's surface) to approximately 8.7 m/s/s. This would cause an astronaut weighing 1000 N at Earth's surface to be reduced in weight to approximately 890 N when in orbit. While this is certainly a reduction in weight, it does not account for the absolutely weightless sensations that astronauts experience. Their absolutely weightless sensations are the result of having "the floor pulled out from under them" (so to speak) as they are free falling towards the Earth.

    Still other physics students believe that weightlessness is due to the absence of air in space. Their misconception lies in the idea that there is no force of gravity when there is no air. According to them, gravity does not exist in a vacuum. But this is not the case. Gravity is a force that acts between the Earth's mass and the mass of other objects that surround it. The force of gravity can act across large distances and its effect can even penetrate across and into the vacuum of outer space. Perhaps students who own this misconception are confusing the force of gravity with air pressure. Air pressure is the result of surrounding air particles pressing upon the surface of an object in equal amounts from all directions. The force of gravity is not affected by air pressure. While air pressure reduces to zero in a location void of air (such as space), the force of gravity does not become 0 N. Indeed the presence of a vacuum results in the absence of air resistance; but this would not account for the weightless sensations. Astronauts merely feel weightless because there is no external contact force pushing or pulling upon their body. They are in a state of free fall.



  • What is a Satellites

    What is a satellite?   

    Satellites are a common occurrence in today's world and are very important for humankind. A satellite is defined as anything that has been captured by the gravity of a larger object, and is in orbit around that object (the moon is a satellite of Earth). A satellite can be as small as a grain of sand or as big as the moon and these would be called natural satellites.

    You may be familiar with man-made satellites.  These items have been launched into orbit for various purposes which range from providing a living area for astronauts, communications, a study of our oceans/land/atmosphere, and military surveillance/spy satellites.

    In the previous projects you have explored the forces that contribute to centripetal force which are the forces satellites encounter.  Below are some models of important satellites. The NEAR Schoemaker satellite traveled to the NEO (Near Earth Asteroid) Eros to study it, landed on it and is there today.  Have fun making a model of the New Horizons satellite which visited Pluto in July, 2015 and is headed out into the Kuiper Belt now! Neither NEAR or New Horizons are not visible to the naked eye.  The template and instructions for the model follow.  There is also a crossword puzzle about the different types of satellites that can now be found orbiting Earth.

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  • 6. Understanding satellite orbits.

    There are various orbits in to which satellites can be launched. The satellite's purpose determines the best orbit for the satellite.  Some satellites are making a survey of Earth and need to be in particular positions to gather the data.  Some satellites convey communication information through a network of satellites and they will need to be oriented in advantageous positions around Earth to give their most efficient and effective performance .  The various types of orbits are discussed and illustrated below:

    Lynne Zielinski contributes the graphics.

    Satellite Orbits

    This information is adapted and expanded from "Looking at Earth From Space", a Teacher's Guide with Activities for Earth and Space Science, 1994, available from NASA Education Resource Centers.

    Satellites can operate in several types of Earth orbit. The most common orbits for environmental satellites are geostationary and polar, but some instruments also fly in inclined orbits. (Other types of orbits are possible, such as the Molniya orbits commonly used for Soviet spacecraft and not described in this lesson.)

    Geostationary Orbits

    GEO and LEO orbital graphics.

    A geostationary (GEO=geosynchronous) orbit is one in which the satellite is always in the same position with respect to the rotating Earth. The satellite orbits at an elevation of approximately 35,790 km (~22,000 miles) because that produces an orbital period (time for one orbit) equal to the period of rotation of the Earth (23 hrs, 56 mins, 4.09 secs). By orbiting at the same rate, in the same direction as Earth, the satellite appears stationary (synchronous with respect to the rotation of the Earth).

    Geostationary satellites provide a "big picture" view, giving great coverage of weather events.  This information is critical when  monitoring severe local storms and tropical cyclones.

    The geostationary satellites are in the same plane as the Earth's rotation, which is an equatorial plane.  The position of the orbit gives a limitation in the polar region images and they become distorted with poor spatial resolution.

    Take a look at the " View from a Geostationary Weather Satellite" at the end of this section.

    Polar orbiit graphics.Polar Orbits

    Polar-orbiting satellites provide a more global view of Earth, circling at near-polar inclination (the angle between the equatorial plane and the satellite orbital plane -- a true polar orbit has an inclination of 90 degrees). Orbiting at an altitude of 700 to 800 km (430 - 500 mi), these satellites are able to gather information from parts of the world that are very difficult to analyze from the ground. For example, McMurdo, Antarctica, can be seen on days 11-12 of the 14 daily NOAA polar-orbiter passes.

    These satellites operate in a sun-synchronous orbit.  The satellite passes the equator as well as each latitude at the same local solar time each day.  This means that the satellite will be passing overhead at about the same solar time throughout all the seasons of the year. The polar orbiting satellites give regular data collection at consistent times as well as long-term comparisons of data collected.  The orbital plane of a sun-synchronous orbit must also rotate approximately one degree per day to deep pace with the earth's surface. Examples of polar satellites are: Terra, Aqua, and NPP satellites (which collect data on long term climate change and short term weather changes.)

    Inclined Orbits

    Inclined orbits fall between those above. They have an inclination between 0 degrees (equatorial orbit) and 90 degrees (polar orbit). These orbits may be determined by the region on Earth that is of most interest (i.e., an instrument to study the tropics may be best put on a low inclination satellite), or by the latitude of the launch site. The orbital altitude of these satellites is generally on the order of a few hundred km, so the orbital period is on the order of a few hours. These satellites are not sun-synchronous, however, so they will view a place on Earth at varying times. You can find several satellite tracking tools here which will show you what various orbits look like. 

    Equatorial and inclined orbit graphic.

    A graphic of the NSS Enterprise.

    The National Space Society's NSS Enterprise is currently planned to be in a LEO orbit and will launch in 2020-2021.  You will be able to track the satellite during its orbit and prior to its retrieval.  Keep posted on the actual dates of deployment, orbit, and recovery! To learn how to track a satellite, take a look at the Trek-A-Sat project link below:

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