Programs

Mission Scenario

Applicable to all Challenger Learning Center Missions

Application to Mission Preparation

Students will learn where the International Space Station (ISS) orbits in relation to the Earth and determine its position. Students will be able to determine when they can see the ISS from the ground.

Note: These activities are in preparation for subsequent activities that will allow students to take virtual photographs from the ISS of Earth.

The following activities are

• Whole Group/Small Group activities          
• Applicable to the Navigation and Science teams

Summary

In the first activity, students will explore the orbit of the ISS by building a flat map of the orbital track using a small globe. Students will be able to view live orbital tracks using links to NASA and other websites to determine where the ISS in real time. In the second activity, students will chart the path of the ISS to find out when it is over a specific location (i.e. their hometown) using real-time reference charts available from TERC’s Windows on Earth. Students will be able to listen to a podcast that helps them to determine when they can see the ISS overhead and visit a NASA sightings website.

LESSON AT A GLANCE

Subject(s)

Mathematics, Science, Engineering, Technology

Grade/Level  

5-12

Objectives

Students will be able to:

• Use latitude and longitudinal references to describe the location of the ISS
• Model the path of the ISS in three dimensions including direction of travel
• Complete mathematical calculations that help describe the orbital track (path) of the ISS orbit
• Calculate specific times on a sample orbital track (path)
• Use NASA and other web resources (Windows on Earth) to determine the next pass of the ISS over their (or any) location

Time Required         

2 class periods. 50 minutes per class.

Essential Questions

• Where does the space station orbit above the Earth?
• How do we track the space station’s location?
• When will the ISS be above my location?

Lesson Overview

How high is the ISS?

The altitude of the ISS over the Earth varies between 320 to 410 kilometers (km). The reason for the variation is because drag from the tenuous atmosphere at this height above the Earth causes the ISS to lose altitude over time.

Spaceships, such as the Soyuz and the Space Shuttle fly to the ISS and push it back up periodically – this is called a reboost of its orbit. Spaceship flights are sometimes irregular due to funding and schedule constraints and changes. Looking over the chart from the past ten years, note the period after the loss of the Columbia space shuttle when few reboosts were available from a U.S. space shuttle. The chart shows that the ISS is currently maintaining an altitude of 345 kilometers.

Click for larger image

As the ISS adds more modules over time and becomes larger, it needs more fuel to reboost it to the same altitude due to increased mass. The ATV, Soyuz and Progress vehicles are planned to handle reboosts after the Space Shuttle retires.

As you might expect, as the altitude of the ISS varies, the time it takes it to orbit the Earth also changes. This then changes when the ISS is passing over different locations.

At an altitude of 400km the ISS orbit was about 94 minutes long. At an altitude of 345km, the ISS takes about 92 minutes to circle the Earth. Even a small change of a few minutes in the orbital time will alter the orbital path the ISS takes over the Earth.

The speed of the ISS changes with its altitude because it is simply traveling in a larger orbit that is further from the Earth. The size of the orbit determines the orbital period. The ISS currently makes about 16 orbits a day.

The numbers listed above are an approximation. Even the rotation of the Earth isn’t exactly 24 hours, which is why you need mathematics to perform specific calculations to figure out when you will be in any specific location.

In which direction does the ISS travel?

The International Space Station (ISS) orbits the Earth at 51.6° to the Equator, following the direction of the Earth’s rotation from west to east.

The orbital inclination of the ISS is 51.6 degrees from the Equator for many reasons (see our Explore More link about the orbital inclination) however this orbital track offers good coverage of most of Earth's inhabited surface. (You cannot see a lot of the Earth's surface from this orbit though, places such as Antarctica).

Look over the ISS orbital tracking page image below. Similar tracking maps are projected onto the wall screens at the mission control centers in Houston and Moscow.

Click for larger image

The ISS goes over a specific area on Earth approximately twice a day. Once in an ascending track from south to north, the second is in a descending track from north to south. For more about the orbit of the ISS visit NASA: Space Station Orbit Tutorial

As the map above is laid out flat in a two-dimensional view, the orbital ground track appears as a wavy line (like a sine wave), but it is in reality flying in a straight path as it circles the Earth. 

The ISS orbits in a circular orbit around the Earth along a plane (imagine a plate bisecting the globe). If you tie a piece of string around a globe at 51.6 degrees to the Equator, you can see that the ISS travels in a straight path. You can also see that the circle forms a plane that cuts through the Earth at a specific angle (51.6 degrees).

You can do an activity (see Activity 1 below) with your students using a Tennis Ball, a cut out map of the world that forms a globe, a string and a red marker that will allow them to draw the circle of an orbit on the map.  When they lay the maps out flat and put them side-by-side, they can easily see the sine waves that are formed, similar to those seen on the tracking map above.

Why does the ISS pass over constantly different places?

In the image below you can see several orbits repeated over time.

Click for larger image

Check out this link from Windows on Earth
You can generate a multiple of orbital tracks (sine curves) in real time,
(or for any time you determine)

If you look carefully you will notice that it is not over the same place exactly in each 90-minute pass. The ISS does not orbit over the same spot every 24 hours, because the Earth is also turning eastward.

If, for example, it takes 90 minutes for the ISS to complete one full orbit (travelling at 17,500 mph or 28,000 km/hr), when it gets back to its starting point, the Earth’s surface has moved eastward due to its own rotation.

The Earth moves 360 degrees in 24 hours, or 15 degrees per hour. So after each ISS orbit of 1.5 hours, the Earth’s surface has moved about one and a half times 15 degrees (1.5 x 15), or 22.5 degrees, further to the east.

An equation used for calculating the degrees longitude orbital ground track shifts eastward based on a 92-minute orbit is shown below. You can have your students complete this calculation.

Equation

 

This is also the reason why the sighting times (when you can see the ISS from the ground at night)  in different cities vary from one day to the next, or don’t appear at all for a week or so.

 

TEACHING THE LESSON

Materials      

• Tennis Balls (one for each group or student)
• String
• Red Markers
• Scissors
• Student Worksheets

• Internet connected computers

• An LCD projector to share websites with the entiure class

Student Worksheets

• Tennis Ball Globe (pdf)
• ISS Sighting (doc)

Preparation

Students should have a basic understanding of the International Space Station and orbits of satellites. You can review the links listed below as preparation.

• ESA: Speed and Altitude of the ISS Interactive - The dependence between height above ground and orbital velocity for ISS. Then a simple interactivity makes it possible to change height above ground and observe changes in velocity and orbital period. Grades 5 through 7.
• NOAA: Satellites and Orbits - An easy-to-use interactive, animated introduction to satellites and the forces that determine orbits. Highlights a short satellite history, non-mathematical review of basic orbital physics, orbital terminology and orbit types with a focus on Earth observation and environmental monitoring. Includes: reading assignment, classroom demonstration, student activity lab, quiz and lesson instructions. Grades 8 through 10.
• NASA: Orbital Mechanics 101 – NASA Aerospace Scholars curriculum that allows students to explore the orbit of the ISS and space shuttle. Grades 11 through 12.
• NASA EarthKam information website about the ISS - Information for students about the International Space Station.
• NASA ISS Website - NASA's official site about the ISS.
• ESA ISS Website - The European Space Agency's page about the ISS.

Introduction

Begin the activity with students in the full group brainstorming the answer to the following questions.

Why do we need to know where the ISS is at any given time?

Some answers may include:

• The ISS needs to be able to communicate with the Earth and so it needs to be able to talk to satellites and ground stations that receive and send audio and video signals
• The ISS takes photographs of the Earth to study weather and climate, and the astronauts need to know exactly when the ISS is going to be over specific places on the Earth
• The ISS also needs to be aware of large space debris that might be nearby in case it needs to quickly maneuver (a difficult process, which is why NASA tracks large objects very carefully all the time)
• The ISS can be seen at night by people on the Earth and the excitement of watching an orbiting manned laboratory pass overhead gets people interested in figuring out when the ISS will get back to the same place that they can see it again!

Explore some of the basic information about the orbit of the ISS with your students, using the images and interactive links provided.

• The angle of inclination (51.6 degrees)
• The altitude of the ISS (345 km)
• The speed of the ISS (28,000 km/hr)
• How long it takes to orbit the Earth (90 minutes)
• Why the ISS does not orbit over the same place each orbit (due to the rotation of the Earth)

Activity 1 - Orbit

Ask students why they think the orbit of the ISS is at 51.6 degrees. Have students read or review with them the discussion with Flight Director John Curry to explain the choice of the orbital inclination of 51.6.

Use the ESA link as a full class demonstration to help students understand the dependence between height above ground and orbital velocity for ISS.

Have students complete the equation to calculate the degrees longitude orbital ground track shifts eastward.

Show the students the Orbital Tracking Map (image or website) and ask them why they think that the orbit looks like a wave (or sine wave).

With your students in small groups, create Tennis Ball Globes using the NASA Student Worksheets and use a string to make an orbit at about 51.6 degrees to the equator.

Using a red marker draw the line of that orbit along the string (which can be seen as a plane, if done carefully) on your Tennis Ball Globe Map paper. Then unwrap the map.

Put two or more of the unfolded maps side by side to see the sine waves.

Click for larger image

This is called an interrupted map of the Earth and represents the landcover of the Earth. Because of its accuracy, it can be used to wrap around a sphere in three-dimensions. It is similar to the one used in the activity to cover a tennis ball.

For more about the orbit of the ISS visit NASA: Space Station Orbit Tutorial

Activity 2 - Sightings

Ask your students, “If we wanted to see the ISS at night, how can we use the tools we have seen so far to determine if the ISS will be passing above our location at night?”

Have students in small groups, visit each site below and fill out the Student Worksheet: Sightings.

• Visit the NASA tracking page to see if the ISS in day or night right now.
• Visit the Windows on Earth orbital projections website again and look closely at the difference day and night tracks. Note: At the bottom of the page, you can change the date).
• Visit the Heavens Above website to find out when the next sighting is for your town.  Encourage students to ask their parents to try to view the ISS with them, or if the sighting is early in the evening plan an outing with your class.

Extension Activity

As a group, or for individuals as homework, have students listen to the Challenger Center podcast on how to see the ISS at night.