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Getting To and From the International Space Station

Low Earth Orbit

Background Information

Note: Orbits drawn in the diagrams below are as seen looking down on the Earth's North Pole. Orbital motion is counter-clockwise around the Earth's equator.

It is a common mistake to think of objects such as weather satellites and the Space Shuttle as being VERY far away from the Earth's surface...so far in fact that gravity is very weak. The truth is that many weather satellites, and the Space Shuttle, are so close to the Earth that the force of gravity is almost the same as it is on the Earth's surface. Consider the sketch drawn below in which the height of the Space Shuttle is 500km above the Earth's surface.

Low Earth Orbit
When drawn to scale as shown in this diagram below we begin to see that 500km is quite close to Earth's surface. One sometimes forgets that the Earth's radius is about 6300km, and that relative to the size of the Earth, 500km is rather small. Of course in these sketches the Shuttle is not drawn to scale!

Low Earth Orbit
Drawn on a larger scale one can easily see why we say that these are in "low earth orbits". It is important to note that the plane defined by the free orbit of any spacecraft around the earth intersects the centre of the Earth!

Low Earth Orbits
The sketch below shows some of the infinite number of possible, stable, low earth orbits. Those that are drawn in black (left earth) are common orbital configurations, whereas the orbits in red (right earth) are forbidden by the laws of physics and are not stable.

Low Earth Orbits

Blog It!

Try the Challenge Questions below! Write a new blog entry describing your solutions.

Please check the Getting to the ISS and Back topics box before posting. Remember to come back often and discuss your solutions with other students!

Mass of the Earth = 5.974 x 1024 kg
Universal gravitational constant = 6.67 x 10-11 Nm2/kg2
Average radius of the Earth = 6.360 x 106 m

  1. Calculate the distance that the Space Shuttle would travel around the Earth in exactly one orbit at a height of 350km. What would the speed (in km/s) of the Shuttle be if its orbital period (time required for one full orbit) is exactly 91.2 minutes?

  2. Calculate the distance that the Space Shuttle would travel around the Earth in exactly one orbit at a height of 550km. What would the speed (in km/s) of the Shuttle be if its orbital period (time required for one full orbit) is exactly 95.2 minutes?

  3. At a height of 600km above the Earth, the Shuttle's speed is about 7590 m/s. Calculate the orbital period of the Shuttle in this orbit.

  4. At a height of 300km above the Earth, the Shuttle's speed is about 7740 m/s. Calculate the orbital period of the Shuttle in this orbit.

  5. At a height of 400km above the Earth, the Shuttle's speed is about 7678 m/s. Calculate the orbital period of the Shuttle in this orbit.

  6. Using the results from the above 5 questions plot a graph of orbital period (in minutes) on the x-axis, vs the orbital height above the Earth's surface on the y-axis (in kilometres). What conclusions can be drawn from this graph?

  7. Using your graph predict the orbital period of a satellite having an altitude of 700 km above the Earth's surface.

  8. Assume that The International Space Station and the Space Shuttle are in exactly the same orbital plane, are 180o apart in their orbits, and that the shuttle is 25 km lower than the ISS. The Shuttle wishes to rendezvous with the ISS in the shortest possible time. Should the Shuttle move to a higher orbit or a lower orbit to catch up with the ISS? Justify your answer.

  9. Activity: For this you will need a short length of string (or large elastic band), a globe of the Earth and a map (Mercator projection) of the Earth.
    1. Wrap a piece of string (or large elastic band) around the globe to simulate the ground track of a satellite in low earth orbit.
    2. Select at least 15 points on the globe that are covered by (or very near) the ground track.
    3. Plot these 15 points on your Mercator projection map of the Earth.
    4. Repeat this for several other possible low earth orbits, plotting each orbit in a different symbol or colour.

  10. Puzzle: The Shuttle in question one (1) above has a predicted orbital period of exactly 91.2 minutes. As it passes due south of Houston (Mission Control), observers in Houston decide to measure how long it takes for the Shuttle to appear over the same longitude ( i.e. due south) on the next pass. It arrives several minutes late! Assuming no orbital changes have occurred, how can this be explained?

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