Table of Contents

Orbital Mechanics

Student Activity

Manoeuvring In Space: Answer Key

Assignment

1. Suppose you were stranded on a perfectly frictionless and perfectly flat surface. Suggest a method by which you might rescue yourself without soliciting outside help.

   
   
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   ANS: Remove any piece of clothing and throw it horizontally as hard as possible. The reaction to this will cause you to move steadily in the opposite direction. (Newton's 3rd Law gets you moving and Newton's 1st Law keeps you going!)

2. To re-enter the Earth's atmosphere the Space Shuttle performs a brief rocket burn in a direction parallel to the Earth's surface and opposite to the direction of its orbital motion. Explain why a downward thrust is not used.

   ANS: The effect of a downward thrust would be to increase the overall energy of the shuttle which would eventually lead to an increase in the orbital height. Reverse thrust, which slows down the shuttle (which actually causes it to speed up as it begins to descend) is required in order to reduce the orbital energy of the shuttle and allow it to descend towards the Earth.

3. Which orbits are possible and which orbits are impossible in the diagram below. Explain your reasoning.
   
   

   

   ANS: All black orbits are allowed, red orbits are not. All orbits about the earth must have the Earth's centre of mass at the focus of their orbital ellipses. This means that the centre of the earth must be in the plane of the orbit. (Kepler's 1st law)