It gave spacecraft a timed trajectory. It ensured spacecraft would return home safely. It was helpful in planning spacecraft trajectories. It ensured spacecraft would avoid being pulled into the sun.
I gave my students this multiple choice question: When manned space exploration began, NASA used the gravity assist concept so that spacecraft could conserve fuel. Which statement explains why the gravity assist concept was useful for spacecraft to successfully travel and return to Earth?

Only one answer is correct among these 4 choices:
- It gave spacecraft a timed trajectory.
- It ensured spacecraft would return home safely.
- It was helpful in planning spacecraft trajectories.
- It ensured spacecraft would avoid being pulled into the sun.

Could you tell me which one is correct?

It was helpful in planning spacecraft trajectories.