Newton's first law (inertia): an object with no net force acting on it keeps its velocity — same speed, same direction — forever.
- In space there is no air resistance and no road friction. Once the engine is off, the only forces left are gravity and, very weakly, sunlight pressure.
- So a coasting spacecraft does not need to "keep pushing" to keep moving. It drifts at constant velocity, and the fuel gauge does not move at all.
- This is the single biggest fuel saver of the whole mission: most of the Earth–Moon transit is unpowered coasting.
Key contrast: on Earth, holding a constant speed on a highway requires continuous fuel because friction and drag remove energy. In vacuum, nothing removes it.