Why don’t satellites fall to Earth?
Satellites are constantly falling under Earth’s gravity. With enough forward speed, Earth’s surface curves away beneath them: the spacecraft keeps going around the planet instead of meeting the ground.
Gravity does not disappear in space
Gravity continually changes the direction of a satellite’s motion. Without it, the spacecraft would travel in a straight line. Weightlessness in orbit occurs because the station and everything inside it are falling together.
Maintaining an ideal orbit does not require a constantly running engine. Real spacecraft perform maneuvers, and atmospheric drag gradually changes motion in low orbits.
How speed changes the trajectory
The lesson’s examples start from a circular orbit 400 km above Earth. Reducing speed along the direction of travel brings the new path closer to Earth. A moderate increase stretches the orbit; enough speed produces an open escape trajectory.
After a short burn, the spacecraft is still at the same location: its future path changes. Accelerating does not mean instantly moving upward, and slowing down does not mean immediately falling straight down.
Predict the result, then try it
Follow the Falling, Orbit and Speed stages, then open the free experiment. Predict how a maneuver will change the path before adjusting speed. Compare the new trajectory with the dashed line of the previous one.
Once the connection between speed and orbital shape makes sense, try the orbital missions. They ask you to choose when and where to thrust while staying within a limited velocity-change budget.