What must be applied for a satellite to decrease its orbital altitude?

For a satellite to decrease its orbital altitude, retrothrust must be applied. When a satellite is in orbit, it is moving at a certain velocity to maintain its altitude through the balance of gravitational force and centripetal force. If the satellite needs to lower its orbit, it must reduce its velocity. This is accomplished by firing retrothrusters, which are engines that work in the opposite direction of the satellite's motion.

When retrothrust is applied, it generates a force that opposes the satellite's current trajectory, leading to a reduction in speed. As the speed decreases, the gravitational pull on the satellite becomes more dominant, causing it to descend into a lower orbit. This is an essential principle in orbital mechanics, where controlling speed and direction is crucial for maneuvering satellites in space.

Other options like forward thrust would increase the satellite's speed, causing it to ascend to a higher orbit instead of descending. "No thrust" would mean the satellite continues in its current orbit, while "constant velocity" would also maintain its altitude. Therefore, the application of retrothrust is the correct approach to achieve a decrease in orbital altitude.