Final answer:
To orbit at a lower height, a satellite or spacecraft must travel faster than it would at a higher orbit due to the stronger gravitational pull. At a higher orbit, the velocity decreases as the gravitational force becomes weaker. Changing a spacecraft's orbit involves adjusting its speed with thrusters.
Step-by-step explanation:
If you want to orbit at a lower height, you have to go faster than if you want to orbit at a higher height. This is because orbital velocity is inversely related to the distance from the center of the mass being orbited. As explained by the principles of conservation of angular momentum and conservation of energy, a satellite closer to the planet must move faster to balance the greater gravitational pull it experiences. In contrast, a satellite in a higher orbit moves slower as the gravitational pull is weaker at larger distances from the planet.
Directly applying this concept, we can see that when orbiting the Earth, a satellite needs to achieve the circular satellite velocity of approximately 8 kilometers per second to maintain a stable low Earth orbit. For orbits further from the Earth, the required orbital velocity decreases.
Any changes to the speed of a spacecraft in orbit, either increasing its speed to move to a higher orbit or decreasing it to descend to a lower one, require the application of external force, typically from a rocket's engines. When a spacecraft approaches its destination and needs to establish an orbit, the spacecraft is decelerated with a rocket burn to ensure it can be captured into an elliptical or circular orbit.