What is the primary difference between a prograde and a retrograde orbit?

• A. Prograde orbits move opposite to Earth's rotation.
• B. Prograde orbits travel with Earth's rotation; retrograde orbits travel opposite.
• C. Prograde orbits alternate between high and low altitudes.
• D. Prograde orbits are only used for satellites.

Answer: (B)

Prograde and retrograde describe the direction of orbital motion relative to the planet’s rotation. Earth spins to the east, so a prograde orbit travels in the same eastward direction as that spin. A retrograde orbit travels westward, opposite to Earth's rotation. This direction matters because it affects how much energy is needed to reach the orbit: launching east into a prograde orbit can take advantage of Earth's rotational speed, reducing the delta-v required, while a retrograde orbit requires more energy to reverse that direction. Retrograde orbits are chosen for specific mission needs, such as particular inclination or ground-track requirements. The other statements don’t fit: prograde isn’t opposite Earth’s rotation, and orbital direction isn’t defined by altitude changes or by being used only for satellites.