How do we put a satellite in a sun-synchronous orbit?

• A. By firing the apogee kick motor just as the satellite passes closest to the sun.
• B. By establishing an orbit that passes over the equator more frequently in daylight than in darkness.
• C. By positioning the satellite so that the sun's gravity and earth's gravity are in equilibrium.
• D. By using the gravitational pull of equatorial oblation to precess the satellite's orbital plane at a mean rate of 360 degrees per year.

Answer: (D)

Sun-synchronous orbits are achieved by using the Earth’s oblateness to make the orbital plane drift at just the right rate. The Earth isn’t a perfect sphere; its equatorial bulge produces a small gravitational torque that slowly changes the orientation of the orbit, specifically causing the line of nodes (where the orbit crosses the equator) to rotate westward. By picking the right altitude and a near-polar, retrograde inclination, this nodal precession rate can be tuned to match about 360 degrees per year. When the plane precesses at that rate, the satellite passes over the same local solar time on each orbit, keeping lighting conditions consistent for imaging or remote sensing. This is the mechanism that makes a sun-synchronous orbit possible. The other ideas don’t produce the required, repeatable alignment with the Sun’s position: simply firing a motor at sunrise, or aiming for more daylight crossings at the equator, or balancing solar and Earth gravity, don’t establish the steady 360-degree yearly precession of the orbital plane.