What causes the variation in kinetic energy of a satellite in an elliptical orbit?
In an elliptical orbit, total energy is constant, but kinetic energy (KE = 12mv2) varies as speed changes with distance from the center (faster closer, slower farther), balancing the changing potential energy.
Ref: Halliday & Resnick, Fundamentals of Physics, 11th ed., Chapter 13: Gravitation.