In astronautics, a powered flyby, or Oberth maneuver, is a maneuver in which a rocket falls into a gravitational well, and then accelerates when its fall reaches maximum speed. The resulting maneuver is actually a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well. The gain in efficiency is explained by the Oberth effect, which is that the use of a rocket at high speed generates greater mechanical energy. In practical terms, this means that the most energy-efficient time for a spacecraft to burn its engine is at the lowest possible orbital periapse, when its orbital velocity (and so, its kinetic energy) is greatest. In some cases, it is even worth spending fuel on slowing the rocket into a gravity well to take advantage of the efficiencies of the Oberth effect. The maneuver and effect are named after Hermann Oberth, the Austro-Hungarian-born German physicist and a founder of modern rocketry, who first described them in 1927.