A planet’s gravity is far stronger than a spacecraft’s, meaning that when a spacecraft flies past a planet, the planet exerts a far stronger gravitational pull on the spacecraft than the spacecraft does on the planet. The planet gravitationally pulls in and tosses away the spacecraft, transferring some of its momentum to the passing vehicle in the process. And at the same time, the spacecraft actually robs the planet of a little bit of its own momentum. But that’s not all. Planets aren’t static; they orbit around the Sun and rotate around their own axis. So when a spacecraft passes by the planet, the planet’s rotation helps bends the spacecraft’s trajectory.