The asteroid's kinetic energy when it encounters the planet is too great for it to be brought into a bounded orbit by the planet's gravity; its kinetic energy is greater than its absolute potential energy with respect to the planet, meaning its velocity is higher than escape velocity. However, an asteroid's trajectory can be perturbed by another mass that could reduce its kinetic energy. If this brings the asteroid's velocity below the local escape velocity, its trajectory changes from a hyperbola to an ellipse and the asteroid is captured.