Spin stabilization is a simple and effective method of keeping a spacecraft pointed in a certain direction. A spinning spacecraft resists perturbing forces in the same way that a spinning gyroscope or a top does. Also because, in space, forces that slow the rate of spin are negligible, once a spacecraft is set spinning, the rate of rotation stays the same. With a spinner, there are inherent inefficiencies because only some of the solar cells are illuminated at any one instant and because most of the radio wave energy, radiating from the nondirectional antennas in all directions, is not directed at Earth. Thrusters are fired to make desired changes in the spin-stabilized attitude. They may require complicated systems to de-spin antennas or optical instruments which must be pointed at targets.