An optimal deflection method for Earth-Crossing Objects (ECOs) is formulated using a power limited spacecraft. To avoid the Earth impact, ECO's momentum is changed continuously using a power limited laser ablation system carried by the conceptual future spacecraft. The optimal operating duration and deflection angle variation history are estimated for various ECOs by using several power levels of ablation system. It is concluded that the timing of the operation start is very important. It is preferred to be as early as possible because the early start of operation requires relatively short operating duration with small operation angle variations. The method of continuous momentum change is more practical because there will be no fragmentation of ECO or unexpected ECO's orbital motions.