Using nuclear fission as a power source for spacecraft is indeed possible and has been considered for certain missions. Nuclear fission involves splitting atoms to release energy, which can be harnessed to generate electricity for propulsion and other systems on board a spacecraft.
One example of nuclear fission being used for space exploration is the NASA spacecraft called the "Kilopower" project. It aims to develop a small, lightweight, and reliable nuclear fission system for potential use in future crewed missions to the Moon, Mars, and beyond. The system would provide a long-lasting power source, enabling extended stays on celestial bodies where sunlight is limited, such as during lunar nights or in the shadowed regions of other planets.
However, it's important to note that nuclear fission power systems would not enable spacecraft to travel faster than light. According to our current understanding of physics, the theory of relativity states that the speed of light is an absolute speed limit in the universe. Nothing can travel faster than light, so spacecraft, regardless of their power source, would be subject to this limitation.
While there have been some speculative concepts and theories, such as the concept of warp drives, which aim to achieve faster-than-light travel, these are still purely hypothetical and not currently supported by scientific evidence or engineering feasibility. As of now, the vast distances of interstellar space are prohibitively challenging to traverse within human lifetimes using conventional propulsion methods.