In physics and nuclear chemistry, nuclear fusion is the process by which multiple nuclei join together to form a heavier nucleus - it is the same process by which stars release the energy that warms the planets in their orbit. It is accompanied by the release or absorption of energy depending on the masses of the nuclei involved. Fusion power refers to power generated by nuclear fusion reactions. In this kind of reaction, two light atomic nuclei fuse together to form a heavier nucleus and release energy. Until the development fusion power, space travel was simply not feasible - it required far too much energy to be achieved by any other means. Fusion power, a derivative branch of nuclear science, is the sole most abundant source of energy known to mankind. Though they are expensive, all starcraft larger than fighters are equipped with a fusion reactor, of necessity. Fusion reactors are much safer than earlier fission reactors and combustion-based power systems, for damage to a fusion reactor does not cause any sort of violent explosion, but rather simply extinguishes the reaction taking place within. This makes them ideal for use on warships, which are often put into dangerous conditions.