We and our planet are the beneficiaries of millions of nuclear fusion reactions taking place every second inside the sun's core. Without those reactions, we wouldn't have any light or warmth, and probably no life. A fusion reaction occurs when two atoms of hydrogen collide to create a larger helium-4 atom, which releases energy. Here's how the process works:
Fusion can only occur in super-heated environments measuring in the millions of degrees. Stars, which are made of plasma, are the only natural objects that are hot enough to create fusion reactions. Plasma, often referred to as the fourth state of matter, is ionized gas made of atoms stripped of some electrons. Fusion reactions are responsible for creating 85 percent of the sun's energy.
The high level of heat required to create this type of plasma makes it impossible to contain the components in any known material. However, plasma is a good conductor of electricity, which makes it possible to be held, guided and accelerated using magnetic fields. This is the basis for creating a fusion-powered spacecraft, which NASA believes is achievable within 25 years. In the next section, we will look at specific fusion engine projects in development.