Fusion reactions were discovered in the 1930s, but the importance of these reactions was not realised until 1939 when we found out that the Sun’s main source of energy was fusion reactions happening in its core. That is, hydrogen atoms fusing together to produce helium atoms and giving out loads of energy. When we realised that the main source of energy in the universe (stars like the Sun) was driven by fusion, we understood how powerful it was and started to think about how to control it on Earth.
It’s very hard to do fusion because the nucleus of an atom is positively charged, so when you try to stick 2 of them together – they repel each other unless you get them reeeeally close together. It has taken a long time to make progress because of this difficulty – but we have made a lot of progress! The JET fusion experiment in Oxfordshire produced 16 million watts of fusion power in 1997 (about 2 million light bulbs!). Scientists are building a new reactor at the moment that will produce 500 million watts … so we’re nearly there 🙂