The use of Hellerium is absolutely central to Earth's economy, and its sole source is Traders' Guild. Anyone who can afford the prices, uses standarized Hellerium reactors, from huge to small, for both flight and energy generation. Hellerium is digged up by Guild's subpart, SolMine, on asteroids and planets, some of them in our Solar System but most in the neighbouring ones (historically, Mars, but these deposits have all but ran out). Yearly excavation goes into many million tons and is fraught with danger; aliens from different parts of Stellar Empire coming over many light years to steal Hellerium.