Water is one of the most abundant chemicals in our solar system, and one that, when broken down through electrolysis, can be used to form oxygen and hydrogen. Those two gasses can then be burned as propellant, meaning that water electrolysis vessels (using solar power to break down the water) are fueled with a stable, safe, and abundant source of energy, perfect for vessels that need to pop around the solar system (this method does not provide enough thrust to launch from a planetary surface under normal circumstances). While no space company is yet focused on using this method, there being a distinct lack of intrasolar travel required, it is entirely doable with existing technologies. What is lacking is the water refueling stations scattered across the solar system. But once provided (if provided) they would ensure abundant fuel and energy.