For a long time Rocket Propellant-1, or RP-1, reigned supreme as the fuel of choice for the first stage of rockets. This highly refined form of kerosene, which was derived from jet fuel, powered the Saturn, Delta, Atlas, Soyuz rockets throughout the 20th century. It even served as fuel for modern rockets like the Falcon 9.

RP-1 has the benefit of being dense, which means a lot of fuel can be packed into a relatively small tank. However, RP-1 isn’t the most effective fuel at creating thrust, a measurement known as specific impulse. Liquid hydrogen, by contrast, has a really high specific impulse. But because it is not at all dense, it can’t efficiently be used as a first stage fuel.

This is one reason why a number of major new rocket engines developed during the last decade, including SpaceX’s Raptor and Blue Origin’s BE-4 engines, have been designed to use methane as a fuel. It represents a compromise between RP-1 and hydrogen—not quite as dense as the former, and with not quite as high a specific impulse as the latter. Methane is also useful if you want to go to Mars, because it is relatively abundant in the red planet’s thin atmosphere and could be used to refuel an ascent vehicle.