Occupy Mars? No, occupy Bennu first!

When considering future Mars journey, we can easily compare it to the voyage of Christopher Columbus and the three small ships that sailed into the unknown. Hoping to find shorter path to Asia. But in reality, future trips to other space bodies are nothing like voyages into the unknown territory. In many ways, the paths and challenges to the Moon and Mars are well known. The real issue is not what we will find. Why we need to go is crucial question. But in this post I will concentrate on how.

Lunar Gateway provides many benefits as the main staging point for launching cargo and crew missions to Mars using the Starship. But the main issue is transporting huge amount of propellant from Earth to the edge of the Earths gravity well. That is why initial plans presented by Elon Musk focus on staging at LEO, where all of the propellant would be provided through rapid cadence of tanker Starships.

Lunar gateway could be supplied with water mined on the lunar poles, but it requires 2.5km/s of delta V. And carbon might not be easily available. Here comes the usefulness of asteroid mining.

Consider just a single near Earth asteroid, Bennu itself. Assuming 10% utilization of its mass for Starship propellant, it contains enough propellant for 6000 fully fueled Starships. It is rich in carbon and water. Plus the availability of earth like solar insolation 24/7. No nasty lunar nights. All of this propellant can be brought back to the Lunar Gateway in NRHO orbit for less than 1km/s of delta-V.

By NASA/Goddard/University of Arizona, Public Domain

In addition, Bennu has a certain hazard to hit the Earth in the next hundred years. Mining it could be used to gradually adjust its orbit. Not to mention rare Earth elements. Although asteroid mining will require lot of technology development, it makes a stepping stone that can be replicated on many other asteroids all the way to Mars and beyond. 

There is just one (but big) problem with asteroids, compared to the Moon. Its lack of (significant) gravity. Mining Bennu requires techniques that simply cannot be tested properly on the Earth. There is a reason why asteroid sample taking missions (Hayabusa, Rosetta and OSIRIS-REX) missions happened decades after Vikings on Mars. It will take a lot of technology development and testing that cannot easily be performed anywhere else. But once technology is perfected, propellent from the NEA will become much cheaper. Simply put, each kg of propellant at Lunar gateway requires at least a hundred times more propellant from the Earth. Fully fuelled Starship at the Lunar gateway will require a hundred Starship launches. No matter how cheap the launches get, it is too much. 

Sure you can do optimizations. Squeeze a second or two of Isp here and there. Use optimized tankers. Use electric propulsion between LEO and the edge of Earth gravity well. Or just move it to the nearest suitable asteroid with appropriate mining equipment. Thus Lunar Gateway is ideally positioned to become a fuel station for future ships. And NEA (including Bennu) are the most suitable as propellant, oxygen and water sources for this fuel station. From the edge of Earth Gravity well whole solar system becomes easily accessible.