NASA/JPL-Caltech
Artist’s impression of the asteroid belt
Building cities in The Expanse style is harder than it seems, given that the weight of the fuel used by rockets to go beyond Earth’s orbit far outweighs the weight of the payload itself. Therefore, removing materials from asteroids is the only viable alternative.
Rome wasn’t built in a day, and a city on Mars will probably take even longer to build than Rome itself. At the time of the first Martian colonizers, it is likely that all of humanity’s industrial capacityincluding infrastructure to produce critical materials such as metals, is concentrated in the Earth-Moon system.
Although Mars has iron, it also lacks many of the materials needed to produce advanced materials such as boron and molybdenum. To alleviate this resource bottleneck, a new one, available in pre-publication on arXiv and led by Serena Suriano and a team of researchers, offers an alternative solution that seems obvious in theory but difficult in practice: extract the necessary material from Main Belt asteroids.
The difficulty in performing this extraction lies in orbital mechanics. It’s not as simple as pointing a spacecraft at a space rock, using the gas to get there, collecting the material and quickly returning to Mars. Requires a complex orbital maneuver and will likely also require the creation of deep space gas refueling stations.
To ground their logistics in short-term realities, the authors needed to use a supply chain with an existing cargo ship — specifically with technical specifications similar to those of SpaceX’s Starship. This theoretical vehicle has a dry mass of 120 tonnes, a payload capacity of 115 tonnes and a fuel capacity of 1100 tonnes. If this discrepancy between payload capacity and fuel capacity seems shocking, it is a good reference to complexity of the rocket equation — the weight of the fuel used to go beyond Earth’s orbit generally far exceeds the weight of the payload itself.
Fully fueled, this theoretical spacecraft can generate a maximum “delta-v” (or speed variation) of 6.4 km/s. And this is where the challenges arise. According to the authors, there are no metallic asteroids close enough to Mars so that a spacecraft can be launched, extract the metal and return to Mars low orbit (LMO) on a single tank of fuel. Most would require a delta-v between 10 and 12.8 km/s — roughly double the spacecraft’s capacity.
To solve this problem with orbital mechanics, the authors suggest a multi-stop supply chainin which the ship itself would make two stops. The first would be on a metallic asteroid to collect its extracted cargo. The second would be on a C-type asteroid, where “volatiles” such as water and hydrocarbons are abundant, and where propellant would be shipped to refill the system’s tanks using a technique called in situ propellant production (ISPP).
After refueling, the spacecraft could then return, with its cargo, to LMO orbit. According to the article, there are 22 distinct pairs of metallic asteroids and type C that align with the 6.4 km/s delta-v limit over a twenty-year launch window starting in 2040. Over those twenty years, a single spacecraft could use this multiple-stop schedule to deliver about 200 tons of metal back to Mars.
Why so little if the ship itself has a payload capacity of 115 tons? Simple: it would take a long time to carry out all these orbital maneuvers. One single trip in this two-stop architecture it would take about a decade. Part of this is due to orbital mechanics themselves — Mars and the asteroids need to be aligned correctly, which can take years. But another part is due to the slow ISPP (Integration of Particles on Mars) process.
According to Robert Zubrin’s Mars Direct 2.0 plan, the average ISPP rate would be about 2 kg/day. That’s right, kilos, not tons. To fill a 1100 ton propellant tank, it would be needed more than 1500 years — obviously, an unfeasible calendar. Therefore, drastically increasing the capacity of our ISPP (Integrated Steam Propulsion System), which is largely based on energy constraints, is fundamental to the functioning of this system.
But there may be another technology that solves the problem: non-chemical propulsion. This entire two-stop methodology is based on the premise of using chemical rockets. Systems like the solar electric propulsion or solar sails could fundamentally alter the calculations of this entire logistics system. As the authors correctly point out, we are still in the early stages of these technologies, so the possibility that they will be ready for a Mars resupply mission in 14 years is optimistic at best.
So for now, this “gas station” logistics system is the best we have. This article proves that is it physically possibleeven if it is extremely slow. And it presents clear technical obstacles that we need to overcome to make it faster. With the possibility of a city on Mars becoming increasingly real, overcoming these obstacles is becoming increasingly important, especially if we want to build it faster than Rome.
