Universe Today
Rome wasn’t built in a day, and a city on Mars is likely going to take even longer to build than Rome itself. At the time of the first Martian colonists, it is likely that the entirety of humanity’s industrial capacity, including the infrastructure to make critical materials like metals, will be based in the Earth-Moon system. While Mars has some iron, it also lacks many of the materials needed to make advanced materials, like boron and molybdenum. To alleviate that resource bottleneck, a new study, available in pre-print on arXiv and led by Serena Suriano and a team of researchers, offers a workaround that seems obvious in theory but difficult in practice - mine the necessary material from Main Belt asteroids.
The difficulty in actually doing so lies in orbital mechanics. It's not as simple as pointing a ship at a space rock, hitting the gas to get there, collecting material once there, and high-tailing it back to Mars. It requires an orbital dance and will also likely necessitate the creation of deep-space gas stations.
Grounding their logistics in near-term reality required the authors to use a supply chain with an existing cargo spacecraft - specifically with technical specifications mirroring SpaceX’s Starship. This theoretical vehicle has a dry mass of 120 tons, a payload capacity of 115 tons, and a fuel capacity of 1,100 tons. If that discrepancy in payload capacity to fuel capacity seems shocking, it’s a good nod to the tyranny of the rocket equation - the weight of the fuel used to get anywhere beyond Earth’s orbit usually far outweighs the payload itself.
Fraser talks about a realistic Mars mission.Fully fueled, this theoretical vessel can generate a maximum "delta-v" (or change in velocity) of 6.4 km/s. And this is where the challenges arise. According to the authors, there are precisely zero metallic asteroids close enough to Mars that a spacecraft could launch, mine the metal, and return to Low Mars Orbit (LMO) on a single tank of gas. Most would require a delta-v of between 10 and 12.8 km/s - roughly double what the spacecraft is capable of.
To solve this problem with orbital mechanics, the authors suggest a multi-hop supply chain, where the spacecraft itself would make two pit stops. First would be to the metallic asteroid to pick up its mined cargo. Second would be a C-type asteroid, where "volatiles" such as water and hydrocarbons are abundant, and where propellant would be onboarded to refill the system’s tanks using a technique called in-situ propellant production (ISPP).
After refueling, the spacecraft can then make the journey back, with the cargo, to LMO. According to the paper, there are 22 distinct pairs of metallic and C-type asteroids 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 ship could use this multi-hop schedule to deliver around 200 tons of metal back to Mars.
Fraser talks about how hard it is to land heavy payloads on Mars.Why so little if the spacecraft itself has a 115 ton payload capacity? Simple - it will take a very long time to actually do all of this orbital dancing. A single trip in this two-stop architecture would take around a decade. Part of that is due to the orbital mechanics themselves - Mars and the asteroids have to be aligned correctly, which could take years. But another part is due to the slow process of ISPP.
According to Robert Zubrin’s Mars Direct 2.0 plan, the average rate of ISPP would be around 2 kg/day. You read that right - kilograms, not tons. In order to fill up an entire 1,100-ton propellant tank would take over 1,500 years - obviously not a feasible time schedule. As such, dramatically scaling the capability of our ISPP, which is largely based on power constraints, is a key feature to getting this system to work.
But there might be another technology that saves the day - non-chemical propulsion. This entire two-stop methodology is based around the premise of using chemical rockets. Systems like solar electric propulsion or solar sails could fundamentally change the math of this entire logistics system. As the authors rightfully point out, it's still early days for those technologies, so the possibility of them being readied for a Mars resupply mission in 14 years is hopeful at best.
So for now, this “gas station” logistics system is the best we have. This paper proves it is physically possible, even if painfully slow. And it offers clear technical hurdles that we have to clear in order to make it faster. With the possibility of a city on Mars becoming closer every day, overcoming those hurdles is becoming increasingly important, especially if we hope to build it any faster than Rome was.
Learn More:
S. Suriano et al. - Asteroid Mining to Sustain a Mars Colony: A Logistics Point of View
UT - Mining the Solar System to Build a New World
