If and when we ever start an asteroid mining industry, one of the most important decisions to be made in the structure of any asteroid mining mission will be how to get the resources back to where all our other infrastructure is – somewhere around the Earth. That solution will typically focus on one of two propulsion methodologies—chemical rockets, like the ones we already use to get us into space, or solar sails, which, while slower and unable to get us into orbit, do not require any fuel. So which propulsion methodology is better for these future missions? A study by researchers at the University of Glasgow looked at these two scenarios and came up with a clear answer – solar sails.
When answering these types of theory questions, it is essential to set limits on the answers. For example, billions of asteroids exist in the Solar System, so it is more realistic to consider only those known as near-Earth asteroids (NEAs). But even so, there are over 30,000 known NEAs. It would have been impossible for lead author Merel Vergaaij, then a Ph.D. university student and her colleagues to calculate optimized trajectories for each of them.
So they divided the area around the Earth into generalized orbital parameters – semimajor axis distance, eccentricity and inclination. With these three parameters, it is easier to understand what the general transfer orbit of an asteroid would look like in this region of space, some of which would be quite close to Earth on its orbital path.
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Other constraints were needed, such as calculating only the costs and benefits of transferring material back from the asteroid, not mining the asteroid itself. Getting the mining equipment there and setting it up was considered beyond the scope of this article. In addition, there had to be time to wait for the asteroid itself so that the return mission would have time to stockpile the material it would carry.
This material, in this calculation, was volatile. Volatiles such as water have been a focal point of discussions about asteroid mining, as they form the basis of rocket fuel that will be needed to explore deeper into the solar system and cost a lot of money to extract from Earth itself . Additionally, chemical rockets can then use some of these volatiles as their own propellant to return back to the Earth system.
A few more constraints arose, including sending the volatiles back into Geostationary Orbit (GEO), making some assumptions about launch costs based on projected Starship costs, and determining that all-important metric of economic research, net present value (NPV). NPV is the result by which decisions will be evaluated and based on various calculated factors. These would include various costs such as start-up costs, development costs, production costs and operating costs. Revenue will be calculated based on the expected value of the volatiles delivered into orbit. When the revenue exceeds the cost of the mission, the NPV becomes positive, which in this case indicates whether a mission to an asteroid in that area would be worthwhile.
To make this determination, the authors used a technique called a genetic algorithm to solve an optimization problem. Essentially, they gave the algorithm a bunch of parameters, such as orbital mechanics, spacecraft masses, and the amount of volatiles returned, and told the algorithm to optimize the all-important NPV value. The result of the algorithm was very clear – solar sails have positive NPVs for a wider variety of areas located in Earth’s orbit.
Mainly this is due to some weaknesses in chemical missiles. They had to use some of the material delivered to them to get back to GEO. And while their transfer orbit time was shorter, another factor from NPV, the discount rate that reduces the amount of a resource’s expected value the further into the future it is sold, doesn’t take enough of the value away from what solar sail can return, which would make it on par with a chemical missile.
There were still some regions of near-Earth space where even solar sails weren’t profitable, so the authors suggest that future asteroid miners are looking at asteroids in the specific regions they call potentially profitable if they’re looking for their first big object to digging. In addition, the researchers made some modifications to their original basic mission structure, such as stopping a lunar portal, adding a second trip, and running a series of variable simulations, known as Monte Carlo simulations, that would test the extent to which these different schemes were profitable.
The volatiles returned by the asteroid will be used for orbital refueling, as described in this UT video.
Both adding a second trip and stopping the Moon Portal instead of GEO added significant value to each type of mission architecture. Monte Carlo simulations also showed that their profitability is consistent with light input costs and variations in output revenue. Overall, although there are potentially winning purposes for each type of propulsion system, solar sails seem to be the clear winner between the two. Now it’s up to those hoping to build the first asteroid mining empire to listen.
Learn more: Vergaaij et al. – Economic Evaluation of High Thrust Propulsion and Solar Sail for Near-Earth Asteroid Mining UT – What is a Solar Sail? UT – How do you keep a solar sail stable? UT – A new kind of solar sail could allow us to explore hard-to-reach places in the solar system
Lead image: An artist rendering of an asteroid field. Credit: NASA/JPL/Caltech
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