Introduction
Deep space exploration comes with many challenges. The biggest among those is fuel cost and inefficiency due to the high fuel mass required. The rocket equation relates the required delta v (change in velocity) to the required payload mass.
Δ𝑣= 𝑣𝑒 · 𝑙𝑛(𝑚𝑜𝑚𝑓)
The rocket equation logarithmically relates mass ratio to velocity change, meaning even a small increase in required velocity would demand disproportionately more fuel.
What this research paper aims to cover is the use of ISRU (In Situ Resource Utilization) as a solution to this challenge.
ISRU can be defined as the act of extracting and utilizing local natural resources on other astronomical bodies to support space missions and settlements. ISRU isn’t a very new concept and has been used in a few missions, notably the OSIRIS-REx (now called OSIRIS APEX) mission, which conducted a Touch-and-Go sample event on the surface of a Near Earth Asteroid (Bennu) and collected samples of its surface and delivered them back to Earth. https://science.nasa.gov/mission/osiris-rex
The thesis of this paper is to understand the feasibility of such operations and whether these NEOs can be used as sites for refuelling for space missions for deep space explorations.
Background
There are mainly 3 types of asteroids:-
1. C type (carbonaceous): These are the asteroids that have hydrated minerals and have a water content of about 5-20%. They contain organic carbon, water ice and other solar system materials.
2. S-type (stony): They have lower water content and contain some metals too.
3. M type (metallic): These contain mostly metals and have negligible water content
This paper will focus on C-type asteroids, including Bennu, 2008 EV5 and other asteroids.
A stopover concept would decrease the initial fuel mass from Earth. The probe could land at the NEO and collect icy regolith from the surface. Inside the same probe, microwave heating would be used to extract water from the regolith and then split the water molecules into their
constituent gases and store them in different chambers. These gases would be combined and burnt to generate thrust as and when needed by the probe. This would make a significant step towards advancing in deep space exploration.
Feasibility Analysis: The main problem is the feasibility of such a model and whether it would actually work in the medium to long term.
A. Technical Feasibility:
Gravity on such asteroids ( 𝑔𝑎) is extremely weak ( 10−4to 10−6of Earth’s gravity). So the inter-atomic forces between the grains of regolith are much greater than their weight due to the asteroid( 𝑚𝑔𝑎). This causes the regolith to behave like a fluid and causes challenges like difficulty in anchoring a probe to the asteroid and the containment of the regolith. When the probe lands, these grains gain enough velocity to overcome the gravitational force of the asteroid and go out into space.
However, these challenges can be overcome by using screw anchors or harpoons that hold the probe steady for enough time to collect enough regolith, and also adopting a low momentum approach so that the grains don’t go flying out into space. Microwave heating is the best approach to capture the water vapour from the regolith as it heats the regolith in a sealed chamber, capturing all water vapour emitted.
The last of these challenges is the amount of water that can actually be extracted from the regolith. Asteroids like Bennu or 2008 EV5 have about 1-5% of water by mass, which is a fairly large amount. Thus, if the efficiency of extraction is 100% the amount of water extracted would still be fairly less. To counter this, regolith extraction must be conducted in bulk or in amounts needed to continue to the destination of the probe. This would mean a space probe larger than the current sizes would have to be developed; however, since the initial launch mass would be less due to the less fuel, there wouldn’t be a very large change in the mass of the probe.
B. Economic Feasibility:
This is the main point of this entire model. If water can be extracted from the asteroids and can be used as fuel for space missions, then it would result in a lower launch mass from Earth, which would result in major cost savings. Currently, it costs about $2,000-$10,000 per kilogram of material to send to space. https://www.nextbigfuture.com/2025/01/spacex-starship-roadmap-to-100-times-lower-cost-launch.html.
However, studies show that extracting about 120 tons of water from space rocks represents a $2.4 billion in launch savings. https://nss.org/wp-content/uploads/2018/04/ad-astra-asteroid-mining.pdf
This would require a huge initial investment in ISRU technology and potential robotic mining missions. But the long-term benefits of this would not compare to the initial investment. It
would open up new avenues for deep space exploration, allowing us to explore bodies in our solar system like Jupiter trojans, the dwarf planet Ceres and many more.
Future Developments:
This is still quite a hypothetical idea, but it can surely be achieved by conducting first small-scale ISRU missions on the ISS or the moon, and actually sending a probe to collect regolith from a known asteroid in bulk and trying to convert it into fuel in the same probe. If this works, then in the long term, say in the next century or so, fuel stations would be present in space for deep space explorations and further space missions, advancing our capability in space technology.
Conclusion:
Thus, from this paper, we can conclude that asteroid water mining is technologically plausible and practical with investments and developments towards ISRU technology.
However, there is one last problem with such a technique, which is that the duration of these space missions would increase due to stopovers at asteroids. However, the cost savings most definitely make up for the increase in duration.
-Author – Abhijit NC
