Current space missions are relatively simple. Most of the resources needed for the mission are loaded on to the spacecraft before it leaves the Earth. There is usually a single target, and even if other celestial objects are fly-by objectives, the majority of the mission takes place at the ultimate destination. After that, the spacecraft is junked. At the moment, apart from what they take up from the Earth, spacecraft only use the energy from the Sun.
There are now incipient plans to undertake more ambitious missions. These include spaceships that hop between planets, pioneering manned missions to the outer solar system, and establishing permanent bases on the Moon and Mars for facilitating further explorations.
Similar to the first colonists visiting new land masses, these missions will have to use the resources available locally to support the effort. Taking all the required resources from the Earth would be prohibitively expensive. The approach is known as In Situ Resource Utilisation or ISRU for short. Solar energy is available throughout the system. The primary concerns of ISRU are rocket fuel, food, waste disposal and habitation. However, before the required resources are mined and used, they first have to be identified.
NASA plans to send a pioneering manned mission to Mars in the 2030s. Just going to Mars and coming back will take three years, and so there is no time for regular resupply missions, similar to the ones that take up additional consumables and experiments to the International Space Station. SpaceX plans to start setting up a human colony on Mars, with the first payload being being a propellant plant that can convert locally mined resources into rocket fuel. NASA and SpaceX are both in the planning phase of a number of pathfinding missions to identify suitable landing spots for future manned missions to Mars.
SpaceX plans to use its Dragon spacecraft, dubbed the Red Dragon for the Mars pathfinding missions. The idea is to scout the planet and identify suitable landing spots. NASA’s Mars Reconnaissance Orbiter (MRO) is also scanning the surface of the planet for signs of resources. Once promising sites are identified, NASA plans to dispatch rovers to the locations, to confirm the presence of the required resources.
There are two primary requirements for a suitable landing site, for both the colonising and pioneering endeavors. The site has to be near the equator to allow for optimum utilisation of the solar energy reaching the solar panels on the surface. At the same time, there is a requirement for water ice, preferably closer to the surface. The problem is that it is difficult to find large deposits of water ice in the equatorial regions, where the solar energy received is at its maximum.
NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) was an impact lander that slammed into the surface of the Moon in 2009 and found a surprising amount of water near the south pole of the Moon. The Lunar Reconnaissance Orbiter (LRO) has also provided supporting evidence of water on the Moon. NASA is planning the Resource Prospector mission, to validate technologies for harvesting of hydrogen, oxygen and water from the Moon. If successful, it would be the first interplanetary mission to demonstrate mining of local resources.
There are three kinds of rocket propellants that can be manufactured by ISRU. Liquid oxygen and liquid hydrogen have been used in the Space Shuttle and the Saturn V rocket that took humans to the moon. The Japanese H-IIA and the European Ariane 5 launch vehicles both use liquid oxygen and liquid hydrogen. Hydrogen peroxide is used for altitude control on spacecraft. Engines that use liquid oxygen and methane are in development.
On the Moon, it is possible to electrolyse the water ice to produce liquid oxygen and liquid hydrogen. The challenges involved are identifying the sites where abundant water ice is found close to the surface, and extracting it. On both the Moon and Mars, the water ice can be used to produce hydrogen peroxide, which can be used to power not only the rockets, but also the rovers and industrial equipment such as the drills necessary to extract the resources.
SpaceX wants to start off the Mars colony with a propellant plant sent to the red planet on the first colonial ship. A build up of a fuel depot is important to allow the rockets to come back to the Earth, and eventually explore the moons of the gas giants and beyond. The depot will use the Sabatier process. The law materials required are water ice and carbon dioxide, both of which are available on Mars. A high temperature reaction with a catalyst can provide the required methane, along with water. The water can be reused as a raw material for the process. The same approach can also be used to produce rocket fuel on Earth.
Pioneers and the eventual colonists on Mars will have to grow their own crops. NASA has used the Veggie system to grow crops on board the ISS. The system has been used to grow lettuce and cabbage. The system uses a combination of green, blue and red lights to provide the energy for the photosynthesis.
The challenges involved are identifying the specific varieties of crops that are ideal for growing in the low gravity environments. NASA is testing out a number of crops on the Earth to see which ones are ideal for growth in space. Making sure these crops are safe to eat is another concern. Potatoes provide twice the amount of food as compared to seed crops, for using the same amount of light. Potatoes are a good source of carbohydrates, while soybeans are a good source of proteins. Along with wheat and salad crops, a well balanced diet can be provided by the greenhouses.
The benefits of the portable greenhouses do not end at just providing the food. They can play an important role in waste and water recycling as well as air revitalisation. The plants can convert the carbon dioxide exhaled by the astronauts into oxygen and food. NASA engineers are working on inflatable greenhouses that can function more or less autonomously for months to years on the Moon and Mars.
When Elon Musk presented his plans for making human life interplanetary at the International Astronautical Congress (IAC) in 2016, one of the questions posed to him was about waste disposal. Specifically, if the first colonists would experience conditions similar to a festival in the desert, a struggle with few facilities for sanitation. Waste disposal is a challenge, but also an opportunity to harvest the garbage for scarce resources.
NASA is working on a trash reactor that can convert waste into rocket fuel. The reactor burns the waste at 537 degrees Celsius, which produces methane, oxygen and water. Using this reactor, about 4.5 kg of waste can be converted to 3.17 kg of fuel.
Another experimental waste disposal device, known as the Vortical Oxidative Reactor Technology Experiment (VORTEX) also burns up the waste. The system can be used to dispose organic waste with ash, water, heat and carbon dioxide as the by-products. Two of the products, ash and carbon dioxide can be used by the plants, as fertiliser and for photosynthesis.
The water can be used for a number of purposes. The crew can use the water for consumption, for the plants, or it can be converted into rocket fuel through electrolysis. The methane produced could also be used as fuel for the incinerator. Additionally, the inedible parts of the plants grown, can be burnt up in the incinerator.
A colony or a pioneering mission to Mars would require the use of construction of structures to house the humans, the equipments, the greenhouses for the food, and other facilities. The simplest approach is to use the locally available soil, or the regolith and convert it into the construction materials required for structures. The structures are then 3D printed by robots.
The structures on the Moon and Mars for human housing and facilities would require thousands of tonnes of concrete. This is to protect the humans, equipment, crops and provisions from a constant bombardment of radiation and micrometeorites. This just goes to show just how much of a protection the atmosphere provides to humans on Earth. It is impossible to ship so much concrete to the Moon and Mars, so the material would have to be locally manufactured. However, conventional approaches to making concrete is an energy hungry process. Additionally, cement mixers can not work in the low gravity environments.
A novel approach involves a new form of concrete known as bio-concrete. The binding agent used is an animal protein instead of limestone. Genetically engineered organisms can produce the protein in biological factories. It is then mixed with the locally available soil using vacuum technology.
The regolith on Mars has properties that make it ideal for making bricks. Research has shown that the iron oxide within the Martian soil acts as a binding agent. With minimal resources, it is possible to convert the Martian regolith into bricks, by just applying pressure. The bricks produced by the process so far, are tiny in size but stronger than steel reinforced concrete.
Researchers have demonstrated the viability of using 3D printers not just for structures, but for tools as well. Simple solvents and biopolymers, combined with the simulants of Lunar and Martian soil, have been used to create “inks” for 3D printing. These can then be used to create flexible and elastic structures, as well as bricks that fit together like LEGO blocks.
The rovers by NASA have identified silicon, calcium, chlorine, iron and titanium on Mars. These can be used to make a variety of materials for use by humans, including plastics, metals, glass and paper. NASA is investigating using robotic probes and machinery to harvest resources such as oxygen, water and hydrogen for life support systems. The Regolith Advanced Surface Systems Operations Robot (RASSOR 2.0) was used to demonstrate such an approach.
ISRO wants to get in on the action and compares setting up a base on the Moon to building a research outpost in Antarctica. The surface of the Moon is covered with soil and dust, the properties of which are considerably different from the soil and dust on the Earth. The dust in particular is so fine, that it moves almost like a liquid. ISRO wants to use the soil as the raw material for 3D printing Igloos. Towards this approach, ISRO has produced soil on Earth which mimics the properties of Lunar soil, at a fraction of the production cost of similar materials in the United States.
A permanent presence on the Moon can allow for testing of equipment and a launching platform for missions into the outer solar system. The manufacturing of some equipment, such as solar panels, as well as putting them into space, is theoretically easier on the Moon than it is on Earth.