Callisto is the second largest moon of Jupiter and the third largest moon in the Solar System. Callisto offers some interesting opportunities for colonization, and some preliminary plans have already been made. Callisto could serve as an important waypoint before colonizing the outer planets of our Solar System.
The diameter of Callisto is about 4 800 kilometres, which is about the equal to diameter of Mercury. Mercury is a lot more dense, though, as Callisto is about the third of mass of Mercury. The gravity on Callisto is quite low, and it is only 13 % of that experienced on Earth, but supposedly this would be enough for humans, at least in the short term.
The core of Callisto consists mostly of ice and rocks. The core is surrounded by a large layer of liquid water, and on top of that, there is a crust made of ice. Because of its small size, Callisto doesn’t have a natural magnetosphere.
Orbit and rotation
Callisto is a moon of Jupiter, and it orbits around the Jupiter in about 17 days. Like most other moons, Callisto is tidally locked to Jupiter, so the same side always faces Jupiter. When compared to other big moons of Jupiter, Callisto is orbiting Jupiter quite far, and it is outside the strong radiation zones. Therefore, the amount of radiation is less than on the other moons, which makes it interesting object for human colonization.
Callisto has probably never been geologically active, so with the lack of strong atmosphere, its geographical features are mostly craters. The soil consists mostly of frozen water and rocks. In some areas, the amount of water is proportionally larger, making them practically large ice fields.
It has been estimated, that under the surface, Callisto would have huge seas of liquid water. These underground oceans could allow life to exist in their current state, but for human colonization they probably wouldn’t play a large role. However, the seas could also work as a location for permanent habitats.
Callisto has a very thin atmosphere, which consists mainly of carbon dioxide. The atmosphere is so thin, that without replenishing, it would disappear in to the space in only a few days. It has been estimated, that its constantly being renewed by carbon dioxide released from under the surface. There are signs that the atmosphere would also contain oxygen, but our measurements haven’t been able to detect this yet.
Callisto is located far from the Sun, so it is a cold place. On average, the temperatures are about -140 °C. The coldest temperatures are about -190 °C, and even the warmest temperatures are only about -110 °C. Therefore, the human habitats should be well insulated. However, cold temperatures aren’t a problem we couldn’t circumvent.
Callisto is orbiting Jupiter, which is quite far, so traveling there doesn’t happen in days. With advanced technologies, the travel could be done in a couple of years, and with more technological advancement, even faster. According to a study done by NASA, a round-trip to Callisto could be done in four years, and the astronauts would have time to study Callisto on its surface.
According to plans, it would be possible to build a base in Callisto. The base could help with harvesting the resources of Jupiter, and could also serve as a waypoint before moving on to explore the outer gas giants. Habitats could be built on surface, or they could be partially underground, possibly in the liquid oceans.
According to plans by NASA, the first trips to Callisto could be done in 2040s, and after this permanent habitats could also be built. NASA has suggested that robots and humans would work in cooperation, with the robots doing most of the dangerous tasks.
Because of the distant location, solar power would be quite useless on Callisto. Therefore, nuclear energy would be the best option for fulfilling the energy needs of the base. If we were to use fission reactors, the fuel for them could be brought form the Earth. If fusion reactors are developed to a working condition, the fuel for them, helium-3, could be brought from Jupiter.
Distance to Callisto is about the same than it is to Jupiter. When traveling at the speed of light, messages take from about half an hour to an hour to be delivered, depending on the position of the planets. Occasionally there will be communication problems, because Jupiter and the Sun can be located between the Earth and Callisto. Because of the long distance, robots can’t be easily controlled from the Earth, so they should be autonomous or managed by people close-by.
Callisto has a great resource, and that is water. With water, humans could survive for long periods of time quite autonomously, as it can be used to produce oxygen, also. Other resources of Callisto are not well known, but there is at least ammonia.
According to NASA, Callisto could be used as a location for rocket fuel production, and ships could be sent from there to explore the gas giants and the remote locations of the Solar System. The exact composition of Callisto is not known well enough, so we can’t say if this idea is feasible.
There are some plans for terraforming of Callisto, but there are several problems that prevent it from happening. First of all, Callisto is far from the Sun. Therefore, we should make it artificially warmer by heating it up with mirrors or causing a strong greenhouse effect. Because Callisto doesn’t have an atmosphere, we should create one, but as the gravity is quite small, there is a danger that it would escape in to the space.
Because of the small gravity, we could try to use heavy gases, such as xenon and krypton. Because these gases are heavy, they wouldn’t escape to space that easily. However, these gases are not common in the Solar System, so we should be able to produce them somehow. Another option would be to convert the ammonia of Callisto to nitrogen, for example with the help of bacteria.
After creating the atmosphere, a strong greenhouse effect should be created. We could use strong greenhouse gases, such as sulfur hexafluoride. The amount of sulfur hexafluoride in the atmosphere should be carefully controlled to keep the climate as stable as possible.
After the moon would be warmer, the huge underground oceans could pose a problem. If the surface ice would melt, the whole moon could become one big ocean. This would mean that the habitats would need to be built on ocean. Therefore, it might be a good idea to keep the temperatures relatively low, so the habitats could be built on ice glaciers.
Oxygen could be produced by electrolysis. Because the process is extremely energy intensive, creating an oxygen rich atmosphere for the whole moon would be really expensive. Therefore, it might be a good idea to not add oxygen to the atmosphere. People would need to use oxygen masks, but otherwise they could roam freely on the surface of the moon.
Callisto is a moon of Jupiter, which is located relatively far from the Earth. Callisto could serve as a good base to help with resource harvesting and exploring the outer planets. For humans, Callisto has many good qualities. It’s not volcanically active, there is plenty of water and it’s outside the radiation zones of Jupiter.
Problems include the distant location, coldness and lack of atmosphere. These are not problems we could not overcome, but building human habitats requires very costly operations. Robots could help with the building, doing to most dangerous jobs instead of humans.
Callisto could be terraformed in the future, but this wouldn’t be an easy task. The lack of atmosphere is a problem, so we should create it artificially, and also make sure it wouldn’t escape to the space. In the future, with more advanced terraforming technologies, terraforming Callisto could be possible.