The discovery image of Halimede
Halimede is an outer irregular, retrograde moon of Neptune. It could be a captured asteroid or a fragment of the moon Nereid. Its diameter is around 60 km. It appears neutral grey in color and is made of water ice with some impurities. Not much is known about Halimede, except that it orbits Neptune in a highly inclined plain, retrograde and with an elliptical orbit. Being so small, it cannot be spherical. Also, it is not tidal locked. Given its small size, there is a high chance that it has a chaotic rotation.
It orbits Neptune in 1879 days.
Importance[]
In order to build colonies on the moons of Neptune, to terraform Triton and to paraterraform Nereid and Proteus, we need a space station. Very large ships, over 2 km long, cannot land or if they can, it is very risky. It is more easy and cheaper to transport stuff with a big ship between space stations then to use many small ships. Passengers and cargo can then be transported to each moon of Neptune with the help of smaller ships.
Challenging Conditions[]
At Neptune, the Solar Constant is very low. Solar panels are useless and the average temperature is -200 C. So, any base will require a nuclear generator to produce energy and heat. But this is not the main problem.
Navigation between the moons of Neptune is very difficult. Some moons are prograde, others are retrograde. Only the small inner moons orbit in the same plain and clearly have circular orbits. Triton, the largest moon and the only one that can be terraformed, is retrograde. Proteus, the second largest moon, is prograde and has a circular orbit. Nereid, the third largest moon, has a very elliptical orbit. Also, all the outer moons of Neptune have elliptical orbits and are very far.
In these conditions, it is very hard, nearly impossible, to find a suitable location for a space station. If one of the inner moons is selected, a ship going to Triton will have to use huge amounts of fuel to change trajectory course. With its highly elliptical orbit, Nereid sometimes gets too close to Triton, so that a ship will also require huge fuel consumption to change orbit.
The only solution for the Neptunian system is to use an outer satellite, which rotates very slow. Halimede is also orbiting retrograde, like Triton, but is far enough to orbit slowly enough for ships arriving from other moons to change course. There also are other outer moons, but their orbits are too far away.
Flight Windows[]
Trade Routes to Neptune must follow the flight windows to other planets:
Mercury - Neptune: 88 Earth days Venus - Neptune: 226 Earth days Earth - Neptune: 367 Earth days Mars - Neptune: 695 Earth days Ceres - Neptune: 4.77 Earth years Jupiter - Neptune: 12.8 Earth years Saturn - Neptune: 35.7 Earth years Uranus - Neptune: 174 Earth years.
These flight windows are calculated based on ideal flight routes, without counting an additional flyby.
As one can see, to the inner planets, flight windows occur almost at each local year. Flight windows to the other giant planets are more rare. Probably, ships will travel between the giant planets at every 1000 Earth days, at the expense of more fuel and longer times.
Based on this, we can calculate that a ship will depart from Neptune every 8.74 launches every Earth year or one launch every 42 Earth days. A space event (launch or landing) will occur at about every 21 days.
Inside the Neptunian system, flight windows are impossible to calculate because moons have sometimes very elongated orbits. Ideal windows are rare, at over an Earth year. Because of this, ships will fly more frequent, at the expense of fuel. Probably, there will be a ship at every 30 Earth days or so.
The main destinations will be Triton, Proteus and Nereid. With a launch every 30 Earth days, there will be 37 launches every Earth year or one at every 10 days. Together with arrivals, there will be one local space event at every 5 Earth days. This is the lowest rate of local space traffic within all planets that have moons.
The Base[]
Halimede base will face some challenging conditions. First of all, solar panels are useless. The base will instead use a nuclear generator to produce heat and energy. Secondly, almost everything will be frozen. Ships that use chemical engines will have a problem, as their propellant and oxygen tanks will need to be heated before takeoff. In addition, all fuels (for example hydrazine) will need to be heated before loading to a ship. Even ships that use a ion engine will require this, because their most used propellant, xenon, is also frozen at these temperatures.
Neptune offers only one moon that is suitable for terraforming, plus two additional moons that are suitable for large colonies. Therefore, the amount of goods and passengers shipped here will be lower then for Jupiter and Saturn and maybe even lower then in case of Uranus.
Halimede station will also face another problem. Because of complex orbital parameters, ships involved in local traffic will not be frequent. Passengers and cargo will have to wait sometimes for long. Space stations orbiting other planets have longer periods of time when not much people are stationed. In case of Halimede, there will be people waiting. So, the base must provide them with food, oxygen and drinkable water.
Because Halimede is far from another moon, oxygen and food cannot be brought frequently. So, the base must have its own enclosed ecosystem and must be able to produce all what people need.
A major problem comes from the fact that, if anything bad happens, it is hard to evacuate everyone. For this reason, the base must have enough stored food. Since the moon is made of water ice, drinkable water and oxygen can be produced in situ. Also, if anything bad happens to the nuclear generator, everyone will freeze soon. So, it is important that a second generator must exist, able to replace the main generator for a long time.
Keeping people in here for a long time also creates problems. Halimade space station must offer people something to do and medical services.
Spaceport[]
What is distinctive for Halimede, is that there will be long waiting times and that there will not be too much traffic. It is expected that ships will carry both passengers and cargo. The base will only need two platforms for large interplanetary ships and three platforms for local ships.
Each time a ship arrives, at first, the passengers will be served. After that, cranes will take care of the cargo. Before a ship departs, at first, all cargo is loaded. Then, passengers get inside. Moving passengers can be done with local flying cars, a task very easy in low gravity. Moving cargo will be done with cranes.
Given the low temperatures, almost all goods will be solid. Most of the cargo will be shipped with containers. Some special containers will require a certain inner temperature (for example, above freezing) and will be shipped fast, token to a storehouse where they can be heated or connected to a source of electricity.
Delta-v[]
The Delta-v shows the required change of velocity for a ship to travel from point A to point B. For Halimede base and Neptunian system, the delta-v budget could look like this:
Mercury Space Station – Neptune Halimede 24.564 Venus Space Station – Neptune Halimede 19.808 Earth Space Station – Neptune Halimede 18.033 Mars Phobos – Neptune Halimede 14.287 Ceres Space Station – Neptune Halimede 9.849 Jupiter Himalia – Neptune Halimede 8.619 Saturn Helene – Neptune Halimede 9.978 Uranus Perdita – Neptune Halimede 5.535 Neptune Halimede – Pluto Styx 1.114 Neptune Halimede – Eris low orbit 2.440 Neptune Halimede – Sedna low orbit 3.595
Neptune orbit (1000 km) – Neptune clouds 16.969 Neptune orbit – Proteus 7.696 Neptune orbit – Triton 10.297 Neptune orbit – Nereid 7.785(!) Neptune orbit – Halimede 7.210(!) Proteus – Triton 6.964 Proteus – Nereid 4.280(!) Proteus – Halimede 3.877(!) Triton – Nereid 4.183(!) Triton – Halimede 3.698(!) Nereid – Halimede 1.096(!)
Note: Values marked with a question mark (!) are for orientation. Nereid and Halimede have elliptical orbits, so the values vary significantly.
For comparison, an Earth surface - Moon surface flight will require a delta-v of 14.466.
Flights towards the inner planets require a huge delta-v, which can be reduced by gravity assists of Jupiter, Saturn, Earth and Venus. Flights to the outer planets come with a very small delta-v, but at such values, they will require decades or even centuries of waiting times. So, ships will spend more fuel to get faster to destination.
Delta-v requirements to travel between the moons of Neptune is not too high (in ideal positions). If perfect alignments don't occur, still, with a delta-v budget below 6, it is possible to travel at reasonable costs.
Feasibility[]
Because the moons of Neptune have complex orbits, flight windows are rare. Switching from a prograde to a retrograde moon requires high amounts of fuel. Ships will not wait for the ideal window to appear, since it will take years. Instead, they will more often, using more fuel.
Neptune has only one moon that is suitable for terraforming and two moons that are large enough to be of high interest. However, all moons appear to be made of water ice, tholins and volatiles. Temperatures are too low and solar panels are useless. Because of all these conditions, there will not be as many people willing to go to Neptune then to other, inner planets. With smaller cargo and passenger payloads, the price per kg or the price per person will rise.
In addition, because Neptune is very far, ships will request a long time to travel all the way to here.
People and cargo will need to stay for longer on Halimede then they will need on Perdita (Uranus), on Helene (Saturn) or on Himalia (Jupiter). They will also need to pay more for the time they stay.
Because of all these factors, Halimde base will operate at higher costs. But will it be feasible? Certainly, it will. It will be cheaper to send a single large ship to ferry between Earth and Halimede, with three smaller ships to carry all the goods around Neptune, then it will be to send three average-size ships to reach Triton, Proteus and Nereid. With its small gravity, Nereid could allow an interplanetary ship to dock, but the other two moons are too big. So, ships will need to dock somewhere anyway.
Alternatives[]
An alternative to Halimede could be to build an orbital space station. The best location should be at a similar distance, but with an equatorial and circular, retrograde orbit. Building the station will be more expensive then building a base on a moon. Maintenance will be more expensive. However, ships will travel at lower prices.
Another, more expensive alternative, is to change the orbit of Halimede into a circular and equatorial one.