As humans expend into the Solar System and then beyond, in thee Universe, there will be a need to trade. Some places will provide us with metals and heavy metals, others with water, others with minerals needed for life. In other places we might find no water, while in others we will find excellent conditions to grow plants and to produce food. Industrial corporations will also need trade routes. On the other hand, as we keep producing garbage, toxic waste and scrap, we will need to dump or recycle all. Third, there will be tourist attractions that will be worth visiting.
Depending on conditions, each part of the Solar System (and other solar systems) will require different types of transportation.
Space Bases Edit
It will be more easy to send one large cargo freighter from Earth to Jupiter System or to Saturn System then to send many small ships towards each moon instead. Large ships can be over 1 km long. At that size, a ship will be almost impossible to land on a planet or a large moon, the gravity will tear them apart. However, a giant ship can travel without problems in a world without gravity and can dock to large orbital stations and asteroid bases. From there, it will be more easy to move cargo with smaller ships, able to land on their target planets and moons.
Mercury. For Mercury, an orbital station can exist. However, because of the proximity to the Sun, there are strong gravitational perturbations. This is why Mercury has no moon, there are no stable orbits. The Messenger orbiter has crashed into Mercury when it ran out of fuel. So, an orbital base around Mercury will have to use fuel from time to time to remain in a stable orbit.
Venus. Around Venus, a satellite will be slowly influenced by the Sun and Earth. However, it can remain in orbit for a very long time. Space ships that explored Venus were deliberately deorbited at the end of their mission. An orbital station that will circle the planet around one Earth day would be safe for millennia.
Earth. We have already one international space station, but it needs to turn on its engines from time to time, to remain at its current orbit. At a higher altitude, the atmospheric drag does not exist. An orbital station placed at a higher altitude would be able to also service colonies on the Moon.
Mars. Mars has the advantage of two small natural satellites. We can use any of them as a space base. Their orbits are safe for millennia. Perhaps we could use Phobos as the main logistics base and Deimos as a backup and quarantine station.
Ceres. As shown by Dawn spacecraft, orbits around Ceres are safe. Dawn itself will remain a perpetual satellite of Ceres. So, it is possible to set an orbital base there too.
Jupiter. The giant planet has many small moons both between the planet and the Galilean moons and outward from them. The inner moons are, however, inside Jupiter's deadly magnetosphere, so there would not be safe for a colony. As for the outer moons, many are retrograde or have high angle orbital planes. From all the outer moons, by far the best candidate for a base is Himalia.
Saturn. From all the planets, Saturn has the largest cohort of moons. The inner moons are inside the main rings or other diffuse rings. The outer moons are usually retrograde. Still, there are a few moons that are co-orbitals (Trojans) of some of the larger moons. Their orbits are cleared of dust and other space debris. One of the best candidate for a space base is the moon Helene, a Trojan of Dione.
Uranus. What is interesting is that Uranus has some small inner moons (between the planet and Miranda). The innermost moons are inside the rings, but there is one, Puck, which could be just perfect for a base. The outer moons are usually retrograde, with the exception of Margaret, which has a high orbital angle plane. Together with Puck, another small moon, Perdita, might also be suitable for a space base.
Neptune. Compared to other planetary systems, Neptune is the most complex. The largest moon, Triton, the only one suitable for terraforming, is retrograde. There are two other moons larger then 100 km, both prograde: Proteus and Nereid. However, Nereid has an extremely elliptical orbit. Navigation between these celestial bodies is extremely difficult. Neptune also has inner moons (prograde, very close to the rings) and outer moons (both prograde and retrograde, with high angle orbital planes and highly elliptic orbits). Building a space station on any of the small, inner or outer moons, would be possible, but not feasible. Since Triton is the largest moon and the most important destination for settlers, it would be better to build an orbital station with a retrograde and circular orbit, sharing the same orbital plane with Triton. However, to make the base functional also for Proteus and Nereid, it has to be further away from the planet, where changing the orbital direction does not require so much fuel consumption.
Pluto. From all the Kuiper Belt objects, Pluto is the most known and probably the best target for future settlers. Pluto has the advantage of four small moons, that can be used for building an orbital station.
Today, on Earth, large ships travel across the oceans for long distances. Then, freight is transported by train or by truck from docks to inland areas. Inside a city, freight is moved from railway stations or warehouses close to highways with smaller cars to each place where it's needed. Sending cargo with small vehicles over long distances is not feasible. Still, goods that need to be delivered fast are shipped by airplanes and from each airport to their destination they are moved by small vehicles. In the same way, depending on costs, passengers are usually transported by air or by train on long distances, then by smaller trains, busses or minibus services for shorter distance. Similar methods might prove feasible for interplanetary and interstellar travel.
Interplanetary cargo freighters. Launch windows don't occur too often (see data below). Freight can wait even years for departure and other years will be spent in deep space. Also, it is cheaper to launch a giant ship once then to send many small ships. An interplanetary cargo freighter should be very big, measuring over 1 km long. Some models might be even up to 10 km. Such big vehicles are unable to land on a planet. Instead, they will be docked at an orbital space station or at a small satellite. At least for the first period of space travel, interplanetary cargo freighters will use the most low-cost trajectories. They will transport cargo, but they might be as well equipped for passengers.
Interplanetary passenger transport. People will not want to wait for the slow, low-cost trajectories used for cargo. Instead, they will prefer faster routes, but not the fastest. Still, with current technology, a space voyage will take months or years to complete. A cargo freighter from Earth to Saturn will probably use multiple Mars and Jupiter flybys, but a passenger ship will take the direct route. Even so, launch windows will not occur much more often.
Planetary space transport. There will be smaller spaceships connecting space stations with the planet or with its moons. These ships, designed for freight, for passengers or both, will offer local transport services. Ships that will service larger planets and moons will be equipped with thermal reinforced fuselage for aerobreaking, with parachutes or wings for landing and also with strong engines, for liftoff. Ships that will connect bases with asteroids and small moons will be less robust.
Redirect ships. This is a special category of space vehicles. They are small, but with highly efficient engines. They are designed to move asteroids and comets. Redirecting celestial objects is vital to avoid collisions, to bring water and volatiles to terraformed bodies with untable atmospheres and to bring metal asteroids to metallurgical corporations.
Interplanetary small ships. It makes no sense to send a huge cargo freighter towards a small asteroid or a Kuiper Belt object, where colonies are smaller and the needed amount of goods is small. While mining industrial corporations might still request a huge cargo freighter, small settlements will only need smaller vehicles. However, unlike those used in planetary transport services, these ships will be able to travel long distances.
Personal ships. Probably at a certain point personal ships will be like personal cars today. At that moment, interplanetary travel will be completely different.
Inner, Rocky Planets Edit
This includes Mercury, Venus, Earth, Luna and Mars. For all 5, the major problem is the escape velocity a spaceship must achieve in order to detach from their surface. As for now, the only feasible technology to leave a planet is the use of chemical engines. Landing on the surface can be more easy, because if terraformed, all will have atmospheres, to allow aerobreaking. The escape velocity is as follows:
Mercury: 4.3 km/s Venus: 10.3 km/s Earth: 11.2 km/s Moon: 2.4 km/s Mars: 5.0 km/s.
As one can see, detaching from Venus and Earth will be the hardest of all. For Mercury and Mars, the needed amount of fuel will be half. For Moon, it will be only 1/6 of Earth's. This clearly shows that sending goods from Earth into space will be more expensive then sending goods from Mars.
A good way to facilitate space traffic will be to build orbital stations around each planet. For Mars, this will be more easy, since we can use its two small moons. In this case, large landing ships will regularly connect the planet with the orbital station, facilitating transport of cargo and people. Since bases will be close enough to the planet, launch windows will occur daily (for an Earth's 24 hours). Also, ships will return daily from the station to the ground base.
Traveling between planets will not be so often. The planets must be aligned in a specific way to allow the cheapest and most effective way to travel. This is called the launch window. Passenger ships will need to travel faster, sometimes using a more expensive route, while cargo ships will use the most low-cost effective route. This might include a number of gravity assists from other planets.
Traveling between Venus, Earth and Mars can be done with the low-consumption ion engines. Traveling to Mercury, because it has to break Sun's great gravity, will have to use something more powerful, probably including gravity assist from Venus. Sending a ship from Mercury will be more possible with a solar sail.
Good launch windows will occur at the following rate:
Mercury - Venus: 143 Earth days Mercury - Earth: 115 Earth days Mercury - Mars: 101 Earth days Venus - Earth: 584 Earth days (1 year and 219 days) Venus - Mars: 337 Earth days Earth - Mars: 777 Earth days (2 years and 47 days).
These windows are calculated in a simple way, when the planets are aligned. However, many other launch windows exist, depending on what kind of propulsion you use, on gravity assists or on deep space maneuvers.
Note that, the bigger the difference between orbits, the more often launch windows occur.
Rocky Planets - Giant Planets Edit
The outer, giant planets, move far slower then the inner planets. As the inner planet makes a complete rotation along the Sun, the outer planet moves only a bit. So, if we wait the inner planet to move a bit more, both will be in the right place. Simple calculations show that the launch windows occur at a rate a bit longer then the inner planet's year:
Mercury - Jupiter: 90 Earth days Mercury - Saturn: 89 Earth days Mercury - Uranus: 88 Earth days Mercury - Neptune: 88 Earth days
Venus - Jupiter: 237 Earth days Venus - Saturn: 230 Earth days Venus - Uranus: 227 Earth days Venus - Neptune: 226 Earth days
Earth - Jupiter: 398 Earth days Earth - Saturn: 377 Earth days Earth - Uranus: 369 Earth days Earth - Neptune: 367 Earth days
Mars - Jupiter: 816 Earth days Mars - Saturn: 733 Earth days Mars - Uranus: 702 Earth days Mars - Neptune: 695 Earth days.
As one can see, launch windows occur sometimes more often between inner and outer planets then between inner planets. Spaceships launched from the inner planets can use the advantage of ion engines and solar sails, to achieve the desired speed. As they depart from an outer planet, ships will use the advantage of low solar gravity, therefore they can change their orbit more easily. When a ship reaches a gas giant, its gravity can capture the ship more easily. By contrast, as the ship heads for a rocky planet, it will reach its target with great speed. To enter in orbit, the ship can get closer to the atmosphere and use aerobreaking.
Between Giant Planets Edit
The gas giants move much slower. The best launch windows are more rare, as seen below:
Jupiter - Saturn: 19.9 Earth years Jupiter - Uranus: 13.8 Earth years Jupiter - Neptune: 12.8 Earth years Saturn - Uranus: 61.8 Earth years Saturn - Neptune: 35.7 Earth years Uranus - Neptune: 174 Earth years.
At this point, it is clear that using the low-cost orbits will not be a good option for the giant planets. Transport companies would like to have a flight at least every 5 Earth years. So, they will look for any possible routes. Because of the high distances, ships will need a lot of time to travel.
Around Gas Giants Edit
The systems of moons that orbit all 4 gas giants offer unique advantages. First of all, the moons have low gravity. Launching a spaceship from their surface will require far less costs then in case of planets. Please take a look at the following values:
Earth: 11.2 km/s Io: 2.558 km/s Europa: 2.025 km/s Ganymede: 2.741 km/s Callisto: 2.440 km/s Titan: 2.639 km/s Triton: 1.455 km/s.
For other moons, the escape velocity is even slower, usually below 1 km/s.
Launch windows will also be often, at intervals of less then 100 Earth days. The trip will also not take too much, about a few days or up to an Earth month. In these conditions, transport companies can offer scheduled service between all colonized moons. Since there are not many stable orbits around the moons, there will not be many orbital stations. However, it should prove highly effective to use a small natural satellite as a station for interplanetary travel.
Many industrial corporations and mining facilities will be on asteroids. The main advantage of asteroids is their very low gravity, that makes them excellent targets for spaceships.
Each asteroid has its own launch windows towards other asteroids and planets. However, we can expect more cargo and less passengers at an asteroid. Cargo will be transported when conditions allow at lowest cost, probably as large convoys of ships. Passenger ships will not detach towards all directions, but more often towards one larger base of a nearby planet.
A major problem is that asteroids have low gravity. So, the spaceship cannot be captured by their gravity. Ship trajectory must be calculated so that the spacecraft will reach the target with nearly the same speed the asteroid is moving. So, the ship must use an engine to increase or decrease speed in the vicinity of its target. The most efficient way seems to use ion engines, like Dawn used to reach Ceres and Vesta.
For the largest asteroid, Ceres, the flight windows are as follows:
Mercury - Ceres: 93 Earth days Venus - Ceres: 260 Earth days Earth - Ceres: 466 Earth days Mars - Ceres: 1161 Earth days Ceres - Jupiter: 7.53 Earth years Ceres - Saturn: 5.42 Earth years Ceres - Uranus: 4.88 Earth years Ceres - Neptune: 4.77 Earth years.
Kuiper Belt & Oort Cloud Edit
At that far, ships will find another problem. They will accelerate, to reach their distant targets in limited time, but once they arrive, they have to decelerate, to land. Dwarf planets and asteroids have little gravity and cannot capture on orbit a fast moving ship. Also, at current technology, the cruise will take years (for Kuiper Belt) and decades (if not centuries) for Oort Cloud.
It is less plausible that a scheduled service can exist between the small Kuiper Belt colonies and each planet. Most probably, there will be flights on demand or scheduled flights connecting one inner planet with an outer planet in the Kuiper Belt. At each 12 Earth years, Jupiter is in the right place for a gravity assist, for each Kuiper Belt object. At every 29 Earth years, Saturn is in the right place. And again, at every 59 Earth years, both Jupiter and Saturn can be used for a gravity slingshot. This scheme shows that scheduled transport can exist between the rocky planets and Kuiper Belt.
Inside Planetary Systems Edit
Mercury and Venus don't have moons. Flight windows to their orbital stations will occur very often.
Between Earth and the Moon, flight paths occur nearly every day (the Moon is nearly at the same coordinates in the sky at every 25 hours).
Around Mars, flight windows between Mars and the moons and flight windows between the moons occur daily.
Around Jupiter, perfect alignment between moons occur from time to time:
Almathea: about twice every Earth day
Io - Europa: 3.52 Earth days Io - Ganymede: 2.35 Earth days Io - Callisto: 1.97 Earth days Io - Himalia: 1.78 Earth days
Europa - Ganymede: 7.04 Earth days Europa - Callisto: 4.51 Earth days Europa - Himalia: 3.60 Earth days
Ganymede - Callisto: 12.5 Earth days Ganymede - Himalia: 7.36 Earth days
Callisto - Himalia: 17.9 Earth days
For Saturn, there are also many launch windows between moons:
Mimas - Enceladus: 2.95 Earth days Mimas - Tethys: 1.88 Earth days Mimas - Dione: 1.44 Earth days Mimas - Rhea: 1.19 Earth days Mimas - Titan: 1.00 Earth days Mimas - Hyperion: 0.99 Earth days Mimas - Iapetus: 0.95 Earth days Mimas - Phoebe: Earth days
Enceladus - Tethys: 4.79 Earth days Enceladus - Dione: 2.74 Earth days Enceladus - Rhea: 1.97 Earth days Enceladus - Titan: 1.50 Earth days Enceladus - Hyperion: 1.46 Earth days Enceladus - Iapetus: 1.39 Earth days Enceladus - Phoebe: 1.37 Earth days
Tethys - Dione: 5.94 Earth days Tethys - Rhea: 3.24 Earth days Tethys - Titan: 2.14 Earth days Tethys - Hyperion: 2.07 Earth days Tethys - Iapetus: 1.93 Earth days Tethys - Phoebe: 1.88 Earth days
Dione - Rhea: 6.89 Earth days Dione - Titan: 3.30 Earth days Dione - Hyperion: 3.14 Earth days Dione - Iapetus: 2.83 Earth days Dione - Phoebe: 2.72 Earth days
Rhea - Titan: 6.30 Earth days Rhea - Hyperion: 5.74 Earth days Rhea - Iapetus: 4.79 Earth days Rhea - Phoebe: 4.48 Earth days
Titan - Hyperion: 60.1 Earth days Titan - Iapetus: 20.0 Earth days Titan - Phoebe: 15.5 Earth days
Hyperion - Iapetus: 29.1 Earth days Hyperion - Phoebe: 20.4 Earth days
Iapetus - Phoebe: 65.8 Earth days
Around Uranus, many launch windows occur between its moons:
Perdita - Puck: 1.19 Earth days Perdita - Miranda: 0.963 Earth days Perdita - Ariel: 0.832 Earth days Perdita - Umbriel: 0.743 Earth days Perdita - Titania: 0.665 Earth days Perdita - Oberon: 0.657 Earth days Perdita - Sycorax: 0.633 Earth days
Puck - Miranda: 1.28 Earth days Puck - Ariel: 1.06 Earth days Puck - Umbriel: 0.916 Earth days Puck - Titania: 0.800 Earth days Puck - Oberon: 0.790 Earth days Puck - Sycorax: 0.755 Earth days
Miranda - Ariel: 2.92 Earth days Miranda - Umbriel: 2.06 Earth days Miranda - Titania: 1.55 Earth days Miranda - Oberon: 1.51 Earth days Miranda - Sycorax: 1.39 Earth days
Ariel - Umbriel: 5.68 Earth days Ariel - Titania: 2.99 Earth days Ariel - Oberon: 2.86 Earth days Ariel - Sycorax: 2.44 Earth days
Umbriel - Titania: 5.60 Earth days Umbriel - Oberon: 5.15 Earth days Umbriel - Sycorax: 3.92 Earth days
Titania - Oberon: 14.7 Earth days Titania - Sycorax: 7.63 Earth days
Oberon - Sycorax: 10.7 Earth days
Around Neptune, everything is far more complicated. The fact that Triton is retrograde and Nereid has a highly elliptical orbit makes all flight routes very difficult. Flying between a prograde and a retrograde moon requires huge amounts of fuel, to change speed direction. Much more easy would be to fly from one moon far away, at the end of Neptune's Hill sphere, change direction there (where orbital speed is very low) and then return to the target moon. In order to save fuel, such maneuvers would require multiple flybys around Triton and other moons, to gain or lose momentum.
Around Pluto, a flight between Pluto and Charon can be done at anytime and with the same fuel consumption, since both bodies are tidal locked. Flight paths between Pluto and the outer moons occur at the same frequency as flight paths between Charon and the outer moons.
Intertellar Travel Edit
Main article: Interstellar Travel
A zero-generation ship will travel towards a nearby star in more then a human lifetime. We already have the Pioneer and Voyager spacecrafts doing this. Reaching Barnard's Star in 500, 1000 or even 10000 years, is not what a space pioneer would dream about. However, it is possible that, at some point, someone will try this. A Soviet sci-fi writer has proposed the use of a comet. There will be a base on the surface of the comet. Settlers will use deuterium (found on the comet) in a nuclear fission reactor. Energy from the reactor will be used to power-up a rocket chemical engine, using hydrogen and oxygen from the comet itself. Settlers will travel for almost 1000 years, slowly increasing speed, until there is nothing left from the comet. Then, at the destination, the colony passes very close to a giant planet and to the star, decreasing the speed via aerobreaking and with the help of gravity assists.
First generation cargo and passenger ships will travel with 5 to 10% of the speed of light. This allows settlers to reach the nearby stars within their lifetime. They will be on one-way trips. Each ship must accelerate to the designated speed, then, once it reaches destination, it must slow down, to allow settlers and cargo to land on a planet, to terraform it.
Second generation ships will move with a significant percent of the speed of light, 50% or even 80%. This will allow us to colonize the stars that are within 10 or 20 light years away. At this step, ships can return to Solar System, to take other settlers or other cargo.
Can trade routes be imagined at this speed? It is hard. What merchant will like to invest in an expensive transport, getting its money back only after 30 years? Communications, even with the speed of light, will take many years to reach back Earth. And even so, the radio signal will be extremely weak. There will be more efficient to send messages onboard the ships, just like was done in the middle ages. The first settlers will fly blind, towards a star system where they don't know well what could be found. Just imagine what this could mean. Dreaming of a perfect democracy, they might end-up sold as slaves at the destination. And nobody would ever know.
Probably, the first and second generation of ships will only transport people and the smallest needed amounts of materials and equipment to build a Terraforming Plant. The trade routes will be paid by states inside Solar System, willing to extend their domains on other stars.
However, third generation ships will fly above the speed of light. At that point, reaching a nearby star or even distant stars within the galaxy, will not be so difficult and might need less then an year. Interstellar trade routes might appear, with regular cargo and passenger ships.
The future Edit
All calculations shown above are made for current technology. However, as colonies will grow and new technologies will be discovered, the time and cost of a space flight will gradually decrease.
The first pioneers in America, Australia, the Pacific or African colonies, were used to receive news from home once a few years. Ocean ships made many months on the way to their destinations. In many cases, settlers received no visitors for over 10 years. However, as colonies grew and became independent states and as technology reached new levels, everything changed.
In the beginning, there will be rare and long flights. We might see that scheduled flights will appear between the rocky planets first, then there will be regular flights between Earth and the gas giants. Then, other scheduled flights will appear between Earth and some asteroids. Soon, inter-satellite transport will develop around the gas giants. Then, by the time scheduled transportation will be needed between gas giants, we will need extra flights between the inner planets. People and cargo will no longer wait up to two years for a flight towards Venus or Mars. Then, regular flights will take us to the Kuiper Belt.
When we will invent vehicles that will travel with up to 50% of the speed of light (see Interstellar Travel), we will also improve transportation inside the Solar System. The problem is that humans cannot resist for many days when acceleration increases above 1G. At an acceleration of 1G, the ship increases speed with 9.8 meters every second. It will reach 35 km/s in an hour and 847 km/s (2.8% of the speed of light) in a day. At this speed, a spaceship will need 6.5 days to reach the orbit of Pluto, plus one day to accelerate and one day to decelerate. A flight from Earth to Venus will only take two days (the time to accelerate and decelerate) and the ship will never achieve 2.8% of the speed of light. It will only accelerate for half of the way and then it will slow down. Until we discover artificial gravity to counter the effect of acceleration, this will be the time limit for interplanetary travel.