The Interplanetary Transport Network is a system used by spacefaring entities to plan and track spaceflights between different Celestial bodies. The ITN consists of multiple planned pathways and sets of maneuvers that allow ships to get from any world in inhabited space to any other world. Gravitational assists and Hohmann Transfers make up the bulk of flight paths. The ITN was originally developed in 2046 by Dr. Joshua Svoboda at the United Nations Space Administration.
The most commonly-used flight path, with ships taking this path at an average of 2 per week, the Earth-Luna ITN flight path was the first to be used, although it wasn't pioneered by the UNSA. The Hohmann transfer-style flight path was first used in the 1960s and 1970s for the Apollo program by NASA and the United States. The path uses roughly 8km/s of delta-v (4km/s on both arrival and return trips), with an average travel time to Luna of between 3 and 4 days. At any time after arriving in Low Lunar Orbit, the ship can then again use a similar maneuver to leave Luna's sphere of influence and return into a highly-eccentric Earth orbit. Once the ships reaches perigee in Earth orbit, a retrograde burn can be performed to circularize the ship's orbit.
The more uncommon of the two Earth-Luna paths, the Cycler route is used by a series of 10 ships in the Earth system. This path is mostly used for autonomous cargo missions, although the path is human-rated, given the proper life-support is available. The flight path utilizes a highly-eccentric Earth orbit to repeatedly do flybys of both Earth and Luna. Once the performing ship is in LEO. A burn using 3.15km/s of dV can be performed to enter the specific cycler-orbit. Assuming the correct trajectory is achieved, the ship can stay in a stable orbit for years at a time, only requiring input in the event of a chance-of-course or any unexpected errors.
Growing in popularity, the Earth-Mars flightpath again utilizes a Hohmann Transfer available during a three-week long span once every 26 months on Earth. Assuming the performing ship is in LEO, a burn using up 3.6km/s can be performed in order to achieve a Mars-transfer trajectory. Travel time between Earth and Mars can vary depending on what is aboard the ship. For any ship carrying living beings, travel time is usually between 6 and 7 months. An unmanned cargo cruiser can sometimes take between 6 and 9 months. After arriving in Mars' sphere of influence, the ship can again perform a series of burns using 2.11km/s of dV to enter into a Low Martian Orbit. Around the same time as Earth-Mars transfers appear, Mars-Earth transfers also appear. This usually causes quite the stirrup in interplanetary travel when the planets align and dozens of ships depart both worlds.
Available at almost any time to any ship in LMO, Transfers from Mars to it's two moons are quite common for ships doing supply runs between Mars' surface and the sphere of one of the moons. To get to Phobos, Only about 1.2km/s of dV is needed, most of which is just used to get on an intercept trajectory. Very little dV is needed to get into orbit of the moon. This is the more popular of the two routes, usually taken by at least one ship every month.
To get to Deimos, a little more dV is needed due to Deimos' higher orbit relative to Mars, totaling in at about 1.7km/s dV needed. The burns required are very similar to that needed to reach Phobos. This route is less popular that the Mars-Deimos route, only taken by a few ships every year. Travel time between Mars and Phobos is roughly 4 days for a round trip, and travel time from Mars to Deimos is roughly 5 days for a round trip.
With transfer windows averaging at about every 20 months, Travel between Earth and Venus has the potential to become more common, though currently isn't very travelled. As of 2110, no civilian-carrying spacecraft has taken this route, as travel is currently restricted to UNSA-registered ships involved with the Aphrodite Program. Travel time between Earth and Venus lasts about 5 months and requires roughly 8.26km/s of dV. Venus is currently the farthest down-system location in the Interplanetary Transport Network, with all other possible stops farther from Sol.
Similar in use to the Earth-Venus route, travel between Venus and Mars is heavily restricted and is only used by ships involved with the Aphrodite Program. From Venus, transfers occur during a week-long span roughly every 11 months. Travel time between Venus and Mars takes 7 months and requires 3.14km/s of dV from Venus departure to arrival at Mars.
For travel departing Mars for Venus, windows also appear roughly 11 months with a 7 month travel time. This is only used for ships involved with the Aphrodite Program who have recently completed a run between Venus and Mars and are returning back to Venus to pick up more carbon dioxide. This time, almost double the dV is needed to cancel out Mars' orbital velocity for ships departing to Venus, totaling at about 6.08km/s.
The ITN route between Earth and Jupiter has never been attempted with a manned spacecraft, with the Zeus-1 mission being the first to attempt it in November of 2110. Transfers can occur during a week-long span once every 17 months. The route requires roughly 8.74km/s of dV, with travel time taking roughly 27 months.