Gravity and rotational Intertia assisted travel

There are methods to reduce the amount of energy used to travel in space by taking advantage of the gravity and rotational inertia of massive bodies. To use these methods, a celestial navigator must know much about the mass and velocity of these large objects, such as planets as moons, and even stars.

The first of these is called 'gravity assist' or 'gravitational slingshot'. This utilises a close approach to a massive body to change the spacecraft's trajectory relative to the massive object, which can be seen as an acceleration from the point of reference of other celestial bodies. The energy imparted to the spacecraft is subtracted from the linear momentum of the massive body, but the difference in mass between the two makes such loss negligible. Though, it may be worthy of note that this effect might be used to dispose of space junk, by a flyby of a spaceship to impart velocity at an appropriate angle to nudge the junk to fall into the atmosphere and burn up.

The second method is to use the LaGrange points, at which trajectories can be changed with little or no expenditure of energy. These points in space are caused by the orbit of one massive body relative to another, and can be expressed as the 'three body problem', with, for instance, the Sun, Earth, and Spacecraft representing the three bodies. These points have the peculiar property in that an object can orbit them, in spite of their being no object at the center and therefore no gravity. There are an infinite number of paths leading away from a LaGrange point, so a celestial navigator merely needs to pick one by leaving on a particular trajectory.

The Interplanetary Transport Network (ITN) utilizes a path through various LaGrange points in a system of massive bodies, such as a star system. The various courses are calculated ahead of time, by knowledge of system's bodies, their courses, and masses. The ITN is calculated for the solar system. However, the lowest energy course between to places is most likely not the most direct one, so the "(almost) free rides" on the ITN are slow. It reminds me much of taking a city bus system, and having to transfer buses many times. A direct route would be more like a helicopter from point A to B, and takes much more energy but can be completed much more quickly.

So, that's all well and fine for travel within the solar system. We can ship freight (ala "Surface mail") for little energy expense, but when you need to get their in a hurry, you need a more or less direct route (ala 2nd day air, or overnight) and have to expend a lot more energy.

What about Interstellar Travel? Well, we can certainly change direction from different points in the solar system, but when what we want is so far away, why not just launch directly from Earth (or some other leaving point) in that direction? I suppose we can hope to get to the outer edge of the solar system, where the sun's gravity is less and therefore less energy is needed to overcome it. And what about Gravity Assist? According to Wikipedia, the Gravity Assist as described here is only useful for travel with the sun as a reference point, and not travel outside the solar system. Though, and object coming from elsewhere in the universe, could use the energy of the sun's orbit around the Milky Way as an energy source, but that the means to do this are beyond our current level of technology. Of this, I am not sure. Of either the conclusion that Gravity Assist cannot help us in interstellar travel, or that use of the gravity of the orbit of the Sun around the Milky Way is beyond our means. For me, it means that I have to understand more of celestial mechanics in order to either believe what I've been told, or to disprove it or to find a loophole.

Even if the limitations of the gravity assist as currently understood by celestial navigators is true, there is still that possibility of 'swinging by' other star systems on a journey, we just need to figure out how.