How space flight works

Pioneer simulates the physics of spaceflight to a high degree, so it is useful to know some concepts in for succesfull navigation and adventures. This article gives a brief explanation about the peculiarities of space flight, including momentum, deltaV, orbits and such. It also provides links to further readings around a web.

Movement and momentum

Pioneer simulates the motion of spacecrafts according to the Three laws of motion described by Sir Isaac Newton. Sometimes this is called Newtonian flight model.

1. In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force.

For our case this means that in space our spaceship will not stop moving ("drifting") when the engines are turned off, and we need to actively decelerate using thrusters. Similarly if we want to alter our course, we also need to apply thrust to the appropriate (opposite) direction.
This is because there's no atmosphere or other medium in space to slow as down gradually, as it would inside the atmosphere of a planet.
It is an important thing to get the hang of, because we need to start decelerating in time if we want to stop at our destination for example.
Same goes for rotation, which will happen, if the vector of thrust doesn't line up with the center of mass of an object. So if you want to rotate your craft, place thrusters off-center, but don't forget to place some on the opposite side, so you can stop rotating too.

2. In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object:F = ma.

In practical terms this means that the acceleration of a spaceship is governed by both it's mass and the thrust of it's engines. So with a less powerfull thruster it will take longer to change the direction the ship is flying, compared to a more powerful thruster. The thrust of the thruster is governed by the amount of it's exhaust and the speed that exhaust is expelled.
Useful to note that the maneuvering thrusters of spaceships are usually much weaker than the main thrusters, so it would take quite more time to decelerate using the sideways thrusters than it would by rotating the ship and using the main thrusters.
Also the acceleration of the ship is depending on how much cargo and propellant it has in it's belly. 

3. When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

This means that if for example something collides with something, they will push each other away proportionally to their mass. But don't try this with your spaceship unless you want to wreck it.

Also this is what makes rockets and any other type of engine work: throwing out material trough the nozzle of the rocket propells it the other way. Much like a garden hose on the loose. 
This is the other principle that governs the performance of the engines of spaceships: more exhaust expelled means bigger thrust, and the faster they are expelled, the more "fuel efficient" the engine. Ignoring the associated heat problems though.

Galilean relativity

Galilean invariance or Galilean relativity states that the laws of motion are the same in all inertial frames.

From practical standpoint for us, this means that any motion only makes sense relative to something. If I'm standing on the ground, the building next to me is practically motionless to me. Seen from a speeding train, we would appear to be moving at the same speed. But viewed from the Moon, we all fly around at quite a speed, and also rotating with Earth. And from the point of the Sun, Earth is flying around it in an eliptical path, and Moon is chasing it on a spiral path. And this whole system draws quite a spiral from the point of the center of the galaxy.
Of course there's no dedicated fixed point in space you can measure everything else to. But on the other hand this means you can pick any point as a point of measurement.
In Pioneer the main point of measurement is the star of a solar system (or the baricenter of multiple stars), and called frame of reference, or Frame for short. During flight your Frame will change depending on where you are in the system. If you get close enough to a planet or other body, your Frame will switch to that body automatically. One reason for this is convenience, but also it is needed for mathematical precision.
You can switch your reference frame manually too, by selecting any target while holding [Ctrl].