Gravity Nu 3.0
visualise gravitational force effects in N objects system
create your N object system
program one object in the system
create log of object physical data and use produced data in your or third party software
Software package for visualisation of gravitational force effects between N objects.

Since version 2.07 it can also visualize spaceship/rocket trajectory programs for interplanetary travel (or any other like interstellar, intergalactic etc. if the position, velocity and mass of objects in the system can be defined for some moment in time).

In order to predict/calculate object data (position and velocity) in future or past time (within limits of calculation) one need object mass, initial position and initial velocity vector at some initial moment of time. 

Primary goal is to be as accurate as possible regarding numerical data - all other stuff (textures, themes, etc.) is for better understanding via visualization or just for fun. Although you can see planets and comets in Solar System this is the only free software (we are aware of) in which you can design and track your own objects (for instance, spaceship launched from Earth at some particular moment in time).

Once the initial object mass, position and velocity is defined you can see the behavior of system in time (forward or backward). You can create the system consisting of light (few kg objects) or heavy (planet, star) objects and see what happens in time.

You can use JPL HORIZONS data (EPHEMERIS) if you want to design (copy of) space system consisting of actual objects (planets, satellites etc.). If you use HORIZONS data all objects should share same 'real' initial moment in time (date & time).
Then, you can design 'real' impulse programmed object (spaceship/rocket) which trajectory is changed at some moments in time with thrust impulse applied  (the only present way to change trajectory after launch).