It seems you are confused about the definition of inputs and outputs of functions:
a function should look like this:
if you call this function you have to define input1, input2,... BEFORE you call the function.
When you call orbits the first time, there does not exist a Ro, so that gives an error
BUT in addition you overwrite all the input variables inside the function. you should not define Ro, Vo or del_t inside your functions if you want to take them as inputs (unless you want to transform these inputs, but i doubt that is what you want)
so you need to define initial values for Ro,Vo in the script (before the loop), and delete the values of del_t and Ro inside the function