Question

Once an object enters orbit, what keeps the object moving sideways?

Answer 1

Answer:

The right answer is inertia.

Explanation:

This name is given to the tendency of an object that is moving to remain that way, or if it is resting remain that way until an external force act upon it. Basically it means an object does what they are doing unless a force changes their speed or direction.  

Answer 2

It's the natural tendency of things to keep going unless there's something trying to stop them.

It's usually called "inertia".

Don't get the idea from all of this that things stop unless there's something to keep them going.  The truth is exactly the opposite:  Things keep going unless there's something to make them stop.