I am trying to make renders of solar system planets, using realistic scales for them.
I have made Jupiter and its 4 main satellites. I have the right sizes for all of them but would like the distance between the satellites and Jupiter to be exact also.

Is there a way to lock the distance between two objects? I want to be able to move the satellites, but make sure they always stay on their orbit.

I have tried the track or link options, but they do not lock the distance between both objects (unless there is an option I did not see).

If you are creating an animation you should use "linked" and only move Jupiter around. On top of that you can "Lock" objects in the viewports by clicking on the symbol underneath the "Load Material" icon in the object properties panel and choose "Locked". Another nice one you'll find there is "Main view only". It frees the other viewports of cluttering.
If you first select a moon in the world browser it is still possible to move it.

offrench - you can do the following:

• choose the moon object/sphere
• then switch to the 'numeric' tab in the top right corner
• there are 5 buttons on the left side
• then select the fifth button called 'pivot position'
• here select the button next to the X-field called 'show pivot on/off'
• in the center of the moon object/sphere appears a green dot which is the pivot point of the moon
• go in Top View and adjust the view, that you can see the moon and Jupiter together
• with the mouse move over this green pivot point, click and hold left mouse button and an arrow-cross will appear
• now move the pivot point to center of Jupiter, a green line appears
• since you've moved the pivot point, the X-Y-Z-gizmo moves along
• now you only have to select the rotate symbol and from now on you can rotate the moon around Jupiter keeping it always the same distance
• repeat for all other satellites of Jupiter
   
  For exact positioning of the pivot point copy the Jupiter center X-Y-Z-coords to the pivot coords, but handpositioning should be accurate enough ...

Hope this will help ...

Just to add - now after adjusting the pivot point, you can link the satellite to Jupiter - so this complements your system.

Linked you can move Jupiter and the satellites follow, still always being able to rotate them separate around Jupiter on their exact orbit ...