After doing some more testing, I'm fairly convinced that it isn't possible to attack ships directly. I tried a number of times with the same result, a message indicating an attack is ordered but no battle.

I also had this bizarre interaction: I encountered a colony ship in orbit of a planet, I moved my fleet to intercept and attack the ship before it "deployed" on the planet (which already had an outpost on it). The result was the same as above, nothing happened. So instead, I attacked the outpost on the planet we were orbiting. When I did this, a battle started on the planet that I won without any directed combat taking place, and the player's colony ship simulaneously vanished from orbit.

This seems like a bug. At first I thought maybe the colony ship had landed before I attacked, and I was just seeing a "ghost" in orbit, but if that were the case, the planet presumably would have had some population and an actual battle would have taken place. This is, of course, assuming I'm understanding the combat tutorial correctly.

Can a dev clarify this? Thanks.