Can you ever get a player down to just 2 ships?
No, David MacKenzie of Clever Mojo Games clarified this point in a FAQ on BGG.

Q: The RED player has four red ships on the board...one on the Lunar Mine, two on the Orbital Market, and one on the Terraforming Station. GREEN discards his Plasma Cannon card and removes the red ship from the Lunar Mine.On RED's next turn, the ship on the Terraforming Station returns to the shipyard stock and RED is left with only two ships.
This is a legal move by a strict reading of the rules.. [...] However, it breaks the intent of the rules, which is to insure that no player has fewer than three ships of their own color. During play testing we found it nearly impossible to recover after being reduced to only two ships, so we tried to insure that it would not happen. It now seems that we missed one situation that could lead to this exact problem. The cure is to adjust the wording of the Plasma Cannon's discard power as follows...
Plasma Cannon - You may discard a Plasma Cannon to return one ship belonging to another player to the ship stock. The target player must not be reduced to less than three ships of their own color for use on their next turn.

And a rule book edit

RED has four ships on the board...one on the Lunar Mine, two on the Orbital Market, and one on the Terraforming Station. GREEN would like to discard a Plasma Cannon to attack RED and removed the RED ship from the Lunar Mine, but cannot because doing so would leave RED with only two ships after the RED ship on the Terraforming Station returns to the shipyard stock next turn.