I always found that scene amusing. Dumbest hitman to ever live. He's gonna fire at a random falling object outside the greenhouse, which is obviously not a man, and in the process compromise the environment and get sucked out into space. What a pro.

No, O'Neil knew that somebody was probably still in the greenhouse. The dropped panel provided the moving shadow that the trigger-happy guy shot at to confirm it.
Even in the future, the old hat-on-a-stick tactic still works.