The harpoon is ready to launch. There is a skippable minigame. Try to get the cross-hair to up left square (with the red stripes). Two moves are unavailable. After the minigame use the torch to fuse the rocket.
Unlike normal walkthroughs, in Nice Game Hints you can select just the puzzle you want hints for. Here are listed the puzzles for "Launching the harpoon" in Deponia. The guide you choose only contains information about that puzzle, reducing possible spoilers for other puzzles.