any planet
At the end of the island quest, you receive the Island Medallion. At that time, or if you return to the planet Poptropica for any reason, your spaceship is parked on top of the castle.
You can only come and go using the Blimp at Fort Ridley. However, you can leave from anywhere on Poptropica using the Haunted House card from the Poptropica store. When you enter and exit the Haunted House, you return to the Poptropica map. If you leave Astro Knights island, your spaceship parks on top of the castle. If you leave Skullduggery Island, you return to find the ship docked at Fort Ridley.
56,52
You have to get the fuel rod
You can't get to Jungle planet without a spaceship. You can make a spaceship at the moon.
Any changes to the game, or items, will remain until you return to that island. Islands cannot be reset. When you return from another island, you start on Main Street as before. On Astro Knights Island, if you leave from space, your spaceship parks atop the castle.
If you land back on Poptropica, your spaceship is parked atop the castle. This is also its location when you have beaten the island.
you put it into the spaceship ontop of the mill
Go to Main Street on the island and climb the rope to the Poptropica blimp. If you are on Astro Knights, return to the kingdom of Arturus on the planet Poptropica. Your spaceship will park on top of the castle. If you are on Time Tangled Island, use the Lab (12 o'clock) setting to return to Pendulum's Lab on Main Street. To leave any island directly (see above), get the Haunted House card from the Poptropica Store. Unlike "Dr. Hare's Lab" or "Earth Day", entering and leaving the Haunted House will not return you to where you were, but to the island map.
At the end of the island quest, you receive the Island Medallion. At that time, or if you return to the planet Poptropica for any reason, your spaceship is parked on top of the castle.
I don't think you can leave an island with a horse.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.