Once you fix the saucer in the swamp, you only have enough fuel to reach the nearby Pewter Moon (X-56 Y-52). Once you are there, there are no particular types of spaceship that are much better to build, and you can go to any of the three planets in whatever order you want.
(see related questions)
You do not get or use a bucket on Astro Knights Island.
There is no treasure chest on Astro Knights Island.
There are no bird eggs in astro knights island.
its on the island
How do you talk to the girl in the hay loft at astro knights island?
On every island you have to be the one to save whatever needs saving. That includes the kingdom in Astro Knights Island.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.
on astro knights island
With Mordred
it is in the left
Astro Knights
To return to space on Astro Knights Island after completing the mission there, you can head back to the rocket ship in the observatory and select the option to launch it again. This will take you back to space where you can explore further or complete any remaining tasks.