It is parked on top of the castle!
your spaceship should be on top of the castle..
If you got the spaceship on the Pewter Moon, and you are back in Arturus, it should be parked on top of the castle. If you are in space, deliberately crashing the ship will transport you back to the Pewter Moon. (The only other way off the planet Poptropica is the saucer. If it's gone, you are stuck.)
Get the hovercraft from the mill attic and fix the saucer that is crashed out in the swamp to the right of the mill. Once you set the coordinates to the Pewter Moon, you can travel there. You will build a new spaceship to travel to the three planets that the space knights have gone to. (see the related questions)
That actually happened to me, too! I was so mad! But, the spaceship you made on the moon is actually on the castle. (I'm just making sure, this is for astro-knight island.)
Fly to the three planets where the other knights have gone, and battle the mechanical creatures there.The Jungle Planet, at X-15 Y-15. (Home of the Phoenix Birds.)The Ice Planet, at X-73, Y-83. (Home of the Snow Panther Copter.)The Fire Planet, at X-83 Y-20. (Home of the Fearsome Fire Beast.)
If you didn't get the coordinates from the Queen, you can use the solar system map in the spaceship. There would be no other reason to go back to Arturus until the game's end. But if you want to land, do the same as on any other planet : click on the planet. When you are in Arturus, your spaceship is parked on the castle roof.
Swap your wrecked saucer for a new holo-built ship by talking to the head mechanic at Astrozone (building on the right). You should have the coordinates to the three planets where the other knights have gone (X-15 Y-15, X-73 Y-83, and X-83, Y-20) and they will all appear on your Spacemap aboard your new spaceship. You will need to defeat all three planets to rescue the Princess.
The Queen gives you the coordinates for the planets where the three space knights have gone, but for the saucer Excalibur they are too far away. The nearby moon at X-56 Y-52 is noted on the page from Mordred's journal (under his bed).
Maybe the hindenburg.
Go to the fountain sign and press moon, planet, star, sun. the water will be gone. go down and you will see a secret group of mordred-fans.
The queen gives you the list of planets that the knights have gone to. But your fuel supply won't reach them. (You go to the moon at X-56 Y-52. The coordinates are on the missing journal page - see related question)
The Queen will give you a card with three sets of coordinates, the planets that the three knights have gone to. However, you will need a closer set of coordinates, and there is one on the missing page from Mordred's Journal. (It is under his bed in the Museum, and the numbers are X-56 Y-52.)