Any ideas? What would make it more challanging?
I like Kosek's idea
But I was also thinking that it could be more challenging if you had limited fuel resources. I'm just popping out some numbers, but let's say that the ship can contain 100l fuel. Then if the landing platform only has 200l fuel in reserves, the player won't be able to just fly around carelessly, but will have to fly strategic to save on the fuel. You could also then place some fuel tanks on hard to reach located areas.
It would probably require some tweaking with the numbers, but it would make it a bit more challeging. Its just to easy to fuel up right now.

It could also be interesting if the amount of fuel affects the ships mass and thereby its stearing capability. When fuelled up the ships will fly slow and hard to steer while it will fly fast and react fast to playes input when there's almost none fuel left.

EDIT: I just died on timeout at the first level because I couldn't get the last purple gem. :? You should make sure that the correct coloured gems at least pops up before the times run out. I collected about 20 other gems while waiting for the purple one, but it never came. I would classify this as a flaw in the game
EDITEDIT: Has anybody actually completed the ninth level? I died once again because of timeout. I was collecting gems all the time but I was still missing 7 green gems and two blue. The game only created 3 green gems on 5 minutes. You should rellay look into this Dirk
I as the player didn't think I made anything wrong, but still I wan't capable of completing the game. It's a sign on bad game design according to a book I read