Might want to reverse up and down arrow key functions so that it simulates a stick control - i.e. "down" = pull back on the stick to climb and "up" = push forward on the stick to dive. Constantly found myself climbing when I wanted to dive etc. Also, consider making the spacebar the fire key as it's easier to tap that with the thumb while your other fingers can remain on other keys. Finally, consider putting in some still objects (i.e. asteroids, space hulks) close to the start point so that you can "feel" forward motion during the start of the game experience - I think that will really help sell the feel of space flight to the player right away. Nice game, smooth motion - I'm a fan of the Unity engine.
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