This game is great! There are, however, some buggy issues that I hope you can iron out.
I love the feel and challenge of this game. I think you have done an excellent job of designing a game world that challenges the player and rewards intelligent play. I think the technology progression is balanced and makes sense. It is, however, not very clear what some techs actually do (e.g. computers) which makes it difficult to know how to prioritize them. The actual battle mechanics are good, but do tend to favor spamming probes and other low end spacecraft (is this intentional?). The actual colonization process works very well and makes sense (engine-based limits on expansion, and don't waste resources colonizing if you can't hold it), although there is often simply no time to carefully manage moving resources around your empire. Automating this process might work better (e.g. setting up automatic resource convoys) but I understand that the coding for that might be unrealistic. The economics system makes sense too, things seem to cost about what they seem to be worth and the player is forced to choose between devoting resources to long term economic development or short term defense. the controls work decently enough, but the the dependance on the mouse makes it very easy to accidentally send a fleet to the wrong place. Trying to deal with deselecting icons and selecting icons in real time can be frustrating if you are in the middle of a battle etc. One thing I felt was slightly unbalanced was the difficulty level. Once you get the game mechanics down, it seemed to me that Easy and Normal difficulty are not that challenging unless you are in a corner surrounded by much stronger enemies, as in the last scenario. The Hard difficulty on the other hand, seemed extremely tough (to me) and I do not feel like I could win most of the scenarios. I have no problem with Hard being "hard," and it is possible that I am doing something wrong, but I would personally prefer to play at something in between. I am not suggesting that you water down Hard mode so much as either make make Normal more challenging or add in a difficulty in between the two.
What killed this game for me are the bugs. They didn't bother me so much on Easy mode, since I was just able to work through them, but I have found it is almost impossible to win on Hard mode, because every time I am in a good position something glitchy happens that destroys my chances to win. I have had fleets simply disappear, sometimes a probe will "switch sides" or at least change from a blue to a red icon and become utterly useless, sometimes after I defeat an enemy fleet their icon remains over my planet and will conquer that planet if I move my fleet away. Oftentimes a ship will seem to plot a course off the map and a blue line will appear that cross cuts the entire entire game universe...they do not tend to arrive at their destination. The game will also sometimes freak out and focus off map, so I am glad that you have a center map button. Sometimes it seems like a small fleet will unexpectedly get into battle with an invisible army as soon as I put them into orbit, although this may be pirates (it is not clear to me how pirates work in this game, so I cannot tell). I have had ships simply stop building (not due to tech limitations), one example is when I became suddenly unable to build scout ships on a planet. I could devote resources to their cue and build them on other planets, but they would not actually be constructed. While the bugs are not supercommon, over the course of a thirty minute mission it is almost likely that something inexplicable will happen which is kind of frustrating. If you iron out the bugs and get the world editor online, I think this could be a classic.
Anyway, great job!