Earth

Typing in earth (or one of the letter at least) during space flight gives you the highest level ship and the ability to scroll through the levels. -Cough-

Never found the cheat

This is a really interesting game to play, not just because the gameplay is great, but because it is also educational. There is a pretty funny story behind this game for me. I was not allowed to play games on the Internet in class, but I did in fact play this game in my class because I thought it was educational. The teacher apparently got tired of this and would not allow it. Now, looking back, I can probably not even remember anything that I learned from this. At least you came up with great gameplay and a lot of intelligent effort being put into it.

it is fun

but this game is old and classic and not really fun actually

Could have been more effective...

Although an OK game, this one suffers the same fate of most 'edutainment' titles that have appeared over the years: the fact that the game has nothing to do with the educational aspect, but is rather a basic shooter with quiz questions in between the levels. If you want to make a game to teach something, the learning should be an aspect of the gameplay. That way, people will actually remember things as they get better at the game.
For example, if you want to make a game to teach people about rocks, you could make a physics-based game where you find different rocks in the places that they could be found in real life (different depths underground, volcanoes, etc.) and where the physical properties of such rocks (hardness, density, etc) could have some impact on the gameplay itself. Like, you have to use harder rocks to break certain obstacles or denser rocks to weigh down pulley systems. Just a few ideas I'm throwing out there.
As for the game itself, it's decent. But nobody who isn't already interested in geology will remember anything they learn from this game...

not quite good

Everything was ok except for the actual gameplay. Ship moves too slow. Reminds me of a certain NES game, can't remember the name.