gets better as you progress
I'm rather... on the fence about this game. on one hand, it is a real challenge to get a few of the challenges as 'complete', and there is a lot left up to the player to choose what ship to use, and each has their own benefits and disadvantages, such as the military ship. a smaller amount of O2 is good in situations when you're veering off into space, but it can be a real bitch if you're trying to get the leap of faith award. as such, it is good you have added the 'golden oldie'. This is a good challenge, and not so difficult as to seem impossible to surmount, which adds more game time, and brings a sense of immense satisfaction once you achieve it.
Slingshotting, although risky, is rather simple to use once you have the knack of it, and can be immensely rewarding if there are planets that are close, and in your way when trying to escape the sun. However, this is the only time your physics work to the players advantage.
The physics of the game are satisfactory, for the most part, but the gravitational fields is all over the place. As Mysterious-Pig said, Gravity's effect is rather steep. to put in a better word, after reaching a certain point, it becomes ineffectual, and almost non-existent. to accurately simulate gravity, it would be better if the equation for gravity were more... quadratic. E.G. where G = gravity, D = distance from the planet, and M = Mass: G= M/D. therefore, as you get farther from the planet, it's gravitational effect decreases, not rapidly, but noticeably. note: this might not be entirely perfect, so I wouldn't put EXACTLY that into it, maybe a multiplier here, a division there, if you've payed ay attention in your algebra or quadratics classes, this should be a breeze. You might also want to play Gravitee, Gravitee, and Gravitee wars, just to get a good feel of what gravity should do in a game.
The graphics of the game are decent, better than a lot of games on here, but it still seems as though you put it together with basic shapes in Microsoft Word, or another program that uses those same shapes. The colours aren't too harsh on the eyes, and they aren't too bland either. on this note, you have done a good job.
To the last point, hinted at by the title: replay value. As you start, you would expect it to be similar to Gravitee, or even the popular Worms, so getting a grip on the apparent lack of gravity can be a little frustrating at best, destructive at worst. You will find, as you progress, that slingshotting becomes like second nature, skipping is normal, and overall gameplay becomes kind of... natural.
Overall, a five. hampered by the unrealistic physics, and bolstered by the exceptional replay value. Suggestions: review your physics engine, and maybe study your physics a little more, and don't forget the quadratics.