For a game animation I don’t know, but for a movie animation one way to achieve this (cheaply) would be to use compositing.
Basically, you create the planet, the haze, and the space ship separately. And you create the illusion of the ship approaching the planet and flying through the haze by combining the three strips, two-dimensionally.
The trick is to make the ship look like it is being concealed by the mist. That is done by modifying the alpha, or transparency, of the ship… which you would do using a mask that is based on the mist texture. (Or you might get away with simply modifying the alpha-value with some kind of an IPO.) As the ship approaches the mist and is then seen to fade away, your eye tells you that the reason why this is happening is because the ship is disappearing behind the mist.
The advantage of this technique is that, although it takes more time to plan and to set up, it is very easy to adjust for “just the right” effect. You are, after all, working in two dimensions. Tweaking a parameter and re-running the composite can be done in a few moments.