First things first. After I got this "unique" idea for the front cover I started to play around with Blender 3d and here's what I came up with. I used only 3 layers, one for the big asteroid, one for the smaller ones and the last one is there only to create some atmosphere.

Here's the wireframe version of the final scene through the camera view. Not much to see here, it's pretty boring without any materials or textures.

Same view as in the picture above but with added textures and with the atmosphere layer on. Colors are pretty harsh but I already knew that I was about to continue the project in Photoshop.

Time to jump into Photoshop. I started the new project by painting some stars and creating the basic shape of the earth. I am not sure if you can see any stars in there or in the final product but they are there anyways.

After a few minutes of tweaking I came up with this. I added some glow and some detail and made the earth texture a bit more visible to give a more rounded feel to it.

At this point I basically had only two active layers again. The first one is the earth layer and the second one is the asteroids. I also added some clouds on the earth layer to breath some life into the background. Again it's very harsh looking but it's a starting point.

And here it is after the final adjustments. The only thing I really did here was some color correction, playing with brightness / contrast and finally added some detail again with the brush tool. 

All in all it took less than two hours to transform the initial idea to the final cover art. Even though I will probably tweak it a bit more before it goes to print I have to say I am pretty happy with how the cover turned out.