I like the over-all look of this photo a lot. It is the details that would make this image even more pleasing.
I agree with those who said the edges around her are unnaturally soft. Crisp, but blended edges usually work best when adding a person or object into a background. I use the Blur Tool to make the edges seem part of the location.
The side-lighting is consistent with the desert lighting and the lighting on the planet and moon. However, the contrast ratio is a bit higher on her. The shadows on her seem a bit deeper than other elements, but not too seriously. Her tan on her arms suggests she has been wearing short sleeve shirts, which might not be likely for the wardrobe worn in that tv show.
I agree with the others about the drop-shadow that was added to make it seem she made the shadow. The over-lighting seems like diffused sunlight on the desert, but her face has brighter contrast to suggest a harsher sunlight, as well as her nose shadow being harsh, so the shadow should indeed be a bit harder, longer, and more defined to show her shape in it. Her shadow gets darker as it gets farther from her head, which it should get lighter, since fill light would do that. I also agree with nathancarter that shadows, especially soft shadows, would have sharper edges when near the person who made them, and get progressively softer as the distance increases. Shadows can make or break the believability of a composite, so I suggest you imitate reality as closely as possible.
Compositionally, I am fine with her size. It makes the scene more epic for me. I would prefer to see her closer to the right frame, since she is look to the left. Her pose is very elegant.
The planet and the moon are well blended into the clouds, but the moon seems unnaturally oval-shaped.
If you can refine the details, I think this will become a more stellar image!