Uma das cenas deletadas do filme explica isso. O plano original era ter a gravidade fornecida pela própria carga, uma gota de matéria escura comprimida com o mesmo peso da lua. No final, este ponto da trama parece ter apresentado os escritores com mais problemas do que resolvido e eles simplesmente o cortaram em favor do navio ter uma gravidade inexplicável.
De uma perspectiva de roteiro, o 'Senior Science Consultant' do filme; Professor Brian Cox abordou este ponto em uma entrevista com revista Sci-Fi Online .
Sem rodeios, eles não tentam explicar isso. Ele handwaves-lo com o clássico "É apenas um filme " explicação para má escrita.
It's very interesting. These guys that get really pedantic are really, I think, missing the point about what science is all about. It's about precision, when you're doing it. So when you're doing research it's all about precision and attention to detail and that's the difficult bit, and that's what you learn how to do. But deciding what research field you want to do, and having really good ideas about what to go and measure, and what to try and find out, that's a creative process. I think a lot of the pedants kinda miss that.
Like you say, Sunshine is not a documentary. It's trying to just, in an hour and forty minutes, get across a feeling of what it's like - not only to be a scientist, because obviously there's much more in it than that. So, I found it interesting to watch the kind of people that get upset because the gravity is wrong.