Human beings can survive exposure to a hard vacuum for about 30 seconds, give or take, although generally one will need medical attention immediately thereafter. How long one can survive at the edge of the atmosphere depends on what exact elevation you mean by that, although it's not likely to be very long in any case. But I think Simone knew exactly what she was writing here. I think she intended for Dinah to be using lethal force but to be in denial about it.