These missions are an important part of Frontier and you'll start to be offered these by the military as you rise higher up the rankings. Only the military offer you these, as the equipment you're given to complete
these missions is specialised military hardware, including reconnaissance cameras and nuclear missiles!!
These missions do not have to be done at a precise time - you just have to make sure you complete your
task and get back to base before a particular date.
Follow the guide here at FrontierAstro and you'll be racking up the credits in no time!