Space mission design is about basically designing a spacecraft or probes to a specific destination in space for a purpose or tasks. Example:the mission can be designing a robot/probe to be borned by a rocket all the way to Mars which needs new robotic/probe design, approaches and technologies.
One also required to know how the space environment affects the design of space systems. In space design, one has to consider the space mission life cycle, mission types, communications, cost analysis, programatics, risks and designing different types of spacecraft subsystem (telementry, control, instruments, power, structures, thermal ,navigation and etc). (Credit:edx.org & deseng.ryerson.ca)