NASA, JPL, and other space agencies rely on ever larger, more complex software systems to do challenging science. This trend raises the stakes and drives the need for breakthrough solutions which reduce mission risk. At the Laboratory, the emergence of model-based design reformulates the design process, and therefore the nature of software systems engineering must change accordingly. For example, Mars Polar Lander is thought to have failed when the software turned off its engines too soon - the flaw in the software-system design was introduced when the software requirements were derived from the system requirements; had there been a software-system model, this flaw would have been easily detected with the Model-based approach we advocate. Model-based engineering is used on other missions, but in this paper, we will discuss 3 Mars missions (Mars Science Laboratory, Mars 2020, and Mars Sample Retrieval Lander). Model-based systems engineering promises to shift the way space mission designs are captured, reviewed, and elaborated from an emphasis on drawings and text-based documents to computer processable models. In practice, the Mars projects applied the following techniques to improve the efficiency of the iterative Mars project flight system design process through: Decomposition of the Flight System into Functional Designs, Abstractions of System Elements, System Block Diagram, Software Interface Block Diagram, Abstractions of Software System Elements, Software Sequence Diagram, Parameter Architecture Model, and JPL-developed Computer Aided Engineering for Spacecraft System Architectures Tool Suite (CAESAR). MBSE efficiency assessment using the comparison between the Actual and the Estimates shows that the MSL Flight Software (FSW) defect undetected rate is reduced from 21% to 12% (nearly 2-fold reduction) and Mars 2020 FSW defect undetected rate is reduced from 21% to 3% (7-fold reduction). To FSW, the biggest contribution, by far, is the effective reuse of code as `heritage' from MSL and the adoption of MBSE techniques mentioned above.
