In the wake of the successful PHILAE landing on comet 67P/Churyumov-Gerasimenko and the launch of the first Mobile Asteroid Surface Scout, MASCOT, aboard the HAYABUSA2 space probe to asteroid (162173) Ryugu, small spacecraft in applications related to small solar system bodies have become a topic of increasing interest. Their unique combination of efficient capabilities, resource-friendly design and inherent robustness makes them attractive as a mission element at the frontiers of exploration of the solar system by larger spacecraft as well as stand-alone low-cost approaches to open up the solar system for a broader range of interests.The operators' requirements for cutting-edge missions compatible with available launch capabilities impose significant constraints in resources, timelines, timeliness, mass and size. To create spacecraft feasible within these constraints, the mission design teams need to accept a broad range of equipment maturity levels from fresh concepts to off-the-shelf units. The resulting Constraints-Driven Engineering (CDE) environment has led to new methods which transcend traditional evenly-paced and sequential development.
We evolved and extended Concurrent Design and Engineering (CD/CE) methods originally incepted for intitial studies into Concurrent Assembly, Integration and Verification (CAIV). It is applied in all phases in most of our projects to achieve convergence of asynchronous subsystem maturity timelines and to match parallel tracks of integration and test campaigns. When facing such a challenge, Model-Based Systems Engineering (MBSE) supports design trades and constant configuration evolution due to unforeseen changes. Proactive change and schedule acceleration has resulted from systemlevel CD/CE optimization across interface boundaries by MBSE-aided CAIV.We discuss advantages and constraints of small spacecraft for planetary science and applications, focusing on emerging areas of activity such as planetary defence, on the background of our projects. These include the comet lander PHILAE flown aboard