Start with the question
Missions begin with science or exploration objectives, which flow down to measurement requirements, then to instruments, orbits and spacecraft. NASA projects pass through phases A–F, with reviews such as the Preliminary and Critical Design Reviews.
Budgets and margins
Engineers maintain budgets for Δv, mass, power, data volume, pointing and cost. Early designs carry large margins — often 20–30% on mass — because estimates grow as designs mature. Exceeding a budget late can force descopes or a larger launch vehicle.
Risk and redundancy
Single-point failures are minimised with redundancy and fault protection. Trade studies compare options — solar versus nuclear power, direct trajectory versus gravity assists — on performance, risk and cost.