Spacecraft systems engineering is the process of defining, developing, integrating, testing, and operating spacecraft systems that meet mission objectives and requirements. It involves multiple disciplines such as mechanical, electrical, thermal, propulsion, communication, and software engineering. However, it must also take into account the human element since humans are both the users and operators of spacecraft systems. Human Factors Engineering (HFE) is important for spacecraft systems engineering because it can enhance safety, efficiency, and effectiveness by reducing human error and increasing reliability. It can also improve usability, functionality, and compatibility by aligning them with human capabilities, limitations, and preferences. HFE can support the health, comfort, and satisfaction of crew and passengers by addressing their physical, psychological, and social needs in space. Additionally, it can facilitate communication among human and non-human agents by designing interfaces that are intuitive and consistent.