Core Viewpoint - The Shenzhou 21 mission features a notable astronaut, Zhang Hongzhang, who is a doctoral degree holder and a payload expert, marking him as the second payload expert from China to go to space after Gui Haichao from Shenzhou 16 [1][3]. Group 1: Role of Scientists in Space - Scientists, particularly payload experts, are selected for their ability to conduct important scientific experiments and technical tests in space that cannot be performed on Earth [4][6]. - Payload experts must have a strong background in specific scientific fields and undergo over two years of space training to operate experimental equipment and analyze data in the unique environment of space [4][6]. Group 2: Physical and Psychological Requirements - While the physical requirements for payload experts are lower than for astronauts and flight engineers, they still need to meet certain health standards, including the ability to withstand significant weight during launch and re-entry [7][9]. - Specific health conditions, such as dental issues or chronic diseases, disqualify candidates from becoming payload experts [10]. Group 3: Selection and Training Process - The selection process for payload experts involves a rigorous two-and-a-half-year training program that includes learning about spacecraft structure, emergency skills, and adapting to the physical sensations of space [11][13]. - After completing training, candidates must pass additional evaluations to ensure they can handle the physiological and psychological demands of space missions [13].