Fig. 1

Schedule of proposed microbiome monitoring on the journey to Mars. Shown is the schedule for switching from regular food (x1a) to space food (x1b), mobility of the astronauts (free or quarantine – beginning in x2), gravity (Earth = 1 g, Mars = 0.38 g, and space is 0 g), microbiome monitoring frequency (high, medium and low), and monitoring adjustment in case of an emergency (emergency in x6 leading to x6b). Sampling frequency phases are numbered sequentially (x1–x17) throughout the process and contain the following rationale: maintaining a low sampling frequency during flight to and from Mars (x6, x6d, and x13); switching to medium frequency after familiarization on Mars (x9), after lift-off from Mars (x12) or after landing back on Earth (x16); and high-frequency phases shortly before and after landing on and lift-off from Mars (x7, x8, x10, x11), before returning to Earth (x14), the first month back on Earth (x15), or whenever there is an incident involving the astronauts’ gut microbiome (x6b)