A hatch between it and the descent module can be closed so as to isolate it to act as an airlock if needed, crew members exiting through its side port (near the descent module). On the launch pad, the crew enter the spacecraft through this port. This separation also lets the orbital module be customized to the mission with less risk to the life-critical descent module. The convention of orientation in a micro-g environment differs from that of the descent module, as crew members stand or sit with their heads to the docking port. Also the rescue of the crew whilst on the launch pad or with the SAS system is complicated because of the orbital module.