A Mission Specialist (MS) is a position held by certain NASA astronauts for the Space Shuttle program. A Mission Specialist is assigned to a limited field of the mission, such as for medical tests or technical quests.
Other functions on board are Pilot, Flight Engineer and Mission Commander. Some space shuttle missions included up to three Payload Specialists. While a Payload Specialist was selected for a single specific mission, a Mission Specialist is selected as an astronaut first, and then assigned to a mission.
Mission Specialist is referred to by NASA as MS.
Mission Specialists 1 & 2 (MS 1, MS 2)
MS 1 & MS 2 are assigned the primary task of performing the extra-vehicular activities (EVA) or space walks. They will construct part of the truss structure that will become part of the International Space Station (ISS. MS 3 & MS 4 will monitor their blood pressures and heart rates. MS 1 & MS 2 exit through the orbitor's airlock and must work together quickly and efficiently to complete the truss structure. MS 1 & MS 2 have to remember to depressurize before they go outside and to repressurize when returning.
Mission Specialists 3, 4, & 5 (MS 3, MS 4, MS 5)
MS 3, MS 4, and MS 5 conduct the fluid flow and chemistry experiments on the mid-deck of the orbiter inside the glovebox. The glovebox prevents damage to the liquids and fumes from entering the cabin and causing potential damage to the orbiter or astronauts themselves. MS 3, MS 4, and MS 5 are also charged with monitoring the blood pressures and heart rates of the entire crew before launch.