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Those parts are on board the

SpaceX Dragon spacecraft

The spacewalk would take place April 22 — if Dragon gets there as planned on Wednesday

e Dragon is carrying a new spacesuit, components to fix an existing spacesuit, critical research experiments and food for the six crew members of Expedition 39.

The challenge, however, is making sure the station could be ready even if the primary multiplexer demultiplexer (MDM) fails before spacewalkers can make the backup replacement.

There are more than a dozen MDMs on station, but each one controls different functions

This primary MDM not only controls a robotics mobile transporter, but also radiators and a joint to move the station’s solar arrays, among other things.

. NASA needs to reposition the arrays when a vehicle approaches because plumes from the thrusters can put extra “loads” or electrical power on the system.

Luckily, the angle of the sun is such these days that the array can sit in the same spot for a while, at least two to three weeks

 NASA configured the station so that even if the primary computer fails, the array will automatically position correctly

NASA also will move a mobile transporter on station today so that the station’s robotic arm is ready to grasp the Dragon when it arrives, meaning that even if the primary computer fails the transporter will be in the right spot

If Dragon is delayed again, the next launch opportunity is April 18 and the spacewalk would be pushed back

There are at least two or three spare MDMs on station

the one needed for this particular spacewalk is inside the U.S. Destiny laboratory, which is handily right next to the S0 truss and spacesuit worksite.