Throughout the movie Sunshine we are shown that each crew member has a job that shows his/her importance to the mission.
From what I can gather Capa seems to be of the highest priority, even more so than the captain as he is the one that needs to set the bomb off on its correct trajectory. We are shown this in this scene:
Mace: Were screwed... one of us isn't anyway.
Harvey: What happened?
Mace: The airlock's destroyed. There's only one suit. Capa's taking it.
Harvey: ...Why Capa?
Mace: Because the rest of us are lower priority.
Harvey: I'm Not A Low Priority.
Mace: You're a comms officer on a ship that has no means of communication.
Harvey: I am the captain!, The mission needs a captain to hold it together.
The scene ends with Capa getting the suit.
Then this is the back and forth in an earlier scene:
Kaneda: It's a two person job, fixing the shield. Harvey you're second in command, you're not coming.
Trey: I volunteer.
Mace: No! I volunteer...
Mace: I volunteer Capa.
Capa: [after long pause] ... alright...
So why in this scene was Capa allowed to go on a potentially dangerous mission to fix the shield, I mean any type of space has risks and if Capa was the highest priority, I just don't understand why this, No. 1 , suggested by Mace and No. 2, allowed by the captain? Because, he speciffically says Harvey was not to go, but in the later scene Capa was clearly stated to be more important, even taking into account the communications had been taken out and were not working, b/c Capa was the only one who could release the bomb.