Team psychologist Searle (Cliff Curtis) diagnoses Trey as a suicide risk and only new captain Harvey (Troy Garity), Searle, Capa and Mace are the only ones left to go aboard Icarus I. Mace makes the discovery that everything is fine with Icarus I, except someone has manually overriden the ships controls rendering the pilot functions useless and the cabin pressurizer was destroyed effectively making it possible for only one member to go back to Icarus II. Mace manages to cover himself with ship insulation and make it back along with Capa.
Now with no second payload, not enough oxygen to survive to the destination point and team distrust at an all-time high they must contend with how to deal with the loss of 02. The remaining members: Capa, Mace, Corazon (Michelle Yeoh) and Cassie (Rose Byrne) vote to kill Trey ensuring they survive long enough to release the payload with Cassie being the only dissenting vote. They discover that Trey committed suicide as a result of his guilt.
I will let Alex Garland's dialogue reveal what happens next:
Icarus: Capa; warning. You are dying. All crew are dying.
Capa: We know we're dying. We're OK with it, just as long as we have enough oxygen to reach the payload delivery point.
Icarus: Capa; warning, you do not have enough oxygen to survive until the payload delivery point.
Capa: Please clarify.
Icarus: Twelve hours before crew will be unable perform complex tasks. Fourteen hours before crew will be unable to perform basic tasks. Sixteen hours until death. Time to payload delivery point, 19 hours.
Capa: Negative, Icarus. We have enough oxygen for four crew members to survive.
Icarus: Affirmative. 4 crew members could potentially survive.
Capa: Trey is dead. There are only four crew members; Cassie, Mace, Corazon and me.
Icarus: Negative. Five crew members.
Capa: Icarus... who is the fifth crew member?
Capa: Where is the fifth crew member?
Icarus: In the observation room.
In the observation room Capa discovers that the former Captain Pinbacker is alive and made the transition to Icarus II, but he has gone insane from isolation. Now here is where Boyle easily makes this film his most interesting. Pinbacker sabotaged Icarus I out of the belief that God made it so the Sun should die and no one shall live to re-ignite it.
Boyle makes God the essential villain in Sunshine while represented physically by Pinbacker (Mark Strong). I am still convinced this is why Sunshine didn't receive a wide release here in the States. Aiming to make a sci-fi film along the lines of 2001, and Alien Boyle doesn't quite make it to that peak of cinema, but you cannot fault him for trying.