If you grow up watching The Simpsons there is that moment when you realised that the star of the show wasn't Bart, it was Homer, and it was a deliberate decision by the writers. Similarly it seems to me, that even though the title is 宇宙兄弟 (Space Brothers), the focus of the story is Mutta (六太), Hibito (日々人) is just a supporting character.
It's great to see another cameo by the Ghostbuster inspired scientists, and a mustachioed Peter Venkman/Bill Murray adding a bit of humour.
I'm not sure if I mentioned this last time, but it must be difficult to write a science fiction in the near future when the pace of current technology, and political maneuvering outpaces the events in the story. 3D printing which has made such rapid advances in the past few years, features heavily. And with the manga originally appearing in 2008, NASA has since seen drastic cuts, especially in its manned space mission, that make the future envisioned in Space Brothers difficult to realise. I wonder if these real word changes have lead to the current story line that seems to be scale back the ambition of the earlier volumes. Now we see Mutta attending budgetary meetings, and campaigning to continue the International Space station in an environment of decreased space spending.
Space Brothers is a series that is slowly losing momentum, it would be nice to see Hibito return after a fairly long absence, or to see Mutta finally realise his dream of heading to space, but at the current pace of the series it may not happen for a while.