 What was considered a downside to the space exploration by Si?
What's more important—Man's conquest of space, or one spaceman's life? They also had a banquet for him, complete with speeches by such bigwigs of the Department of Space Exploration as Academician Lofting Gubelin and Doctor Hans Girard-Perregaux. In common with recipients of gold watches of a score of generations before him, Si Pond would have preferred something a bit more tangible in the way of reward, such as a few shares of Variable Basic to add to his portfolio. It was better to retire on a limited crediting, on a confoundedly limited crediting, than to take the two or three more trips in hopes of attaining a higher standard. He'd had plenty of time to figure it out, there alone in space on the Moon run, there on the Venus or Mars runs. His explanation was that a phobia against having his eyes touched prohibited either surgery to remould his eyeballs and cure his myopia, or contact lenses. Automation being what it is, only a fraction of the population is ever called up. He was drafted into the working force reserves, served his time, and is now free from toil for the balance of his life.
 The fear of contracting space cafard.
 The inability to start of family of his own due to being away for long periods of time.  The fear of contracting space cafard.  His fear of being in the ship itself.  Becoming too used to being along for long periods of time.