As others have mentioned, The Hermes didn't follow a Hohmann trajectory. It is an ion propelled craft capable of 2 mm/s^2 acceleration. A very weak thrust but p

As others have mentioned, The Hermes didn't follow a Hohmann trajectory. It is an ion propelled craft capable of 2 mm/s^2 acceleration. A very weak thrust but powerful in that it can sustain this acceleration for looooong burns. If just the sun is accounted for, Andy Weir's trajectories are okay (I believe). However putting planets in complicates things. Ion engines really suck at climbing in and out of deep planetary gravity wells. Neil deGrasse Tyson's trailer for The Martian had Hermes leaving low earth orbit and arriving in Mars orbit 124 days later. Which is nonsense. With 2 mm/s^2 acceleration it would take 40 days to spiral out of earth's gravity well. Not only does this wreck the 124 day trajectory Weir so painstakingly calculated, but most the slow 40 day spiral would be in the Van Allen belts. This astronauts would be killed by radiation.