I have a physics question about the Rich Purnell maneuver in The Martian. This has MAJOR spoilers for the book and presumably the movie, so be warned. It might be a better GQ question, but I figured CS was safer considering the spoiler-y content.
The maneuver is started when the ship is approaching Earth en route from Mars, and involves a slingshot around Earth to redirect them back to Mars. There is then a Mars fly-by and presumably another slingshot using Mars to redirect to Earth. The details of the maneuver aren’t explained, but based on the duration, I don’t think it can include using any other planets. It is a “brilliant” maneuver requiring weeks of calculations by a genius.
During the Mars fly-by, the crew is forced to use steering jets to reposition the ship by ~70 km from the original course, and then use a rapid depressurization to accelerate by 29 m/s.
My question: how precise would a course like this be? Is 70 km and 29 m/s within an acceptable margin of error, or did this completely throw off all the calculations and they are now screwed?