The Rich Purnell Maneuver was a course for the Hermes first proposed by Astrodynamics' Rich Purnell, as a way of getting the Hermes back to Mars to save Mark Watney. It was proposed in a meeting known as 'Project Elrond'.
Statistics of the Maneuver
The Maneuver would start as Hermes approached Earth. Instead of Hermes executing its month long deceleration to get into LEO (Low Earth Orbit), Hermes would instead begin accelerating using its Ion Engines to preserve Hermes' velocity, and gain even more. As Hermes flies by Earth, it re-supplies, then uses Earth's gravity to preform a gravity assist to get Hermes moving even faster.
Earth's gravity then bends Hermes' current trajectory, and sets it on course for Mars. Hermes then spends 322 days travelling to Mars, accelerating the whole time. After the 322 days, Hermes' enters Mars' gravity, and Hermes preforms a flyby of Mars. During the Flyby, Mark Watney uses a modified Ares IV MAV to get to orbit, just as Hermes passes by. The crew then picks up Watney from the MAV Capsule, and brings him in. Then, Hermes preforms a gravity assist from Mars, which puts them on a 211 day trek back to Earth.
On the Way home, Hermes uses it's Ion Engines to slow down, so Hermes can enter Earth's gravity and get into Orbit. Once in orbit, Hermes uses a shuttle to get back down to Earth.
The Rich Purnell Maneuver adds 533 days to the Ares III mission, making it the longest Ares mission, a total of about 1,000 days in space, and the crew of Ares III are the only humans to return to Mars twice in a single mission. The Ares III extended mission also cancelled the Ares IV mission because of Mark Watney's use of the Ares IV MAV.