In The Martian, the Mars mission that leaves Watney behind is Ares 3. The mission’s crew is forced to evacuate Mars after a severe dust storm threatens the Mars Ascent Vehicle, and astronaut Mark Watney is struck by debris during the escape.
Watney’s damaged suit stops transmitting vital signs, so mission commander Melissa Lewis and the rest of the crew believe he has died. With the ascent vehicle in danger of tipping over, they launch without him and return to their orbiting spacecraft, the Hermes. Watney survives, however, and reaches the Ares 3 habitat, where he begins planning how to stay alive until a rescue is possible.
The Ares missions are fictional NASA expeditions created for Andy Weir’s novel and its 2015 film adaptation. Watney grows potatoes using stored supplies, crew waste, Martian soil, and carefully produced water. He eventually travels to the Ares 4 landing site, where he can use that mission’s ascent vehicle to reach the Hermes.
The main mix-up is with Hermes, which is the crew’s spacecraft, not the surface mission. Ares 1 is an earlier fictional mission, while Ares 4 is the later mission whose landing site becomes central to Watney’s rescue plan.