"What's up Hall H!" a giddy Ryan Gosling in a trucker hat and flannel shirt shouted to the crowd of more than 6000 at Comic-Con's biggest venue.
Amazon MGM Studios showed the opening five minutes and several other slightly unfinished scenes from the first third of the film, seven months before its planned release. (Spoilers for that section follow).
It included an extended glimpse at Rocky, the stone-shaped and faceless alien who becomes Gosling's mission partner as they attempt to save the universe from ecological disaster.
Phil Lord, who co-directed the film with Chris Miller, said the relationship between the two beings stuck alone together in space represents the central theme.
"If the universe depended on it," Miller said, "can adult men make friends?"
Rocky is already a cult favourite for readers of Andy Weir's novel, and is sure to be a future staple of Comic-Con cosplay.
Gosling said he got on board immediately after reading Project Hail Mary in manuscript form, and was only partly kidding when he called Weir, who was sitting next to him, "the greatest sci-fi mind of our time".
"I knew it would be brilliant, because it's Andy, but nothing could prepare me," Gosling said. "It took me places I'd never been, it showed me things I'd never seen, it was as heartbreaking as it was funny."
Gosling plays Ryland Grace, a middle school teacher and underachiever drafted for the mission.
The opening five minutes show a gloppy, long-bearded, amnesiac Gosling as he awakes in a pod.
He climbs out, confused. He finds other people in pods who are clearly dead. Then he finds a window and learns he's in space. He gives a mealy-mouthed scream of "Where am ?!"
The movie represents the return to directing, and return to space, of Lord and Miller for the first time since they were fired and replaced by Ron Howard by Disney and Lucasfilm from 2018's Solo.
Like The Martian, the movie goes heavy on the science but takes the messy, kitchen-sink, everything-is-comedy approach Lord and Miller used in films like The Lego Movie.
"This movie is not a Mac, it's a PC," Lord said. "It can be beautiful, it just can't be pretty."
