The blastoff of the space shuttle Discovery on a mission to rendezvous with a Russian space station was delayed Wednesday for at least a day, from early today to early Friday, after routine testing revealed a failure in one of the shuttle’s three critical guidance units.

Ground crews are working to replace the suspect part, hoping to finish soon enough that liftoff will be postponed no more than 24 hours.

“It’s going to be really tight,” said Ed Harrison, a spokesman at the National Aeronautics and Space Administration’s Kennedy Space Center. “But we believe they can pull it off.”

Launching of the winged spaceship, originally scheduled for 12:48 a.m. today, was put off until Friday at 12:22 a.m. at the earliest.