An electrical problem cropped up aboard Discovery again Tuesday after engineers thought they had it licked, and threatened to delay the shuttle’s final liftoff just one day away.

The trouble appeared to be with a backup controller for one of the shuttle’s three main engines.

The controller was sluggish early Tuesday morning, but shuttle officials found everything to be fine. Later, voltage irregularities were noted, prompting NASA to order last-minute reviews and hurriedly schedule a late afternoon meeting of top managers.

NASA spokeswoman Candrea Thomas said it was too soon to know whether Wednesday afternoon’s planned launch would remain on track.

Discovery was supposed to blast off for the last time Monday on a trip to the International Space Station, but gas leaks forced a two-day delay.

NASA has until Sunday — possibly Monday at the latest — to launch the shuttle. Otherwise, the mission will be pushed into December because of unacceptable solar angles.

It’s the next-to-last shuttle flight on NASA’s official schedule as the agency looks toward newer and farther-flying craft. An extra mission may be added next year.

Each of the main shuttle engines has both a primary and backup computerized controller that serve as electronic brains. They are critical parts that must work perfectly before going ahead with a launch.

Earlier, NASA test director Steve Payne said the controller trouble appeared to be caused by debris in the circuit breaker.

As the shuttle team scrambled at the launch site, the rest of NASA celebrated 10 years of continuous human presence at the space station. The six U.S. astronauts and Russian cosmonauts there now fielded calls of congratulations.

“You all are incredible ambassadors,” NASA Administrator Charles Bolden Jr. said in a broadcast hookup from Kennedy Space Center. “What you do is actually a modern-day ’Star Trek.”’

On Nov. 2, 2000, three men moved into the young space station. People from around the world have been there ever since, living and working more than 200 miles above Earth.

On its 39th and final voyage, Discovery will bring six visitors as well as thousands of pounds of supplies, including a humanoid robot. Forecasters said there is a 70 percent chance that the weather will cooperate for the 3:52 p.m. liftoff, if Wednesday’s launch attempt holds.

Payne said there was a lot of excitement as the countdown enters its final stages. Shuttle workers “put their heart and soul into this one,” with the intent of making this last voyage of Discovery as good as the 38 that have come before, he said.

“She’s now poised to take to the skies tomorrow, and when she goes, she’s going to take a little bit of every one of us with her,” Payne told reporters.

Discovery has carried 180 individuals into orbit over its 26-year career, and logged nearly 150 million miles and more than 5,600 orbits of Earth. It is NASA’s oldest surviving shuttle and fleet leader, and will be the first to be prepared for museum retirement.

Online:

NASA: http://www.nasa.gov/mission—pages/shuttle/main/index.html