NASA is fueling its new moon rocket in a final make-or-break test Monday, with hopes of sending astronauts on a lunar fly-around as soon as this coming weekend.

The launch team began loading the 322-foot (98-meter) rocket with super-cold hydrogen and oxygen at Kennedy Space Center at midday.

More than 700,000 gallons (2.6 million liters) had to flow into the tanks and remain on board for several hours, mimicking the final stages of an actual countdown.

The only thing missing from the critical dress rehearsal was the crew. The three Americans and one Canadian monitored the action from nearly 1,000 miles (1,600 kilometers) away in Houston, home to Johnson Space Center. They have been in quarantine for the past 1½ weeks, awaiting the practice countdown's outcome.

The all-day operation will determine when they can blast off on the first lunar voyage by a crew in more than half a century.

Running two days behind because of a bitter cold snap, NASA set its countdown clocks to stop a half-minute before reaching zero, just before engine ignition. The clocks began ticking Saturday night, giving launch controllers the chance to go through all the motions and deal with any lingering problems with the Space Launch System rocket. Hydrogen leaks kept the first SLS rocket on the pad for months in 2022. Launch managers said they're confident the issues are behind them.

If the fueling demo goes well, NASA could launch commander Reid Wiseman and his crew to the moon as soon as Sunday.

The rocket must be flying by Feb. 11 or the mission will be called off until March. The space agency only has a few days in any given month to launch the rocket, and the extreme cold already has shortened February's launch window by two days.

The nearly 10-day mission will send the astronauts past the moon, around the mysterious far side and then straight back to Earth, with the goal of testing the capsule's life support and other vital systems. The crew will not go into lunar orbit or attempt to land.

NASA last sent astronauts to the moon during the 1960s and 1970s Apollo program. The new Artemis program aims for a more sustained lunar presence, with Wiseman's crew setting the stage for future moon landings by other astronauts.

NASA conducts crucial test to decide when Artemis will launch