Final preparations began this weekend at NASA’s Stennis Space Center in Mississippi to load super-cold liquid hydrogen and liquid oxygen into the massive core stage of the Space Launch System heavy-lift rocket for the first time as soon as Monday.

SpaceX launched its 12th Falcon 9 rocket of the year Thursday from California with Taiwan’s Formosat 5 Earth observation satellite. The two-stage rocket lifted off at 11:51 a.m. PDT (2:51 p.m. EDT; 1851 GMT) and deployed the Taiwanese-built spacecraft into polar orbit. The Falcon 9’s first stage booster successfully landed on a drone ship downrange in the Pacific Ocean.

The mission, dubbed Cert-2 by United Launch Alliance, is the final mission required before the rocket can begin launching national security missions. Liftoff is set for the beginning of a three-hour window at 6 a.m. EDT (1000 UTC).