Technicians began the painstaking work of assembling a powerful Atlas 5 rocket today, a launcher that will deploy an advanced satellite to substantially improve U.S. weather forecasting.

A Long March 4B rocket successfully launched an oceanography satellite Wednesday from northwestern China, hours before officials at a different spaceport scrubbed the planned launch of a cargo ship to deliver supplies to the Chinese space station.

If all goes according to plan, engineers in southern Mississippi later this month will load cryogenic propellants into the core stage of a rocket NASA says will launch astronauts back to the moon, exposing its tankage and internal plumbing to extreme operating conditions hundreds of degrees below zero for the first time. The fueling test — a major milestone in its own right — is a precursor to a high-stakes eight-minute test-firing of the Space Launch System’s four shuttle-era main engines planned as soon as November.