Watch a time-lapse of the Crew Dragon access arm installation (members only)

Fueled segments of the solid rocket boosters that will lift off on the first flight of NASA’s Space Launch System have arrived at the Kennedy Space Center in Florida, but officials will hold off on stacking the boosters until the SLS core stage completes a crucial test-firing later this year.

Eric Boe, one of three astronauts assigned to the first piloted test flight of a Boeing CST-100 Starliner commercial crew ship later this year has been removed from the mission due to unspecified medical issues, NASA announced Tuesday. He will be replaced by veteran astronaut Mike Fincke.

While a SpaceX Dragon cargo ship closed in on the International Space Station early Sunday, a Russian Soyuz spacecraft blasted off from frigid Kazakhstan, boosting a fresh three-man crew into orbit for a two-day flight to the lab complex.