The European-built Solar Orbiter spacecraft was installed on top of its United Launch Alliance Atlas 5 launcher Friday at Cape Canaveral, ready for final charging and checkouts before liftoff Feb. 9 to finally begin a more than $1.5 billion science mission first approved by the European Space Agency nearly 20 years ago.

A critical test firing Saturday of the four main engines powering the first stage of NASA’s gargantuan Space Launch System moon rocket is the final major hurdle before the fully assembled booster’s costly, oft-delayed launch late this year on an unpiloted test flight.

The Atlas 5 countdown leading up to Friday’s early morning liftoff from Cape Canaveral with Boeing’s Starliner crew capsule will be several hours longer than usual.