Q&A with Elon Musk, SpaceX’s founder and CEO

Cosmonauts Sergey Prokopyev and Dmitri Petelin floated outside the International Space Station Wednesday, unbolted an experiment airlock from the Rassvet docking compartment and then connected it to Russia’s Nauka multi-purpose laboratory module in a 7-hour, 11-minute excursion.

The mission, dubbed USSF-124, was the eighth National Security Space Launch using a SpaceX Falcon 9 rocket. Liftoff occurred at the start of a four-hour launch window that opened at 5:30 p.m. EST (2230 UTC) on Wednesday.

SpaceX’s launch team loaded liquid propellants into the company’s first upgraded Falcon 9 Block 5 rocket Friday at the Kennedy Space Center in Florida, and the countdown rehearsal culminated in a brief ignition of the launcher’s nine Merlin main engines on the launch pad.