A September power outage at a California facility of SpaceX, the space venture of billionaire entrepreneur Elon Musk, caused a loss of ground control for at least an hour during a mission that included the first private spacewalk in history, according to three people familiar with the problem.

The spacewalk, part of SpaceX's five-day Polaris Dawn mission, was carried out by private astronauts including Jared Isaacman, a fellow billionaire and longtime Musk partner whom incoming President Donald Trump has nominated to be administrator of NASA.

The outage, which hasn't previously been reported, meant that SpaceX mission control was briefly unable to command its Dragon spacecraft in orbit, the people said. The vessel, which carried Isaacman and three other SpaceX astronauts, remained safe during the outage and maintained some communication with the ground through the company's Starlink satellite network.