HAWTHORNE, Aug. 5 -- Starlink subscribers doubled to 12 million in the year through June, SpaceX disclosed Tuesday, as the satellite operator posted second-quarter revenue of $7.8 billion and its chief operating officer made clear the company intends to compete directly with AT&T Inc., Verizon Communications Inc., and T-Mobile US Inc. for the American wireless market.

Shares in SpaceX climbed 9.4 percent during Tuesday's session before surrendering 7.2 percent in after-hours trading after a quarterly net loss of $541 million, or 9 cents per share, landed below analyst expectations. AT&T, Verizon, and T-Mobile each declined after-hours on the news.

The subscriber number that rattled the industry was notable not simply for its size but fo...