Nothing ruins a telecom executive's evening quite like a SpaceX press release. Space Exploration Technologies Corp (SPCX) announced Thursday that it has agreed to acquire low-band spectrum from Grain Management, and the market's reaction was immediate: shares of the big three U.S. mobile carriers went into a slide after the bell.

The Spectrum Deal

SpaceX is picking up the nationwide 800 MHz spectrum portfolio from Grain Management for an undisclosed amount. Undisclosed, of course, because the number would probably make everyone uncomfortable.

"We announced an agreement to acquire a nationwide low-band spectrum license portfolio that will pave the way for Starlink to become a major mobile carrier in the US. With this new spectrum and our Gen2 constellation, Starlink Mobile can ensure Americans have access to high-speed mobile broadband no matter where they are," SpaceX tweeted.

In a blog post, SpaceX said the acquisition addressed a key technical gap for Starlink Mobile to become a major U.S. mobile carrier.

SpaceX said that when it receives final FCC approval, Starlink Mobile will deploy and connect devices everywhere. The company also highlighted that the new low-band spectrum will help Starlink Mobile's signal penetrate walls and other obstacles, which is the kind of thing that matters a lot when your competition has spent decades building cell towers.

"Starlink Mobile can now become the first network operator to deploy both satellite and terrestrial spectrum. One network that provides Americans with a reliable service indoors, outdoors, in cellular dead zones, and everywhere between."

SpaceX CEO Elon Musk shared the news on social media and said this is a "very big deal."

Telecommunications Stocks Fall

SpaceX stock is up 2.56% to $164.68 in after-hours trading Thursday. The same cannot be said for the incumbents.

Shares of mobile carrier companies falling after hours significantly are:

AT&T (T): -7.92%

Verizon Communications (VZ): -6.90%

T-Mobile US (TMUS): -6.78%

So the market is treating this as a real competitive threat rather than a press release with big ambitions. When a company that already owns the launch business starts buying the spectrum your network runs on, investors tend to notice.