NASA has voiced "substantial concerns" about a planned constellation of broadband satellites, saying the commercial spacecraft would increase the risk of collisions in an important slice of Earth orbit.

On Oct. 30, NASA submitted an official comment letter to the U.S. Federal Communications Commission (FCC) regarding a request by Texas-based company AST & Science to operate a network of up to 243 satellites about 450 miles (720 kilometers) above Earth's surface, as Ars Technica's Eric Berger reported last week.

This constellation, called SpaceMobile, will provide broadband service directly to cell phones, if all goes according to the company's plan. To pull this off, the SpaceMobile satellites will sport very large antennas — gear that covers an area of about 9,700 square feet (900 square meters), Berger wrote.

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The SpaceMobile satellites will therefore have quite big cross sections, boosting the probability of conjunctions, or close flybys, with other spacecraft in their neck of the orbital woods, states the NASA letter, which you can find here.

And the space agency cares quite a bit about that orbital region, because it houses the "A-Train," a group of 10 Earth-observation missions operated by NASA, the U.S. Geological Survey and several international partners that travel around Earth in the same path. The A-Train's average altitude is 438 miles (705 km), but the satellites get as close to Earth as 429 miles (690 km) and as far away as 460 miles (740 km).

"Therefore, the AST constellation would be essentially collocated with the A-Train if the proposed orbit altitude is chosen," reads the comment letter, which was signed by Samantha Fonder, NASA representative to the Commercial Space Transportation Interagency Group.

NASA's calculations suggest that gliding safely among the SpaceMobile satellites might require 1,500 "mitigation actions," or spacecraft maneuvers, and 15,000 "planning activities" per year for the A-Train's handlers, Fonder wrote. That equates to about four maneuvers and 40 planning activities every day.

In addition, "this is an orbit regime that has a large debris object density (resulting from the Fengyun 1-C ASAT test and the Iridium 33-COSMOS 2251 collision) and therefore experiences frequent conjunctions with debris objects," the letter adds, referring, respectively, to a 2007 Chinese anti-satellite test and a 2009 smashup between one operational satellite and one dead one.

NASA would therefore like AST & Science to "consider alternative orbit regimes for this constellation, perhaps notably below the A-Train constellation, in order to allow for a more manageable safety-of-flight situation for a constellation of such large satellites," the letter reads.

AST & Science, for its part, maintains that SpaceMobile won't pose an undue collision risk. The company's calculations indicate that each of the network's satellites has just a 1-in-5,000 chance of colliding with another spacecraft at random, without any mitigation actions, over its operational life, Berger reported in another Ars Technica story last week. If AST & Science does get 243 satellites aloft, the probability of a random smashup constellation-wide would therefore be about 1 in 20.

AST & Science founder Abel Avellan also stressed that the company knows what it's doing, even though it has yet to launch any satellites to orbit. (The company is building a scaled-down prototype of a SpaceMobile spacecraft and plans to launch it in the second half of 2021, Avellan told Berger.)

"We're not a bunch of cowboys launching satellites," Avellan told Berger. "This is a serious, well-funded project."

Indeed, AST & Science recently snared about $128 million in a recent "Series B" investment round, Avellan told Berger, and the company's partners include Samsung, Rakuten and the Vodafone Group.

You can learn much more about SpaceMobile and NASA's objection to AST & Science's constellation plan in Berger's two stories, which you can find here and here.

SpaceMobile is not the only big broadband constellation in the offing, of course. Amazon aims to launch about 3,200 broadband satellites to low Earth orbit, and OneWeb has already lofted 74 internet satellites for a planned constellation of at least 648 spacecraft (though the company recently went through bankruptcy, potentially complicating those ambitions).

SpaceX has launched nearly 900 satellites for its Starlink megaconstellation and is already rolling out a public beta testing campaign for its broadband service. And there will be many more Starlink launches to come: Elon Musk's company already has FCC permission to operate 12,000 Starlink satellites in Earth orbit.

Starlink craft fly considerably lower than the A-train, zooming through space about 340 miles (550 km) above the planet's surface.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.