SpaceX has received Federal Communications Commission approval to halve the orbital altitude of more than 1,500 planned broadband satellites in order to lower the risk of space debris and improve latency.

SpaceX's satellite project, named Starlink, aims to provide high-speed, low-latency broadband around the world. In a statement on the new FCC approval, SpaceX said that "Starlink production is well underway, and the first group of satellites have already arrived at the launch site for processing."

SpaceX last year received FCC approval to launch 4,425 low-Earth-orbit satellites at several different altitudes between 1,110km to 1,325km. However, the FCC approval was contingent on SpaceX filing a more detailed debris mitigation plan.

As part of its plan to prevent space debris, SpaceX later asked for permission to operate 1,584 of those satellites at an altitude of 550km instead of the previously authorized 1,150km. The FCC approved the request in an order on Friday but pointed out that SpaceX still has to file a detailed debris mitigation plan for the rest of the satellites.

"Given the atmospheric drag at this lower altitude, this relocation will significantly enhance space safety by ensuring that any orbital debris will quickly re-enter and demise in the atmosphere," SpaceX told the FCC in November 2018 in its application for a license modification.

At the lower altitude, "any orbital debris will undergo rapid atmospheric re-entry and demise, even in the unlikely event that a spacecraft fails in orbit." (SpaceX is designing its satellites to burn up completely during atmospheric re-entry in order to prevent physical harm from falling objects.)

Satellites orbiting at 1,150 km will take "hundreds of years to enter the Earth's atmosphere," but a SpaceX satellite "will take less than five years (even under worst-case assumptions) if it starts at an altitude of 550 km," the company said.

The lower altitude will bring an advantage to broadband users, SpaceX explained. "By operating closer to the Earth, SpaceX would also reduce the latency of its communications signals to as low as 15 milliseconds, at which point it would be virtually unnoticeable to almost all users," the company said. (SpaceX has said latency from an altitude of 1,150km would be 25ms to 35ms.)

Low-altitude trade-offs

There are downsides to using a lower altitude, though.

"The very same atmospheric drag that helps to sweep the orbit clean of debris also forces satellites to work harder to remain in orbit," SpaceX wrote. "Staying aloft requires the satellite to be able to overcome more atmospheric drag. In addition, satellites operating at low altitude see less of the Earth, requiring more satellites to serve a given area."

SpaceX said it has conducted tests that show it can solve these problems. SpaceX said it intends to "operate at this altitude nonetheless based in part on feedback gained from its experimental satellites, which have conducted extensive testing of SpaceX's capabilities to operate in the lower 500km range. Consequently, SpaceX has learned to mitigate the disadvantages of operating at a lower altitude and still reap the well-known and significant benefits."

SpaceX also plans to reduce the number of satellites in the low-Earth-orbit constellation from 4,425 to 4,409. The planned orbital heights of the rest of the 4,409 satellites haven't been changed. Under its FCC authorization, SpaceX must launch at least half of these satellites by March 29, 2024 and the rest by March 29, 2027.

The FCC said it is satisfied by SpaceX's debris mitigation plan for the 1,584 satellites subject to the altitude change. But SpaceX has to submit a more detailed plan for the rest of the satellites.

"Although we find that the orbital debris mitigation plan is sufficient with regard to the space stations that SpaceX proposes to operate under its modification, SpaceX has provided no new information regarding the orbital debris mitigation plans for the other satellites in its proposed system," the FCC said. "SpaceX has only partially satisfied the condition on its authorizations that requires SpaceX to submit, and have approved by the Commission, an updated orbital debris mitigation plan prior to initiation of service."

The new altitude of 550km isn't the lowest that SpaceX plans to use for its broadband service. SpaceX received a separate authorization in November 2018 to deploy 7,518 broadband satellites at altitudes of 335km to 346km. These lower satellites are intended to boost capacity and reduce latency in heavily populated areas. In all, SpaceX has FCC approval to launch nearly 12,000 broadband satellites.