SpaceX is revising its satellite internet initiative, Starlink, and it now hopes to operate some of its spacecraft at a lower altitude than originally planned. In a new filing to the Federal Communications Commission (FCC), SpaceX is asking the agency to modify its license so that more than 1,500 Starlink satellites can operate at an altitude 600 kilometers lower than the company originally requested.

SpaceX argues that this change will make the space environment safer, as it will be easier to get rid of these satellites at this new altitude when they run low on fuel or can no longer function properly in orbit. This update could also explain the unexpected behavior of two of SpaceX’s test satellites for Starlink, which have remained in lower orbits than expected.

SpaceX argues this change will make the space environment safer

Back in March, the FCC approved SpaceX’s license for the first phase of its ambitious Starlink initiative — the company’s long-term plan to launch nearly 12,000 satellites into orbit to beam internet coverage down to Earth. Initially, SpaceX asked the FCC for permission to launch 4,425 satellites into orbits ranging between 1,110 to 1,325 kilometers high. But with this new filing, SpaceX is requesting that 1,584 of those satellites, which were supposed to operate at 1,110 kilometers, be allowed to operate at 550 kilometers instead.

SpaceX says moving the satellites to a lower altitude means it can do more with less. Originally, the company said it needed 1,600 satellites to operate at the 1,110-kilometer altitude, but moving them lower means the company can get the same results with 16 fewer spacecraft. And the lower altitude makes it easy to dispose of these satellites once they’re done in space. At this height, particles from Earth’s atmosphere bombard the spacecraft more rapidly, pushing them out of orbit and dragging them down to the planet. And on the way down, they burn up in the atmosphere.

Making sure these spacecraft come out of orbit in a timely manner is crucial because of the vast number of vehicles that SpaceX wants to put into orbit. A constellation the size of Starlink could dramatically increase the number of operational satellites in space, raising the risk of in-space collisions. A recent NASA study argued that 99 percent of these satellites will need to be taken out of orbit, reliably, within five years of launch, or the risk of satellite collisions goes up quite a bit.

De-orbiting a satellite typically entails bringing the vehicle to a low enough altitude with thrusters where Earth’s air particles and gravity drag the probe down so that it burns up. Now, with this new filing, SpaceX won’t have to significantly move 1,584 of its satellites to get rid of them. The atmosphere at 550 kilometers should do the job within a few years. That’s also helpful in case the spacecraft fails in orbit. Satellites that fail in higher altitudes could turn into unoperational space debris that stay in orbit for long periods of time. At lower altitudes, they can still fail, and the atmosphere will still swallow them up in a timely manner.

The new filing may also explain the behavior of two of SpaceX’s test satellites

And that may go a long way with the FCC, which expressed concerns about how reliable these satellites will be and if they will be de-orbited on time. In fact, when the FCC approved the Starlink initiative, the agency said “it would be premature to grant SpaceX’s application based on its current orbital debris mitigation plan.” However, SpaceX received authorization anyway under the condition that the company would provide an updated plan for how it would de-orbit its satellites on time.

The new filing may also explain the behavior of two of SpaceX’s test satellites currently in orbit. In February, SpaceX successfully launched a pair of test satellites — TinTin A and B — meant to test out the technology needed for the internet-from-space endeavor. However, the satellites did not reach their final intended orbits in space. The goal was to insert the satellites at an altitude of 511 kilometers, and once all of the systems on the vehicles had been checked out, SpaceX would then raise the pair to an altitude of 1,125 kilometers with the satellites’ onboard thrusters, an operation that would take about half a year to complete. The company detailed these plans in a letter to the FCC dated February 1st, 2018, three weeks before the launch

But the satellites never left the vicinity of 500 kilometers, according to Space_Track.org, a site that uses satellite tracking information provided by the Department of Defense. A chart of the duo’s location over time shows the satellites naturally getting lower in their orbits — most likely due to the particles from Earth’s atmosphere dragging them downward, according to Jonathan McDowell, an astrophysicist at Harvard University who tracks spacecraft in orbit. In June and July, TinTin B raised its orbit slightly, suggesting a small burn of its onboard thrusters. TinTin A has barely moved, except for a small burst on October 17th.

This led to speculation of a possible failure, which was first reported by Space Intel Report, and SpaceX told the site that the satellites “were delivered to their intended orbit, communicated with ground stations, continue to communicate with ground stations, and remain in operation today.”

In today’s FCC filing, SpaceX says that it decided to change the orbits based on what it had learned from operating TinTin A and B at the lower altitude. “Operating at a lower altitude offers several attractive features both during nominal operation and in unplanned scenarios,” SpaceX wrote in the filing. The company says this will simplify the design of the spacecraft and reduce the latency in signals to just 15 milliseconds, which “would be virtually unnoticeable to almost all users,” according to SpaceX.

SpaceX acknowledges that there are some downsides to the lower orbit

However, SpaceX acknowledges that there are some downsides to the lower orbit. Because the atmosphere is slightly denser at this altitude, it also means the spacecraft have to work harder to stay in orbit and not get dragged down to Earth prematurely. It will also reduce the amount of Earth’s surface each satellite can cover at a given point in time, so SpaceX will need to modify how the spacecraft transmit their signals.

The FCC still needs to approve SpaceX’s request, but the commission has declared November “Space Month,” so it’s possible there may be some movement on this soon. In the meantime, SpaceX says it plans to launch its first batch of Starlink satellites in 2019. Under the conditions of its FCC license, SpaceX must launch at least half of its 4,425 satellites (potentially 4,409 now) within six years in order to bring its full constellation into use. In October, Reuters reported that SpaceX CEO Elon Musk had done a shakeup of Starlink’s management, in order to meet the program’s deadlines more quickly.