For the first time, we have a complete, representative number for the overall orbital collision risk of a satellite mega-constellation.
Last month, Amazon provided the U.S. Federal Communications Commission (FCC) with data for its planned fleet of 3,236 Kuiper System broadband Internet satellites.
If one in 10 satellites fails while on orbit, and loses its ability to dodge other spacecraft or space junk, Amazon’s figures [PDF] show that there is a 12 percent chance that one of those failed satellites will suffer a collision with a piece of space debris measuring 10 centimeters or larger. If one in 20 satellites fails—the same proportion as failed in rival SpaceX’s first tranche of Starlink satellites—there is a six percent chance of a collision.
More than a third of all the orbital debris being tracked today came from just two collisions that occurred about a decade ago. Researchers are concerned that more explosions or breakups could accelerate the Kessler Syndrome—a runaway chain reaction of orbital collisions that could render low earth orbit (LEO) hostile to almost any spacecraft.
“Six percent is huge,” says John Crassidis, an expert on orbital debris at the University at Buffalo. “At a six percent chance of collision, astronauts would be put into an escape hatch to possibly escape. Even at orders of magnitude less than that, you’d want to do a maneuver to avoid it.”
Those figures are not even the worst case scenario. The FCC also asked Amazon to provide an aggregate collision risk should 15 percent of its satellites lose their ability to maneuver. That resulted in a 17 percent chance of a collision.
Amazon noted in its reply [PDF] that: “[The] failure rate assumptions of 5 percent, 10 percent, or 15 percent of the fully deployed system are well beyond what Amazon would view as expected or acceptable.” It claimed that it would conduct comprehensive on-the-ground qualification of Kuiper satellites and their sub-systems, carry out verification testing in-orbit before raising satellites to their operational altitude, and continually monitor the health of each satellite.
“As a result,” wrote Amazon, “Kuiper System satellites will be designed, manufactured, qualified, and operated to specifications and tolerances that will ensure potential failure rates are well below those proposed.”
This image shows how Kuiper satellites will be positioned to achieve the best possible broadband Internet coverage.Illustration: Amazon
Envisioning a 15 percent failure rate among satellites is new for the FCC. When the commission previously asked other companies that were planning LEO constellations, including SpaceX and Telesat, for collision risks, the agency only asked for data should one, five, or 10 percent of the companies' satellites fail.
When responding to the FCC in 2017, SpaceX only provided a collision risk for the one percent failure scenario, writing [PDF]: “SpaceX views satellite failure to deorbit rates of 10 or 5 percent as unacceptable, and even a rate of 1 percent is unlikely.”
However, three out of 60, or five percent, of the company’s first batch of Starlink satellites failed following their launch in May. These satellites are at a low enough altitude that they will quickly be dragged down into the atmosphere and burn up naturally.
Even higher failure rates than 15 percent are not unheard of. Around 30 percent of Iridium’s first generation of communication satellites died on orbit, and the European Space Agency noted in its latest space environment report [PDF] that only about half of all satellites are properly disposed of at the end of their lifetimes.
If the risk associated with a one percent failure rate that SpaceX quoted is extrapolated to a 15 percent failure rate, Elon Musk’s satellites presented a similar aggregate risk as those belonging to Jeff Bezos, at around 14 percent. However, SpaceX has since altered its planned operational altitude for these satellites, and made design changes, which could reduce the chance of collisions. The company does not seem to have supplied the FCC with updated collision risk figures for its new working altitude of 550 kilometers, which is close to Amazon’s.
Instead, SpaceX recently provided [PDF] collision predictions for its satellites at their initial deployed altitude of 350 kilometers. There is little tracked debris at this low altitude, as objects decay into the atmosphere very quickly. Thus SpaceX’s latest quoted collision risk, nearly 700 times lower than for Kuiper satellites, cannot easily be compared with Amazon’s.
A satellite failing at Kuiper’s highest altitude will take around six years to burn up in the atmosphere.Illustration: Amazon
The FCC is in the middle of a review of new rules it proposed to minimize the chance of catastrophic orbital collisions or satellites endangering people on re-entry.
In the past, satellite operators have been judged on the risk caused by individual satellites, including a requirement that there must be a less than 1 in 10,000 chance that an individual satellite will cause a casualty on Earth. But with plans for broadband Internet satellites that number in the thousands, even small risks can add up.
In December, IEEE Spectrum reported that the aggregate chance that SpaceX’s planned Starlink constellation would cause an injury or death on Earth was 45 percent every six years. SpaceX subsequently announced that it would re-design its satellites so that no components would survive re-entry.
In its latest filing, Amazon did not provide any casualty risk data to the FCC, writing: “Although Amazon seeks to use materials and components that will demise fully upon re-entry into the Earth’s atmosphere, the specific design and composition of certain satellite components remains outstanding. As a result, Amazon is not yet able to provide [the] meaningful estimates requested.” Amazon declined to comment in response to a request from IEEE Spectrum.
The FCC will likely want that information before it allows any of Amazon’s satellites to launch. But other experts worry that, even if it does, the FCC is ill-equipped to deal with the complexities of orbital safety.
“The way that collision probabilities are computed has a variety of flaws in it,” says Moriba Jah, an associate professor at the University of Texas who studies orbital mechanics. “And who in the government is technically competent to provide an independent assessment of the analysis? Nobody is doing that. They’re not checking up on it [or] verifying it in any way. They’re just accepting it at face value.”
SpaceX plans to launch its second group of Starlinks within the next month. Amazon has not announced yet when it hopes to launch its first Kuiper satellite, and its application before the FCC is still pending.
Mark Harris is an investigative science and technology reporter based in Seattle, with a particular interest in robotics, transportation, green technologies, and medical devices. He’s on Twitter at @meharris and email at mark(at)meharris(dot)com. Email or DM for Signal number for sensitive/encrypted messaging.