The US Air Force's top secret, unpiloted space plane X-37B has arrived safely back on Earth after 780 days in orbit, the longest time in a craft.
Originally launched on 7 September 2017, the reusable vehicle landed once again on Sunday at NASA's Kennedy Space Center in Florida, after completing the fifth long-duration mission.
The first launch, which began in 2010, was only designed to fly for 270 days, but now, almost a decade later, these boeing-built space planes – of which there are at least two – have accumulated over 2,800 days in orbit.
"Today marks an incredibly exciting day for the 45th Space Wing," says commander Doug Schiess.
"Our team has been preparing for this event, and I am extremely proud to see their hard work and dedication in today's safe and successful landing of the X-37B."
Exactly what these spacecraft are doing.
A recent statement from the Air Force claims it is simply performing "risk reduction, experimentation and concept of operations" for reusable spacecraft technology; previous announcements have described tests for thermal protection systems, avionics, advanced propulsion systems, reentry and landing.
Some of the details, however, have experts concerned.
"This program continues to push the envelope as the world's only reusable space vehicle. With a successful landing today, the X-37B completed its longest flight and successfully completed all mission objectives," says the Air Force capabilities director, Randy Walden.
"This mission was successfully hosted by the Air Force Research Laboratory experiments, among others, as well as providing a ride for small satellites."
Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, tweeted "The United States or Russia has blatantly flouted the Convention."
Ever since 1962, the United Nations has maintained a Register of Objects Launched into Outer Space to keep tabs on which countries bear responsibility for space objects.
The statement that this @usairfor by X-37 flight deployed small satellites. This would be the first time that the USA or Russia has blatantly flouted the Convention. https://t.co/mplwuvsecv
– Jonathan McDowell (planet4589) October 27, 2019
Well over 90 percent of satellites are registered, McDowell has found in his research, and this is usually done within one or two years of launching. Of all the unregistered satellites up there in orbit, however, the US is responsible for a third of them.
There is, of course, a possibility that the US government hasn't gotten around to registering these new satellites, but McDowell says that seems unlikely given what he knows.
"Top secret (even TS / SCI) does not trump international law and treaty," he Argues.
As Earth's orbit gets busier and busier, a massive point of contention. Not only is there fear of dangerous debris flying around our planet, there is also the problem of secret militarisation.
The US Air Force missions will come under far greater scrutiny. Next year, and once again, the particulars are anyone's guess.