What was the US military's secret space plane doing on its record-breaking mission?

A secret space plane operated by the United States Space Force (USSF) has landed back on Earth after spending a record 908 days in orbit. But what it was doing above our heads remains shrouded in mystery. 

The uncrewed X-37B space plane touched down at NASA’s Kennedy Space Station on Nov. 12 at 5:22 a.m. ET, concluding the sixth mission that it and another identical vehicle have completed since the first flight in 2010. Details on its activities during the record-smashing trip are sparse, but officials claim it was conducting a number of scientific experiments at around 249 miles (400 kilometers) above Earth.

The X-37B was first designed by Boeing for NASA, before being adapted for use by the U.S military. It is an airplane-spacecraft hybrid that in many ways resembles a miniature space shuttle. For its sixth mission, classified as Orbital Test Vehicle-6 (OTV-6), it was launched vertically while perched atop an Atlas V rocket in May 2020. The space plane has now spent roughly 10 years in orbit across all of its missions, covering approximately 1.3 billion miles (2.1 billion km). The new 908-day flight smashes the 780-day record for a space plane in continuous orbit, which was also set by the X-37B during an earlier mission.

Related: 10 things we know about the secret X-37B space plane

"The X-37B continues to push the boundaries of experimentation, enabled by an elite government and industry team behind the scenes," Lt. Col. Joseph Fritschen, X-37B program director at the Air Force Rapid Capabilities Office, said in a statement. "The ability to conduct on-orbit experiments and bring them home safely for in-depth analysis on the ground has proven valuable for the Department of the Air Force and scientific community. The addition of the service module on OTV-6 allowed us to host more experiments than ever before."

The United States Space Force has revealed only a few morsels of information about the experiments conducted aboard the craft during its most recent flight. These include a test by the U.S. Naval Research Laboratory that successfully harvested light from the sun before beaming it back to Earth as microwaves; and the deployment of an electromagnetically steered training satellite designed by U.S. Air Force cadets. NASA also provided an experiment, called Materials Exposure and Technology Innovation in Space (METIS-2), that researched the effects of space on different materials. 

No other details of the experiments on board were revealed, although this hasn’t stopped rivals from engaging in speculation. Dmitry Rogozin, the former head of Russia’s Roscosmos space agency, claimed in an April interview with the Russian state-owned news channel Russia-24 that the craft could be being used for spying or for carrying weapons of mass destruction. Chinese military expert and commentator Song Zhongping echoed this sentiment, telling the South China Morning Post that the craft’s ability to alter its orbit mid-flight gave it the ability to spy on other satellites or on Earth-based targets, as well as launch attacks from orbit.

"If the X-37B can be loaded with small satellites, it can also be loaded with weapons. It may also be able to be fitted with robotic arms to capture other satellites that are in orbit," Song said.

Former Pentagon official Heather Wilson has also remarked on the craft’s ability to alter its orbit, a capability she said was due to the significant drag generated by its low Earth orbit.

"Which means our adversaries don't know — and that happens on the far side of the Earth from our adversaries — where it's going to come up next," Wilson said at the Aspen Security Forum in 2019. "And we know that that drives them nuts. And I'm really glad about that."

China also has a secret space plane, which was launched into orbit from a Long March 5B rocket on Aug. 4. Much like the X-37B, much of what it is doing in orbit is unknown.