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We know what it looks like: a cute little space shuttle. We know where it is: in orbit around Earth. We know who sent it there: the US Air Force, via a SpaceX Falcon 9 rocket in September 2017. Why just don't know why.

The uncrewed Boeing-built X-37B space plane has now been in orbit for 719 days, breaking the previous record of nearly 718 days set during its last mission, which returned to Earth in early 2017.

The Air Force hasn't revealed the exact purpose of the drone, but it did at least let us know when it landed last time. "The X-37B program is the newest and most advanced re-entry spacecraft that performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies," the Air Force said in a boilerplate description it's repeated over the years.

Acting Secretary of the Air Force Matthew Donovan kept things plenty vague when he visited the X-37B facility at Kennedy Space Center in Florida in July. "The X-37B is a key component to America's future in space dominance," he said in an Air Force release.

The Air Force didn't immediately respond to a request for comment about the X-37B's current flight.

The X-37B, which began as a NASA project, made its first flight in 2010. A second model took off on its first mission in 2011.

We don't know X-37B's purpose, but we do know it's been hanging out in orbit for longer and longer periods of time. As for when we can expect it to rejoin us on Earth, that's classified, too. We'll just have to live with the mystery.