The launch itself wasn't a secret: The robotic craft, made by Boeing, took off on its fifth mission on Sept. 7, 2017, atop a SpaceX Falcon 9 rocket that launched from NASA's Kennedy Space Center in Cape Canaveral, Florida, according to a SpaceX statement.

Cees Bassa didn't know at first that he'd spotted the spy craft, which looks like a mini space shuttle when seen up close. But he was intrigued enough to estimate that the object's circular orbit was inclined by about 54.5 degrees and had an altitude of 220 miles (355 kilometers) above Earth. With those estimates, he realized the flying object could be the fifth mission of the X-37B, also called the Orbital Test Vehicle (OTV-5 for the fifth mission), writes Leonard David, the Space Insider columnist for Live Science's sister site Space.com. [The X-37B's Fourth Mystery Mission in Photos]