The U.S. military’s X-37B space plane blasted off Thursday on another secretive mission that’s expected to last at least a couple of years.

Like previous missions, the reusable plane resembling a mini space shuttle carried classified experiments. There’s no one on board.

The space plane took off aboard SpaceX’s Falcon Heavy rocket from NASA’s Kennedy Space Center at night, more than two weeks late because of technical issues.

It marked the seventh flight of an X-37B, which has logged more than 10 years in orbit since its debut in 2010.

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The last flight, the longest one yet, lasted 2 1/2 years before ending on a runway at Kennedy a year ago.

Space Force officials would not say how long this orbital test vehicle would remain aloft or what’s on board other than a NASA experiment to gauge the effects of radiation on materials.

Built by Boeing, the X-37B resembles NASA’s retired space shuttles. But they’re just one-fourth the size at 29 feet (9 meters) long. No astronauts are needed; the X-37B has an autonomous landing system.

They take off vertically like rockets but land horizontally like planes, and are designed to orbit between 150 miles and 500 miles (240 kilometers and 800 kilometers) high. There are two X-37Bs based in a former shuttle hangar at Kennedy.

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