After being delayed a day by bad weather, the U.S. Air Force’s second X-37B robotic space plane blasted off from Florida March 5 on a classified mission.

The unmanned X-37B minishuttle — known as Orbital Test Vehicle 2 (OTV-2) — lifted off from Cape Canaveral at 5:46 p.m. EST atop a United Launch Alliance Atlas 5 rocket.

The space plane was originally scheduled to launch March 4, but cloudy, windy conditions scrubbed two attempts.

The X-37B’s mission is classified, but Air Force officials have said the vehicle will be used to test out new spacecraft technologies. Shortly after launch, the mission went into a scheduled media blackout, with no further public updates.

The launch marks the start of the X-37B program’s second space mission. The Air Force’s other X-37B plane, known as OTV-1, returned to Earth in December after a similarly mysterious seven-month maiden mission.

The X-37B spacecraft resemble NASA’s space shuttles, but are much smaller. The vehicle is about 8.8 meters long by 4.5 meters wide, with a payload bay about the size of a pickup truck bed. By comparison, two entire X-37Bs could fit inside the payload bay of a space shuttle.

The space plane can fly long, extended missions because of its solar array power system, which allows it to stay in orbit for up to nine months, Air Force officials have said.

What exactly the vehicle does while circling the Earth for so long is a mystery, since the craft’s payloads and missions are classified. Partly as a result of the secrecy, some concern has been raised — particularly by Russia and China — that the X-37B is a space weapon of some sort.

But the Air Force has repeatedly denied that charge, claiming that the X-37B’s chief task is testing out new hardware for future spacecraft — instruments like sensors and guidance, control and navigation systems. And that is likely to be the case, experts say.

“It gives the Air Force the ability to test-fly some of this hardware,” said Brian Weeden, a former Air Force orbital analyst who works as a technical adviser for the nonprofit Secure World Foundation.

Weeden suspects the X-37B is testing gear for the National Reconnaissance Office, the intelligence agency that builds and operates U.S. spy satellites. That would explain all the secrecy, he said.

The Air Force’s other X-37B, known as OTV-1, launched last April and returned in December after spending 224 days in space. While its mission also was classified, technology testing was OTV-1’s primary job, too, Air Force officials have said.

Boeing Space and Intelligence Systems builds the X-37B for the Air Force. Originally, NASA used the space plane as an experimental test bed until funding for the project ran out in 2004. The vehicle then passed to the Defense Advanced Research Projects Agency and was ultimately turned over to the Air Force in 2006.