FLORIDA: Elon Musk's private space enterprise SpaceX has launched the US military's most secretive X-37B plan through its hulking Falcon Heavy rocket that returned to the skies.

It launched a mysterious spacecraft for the US military on Thursday evening that will carry out cutting-edge research.

The rocket lifted off from NASA's Kennedy Space Centre in Florida at 8:07 p.m. ET, carrying the military's uncrewed X-37B space plane that operates autonomously -- to unprecedented altitudes. The launch streamed live on SpaceX's website. The side boosters returned to Earth and safely touched down, media reports said.

It is not clear, however, exactly where the space plane is going. The X-37B's activities in space have fascinated the space community as amateur enthusiasts raced to track its whereabouts and share theories about its activities, CNN reported.

Akin to a miniature NASA space shuttle with the windows blacked out, the reusable and fully autonomous X-37B has been known to carry out research on concepts such as relaying solar energy from space for use back on Earth and studying the effects of radiation on seeds used to grow food, reports said.

And this mission -- the seventh trip to space for an X-37B plane -- promises to be even more intriguing than earlier flights.