A U.S. Air Force space plane in orbit for more than a year will come back to Earth this weekend, say officials who remained mum on the mission's purpose.

The robotic X-37B, after 15 months in space, is set to land Saturday at California's Vandenberg Air Force Base, although weather and other factors could change that, officials said.

"We continue to monitor weather and technical conditions day by day to ensure conditions are safe for landing," Vandenberg spokesman Jeremy Eggers told SPACE.com. "At this time, the next available opportunity is Saturday, dependent upon weather and technical conditions. The landing window extends through June 18."

The unmanned X-37B looks like a much-shrunken version of NASA's now-retired space shuttle; two X-37Bs could fit into the payload bay of one of the shuttles now on their way to museums.

The space plane, by comparison, has a payload bay the size of a pickup truck bed.

The Boeing-built X-37B, powered by a solar array that lets it remain in orbit for long periods, is designed to land itself on a runway without the aid of a human controller.