It’s assumed that the X-37B — one of two used by the military — operates a lot like a spy satellite, only more versatile.

LOMPOC, Calif., Oct. 13 (UPI) — The X-37B is coming home, presumably chock-full of secrets. The unmanned Air Force plane — a sort of space drone that has spent the last two years orbiting the planet — is expected to return to Earth on Tuesday. At some point during daylight hours, depending on weather, the plane will land at Vandenberg Air Force Base in California.

“Team Vandenberg stands ready to implement safe landing operations for the X-37B Orbital Test Vehicle, the third time for this unique mission” the Air Force’s Col. Keith Baits, 30th Space Wing commander, said in a press release issued last week.

The Air Force’s announcement of the plane’s return landing arrived with few details, and that’s been the case throughout the project’s brief history. It’s the plane’s third complete mission since it first launched in 2010, the longest yet — a world record at 671 days in space. And as was the case with the first and second missions, no one really knows what exactly the plane’s been up to for the past two years.

It’s assumed that the X-37B — one of two used by the military — operates a lot like a spy satellite, only more versatile. Military satellites are set up for a mission, blasted into space, and left un-tinkered-with for the rest of their lifetime. But because the X-37B is a plane, and can come and go as it pleases, it can be outfitted with a different array of sensors and instruments to complete the top-secret objectives of each new operation.

As is often the case for secretive government programs, the void left by the absence of official information is quickly filled by outlandish conspiracy theories. Some have speculated that the plane is used to listen in on other nation’s spy and telecommunications satellites — a task that hardly requires a next-generation space plane. Others suggest the drone is designed to physically assault or hijack other spy satellites. And a few have pegged the X-37B as the military’s first space bomber, capable of dispensing and targeting a missile to anywhere on Earth within an hour’s time. Such guesses range from ridiculous to unlikely, experts say.

“I think it is primarily an ISR (Intelligence, Surveillance and Reconnaissance) platform for testing new sensor technologies or validating new technologies,” Brian Weeden, a retired U.S. Air Force Space Command officer, told The Daily Beast. “The current [vehicle] on orbit has basically been in the same orbit since launch, with only the occasional maneuver to maintain that orbit. That’s consistent with a remote sensing/ISR mission.”

Given that the X-37B’s orbit passes atop North Korea, Iran, Pakistan, Afghanistan, and China, Weeden’s explanation seems the most plausible.