Earlier today there were a rash of predictions that the Air Force's secret space plane would be returning to Earth today. Multiple signs, including orbital maneuvers spotted by ground observers and a Notice to Airmen (NOTAM) notice warning pilots to stay out of the Kennedy Space Center, hinted that the bird might be coming home. But since then the Air Force has denied the unmanned plane is coming home, at least today anyway.

"The X-37 is still on-orbit," said Capt. Annmarie Annicelli, an Air Force spokeswoman at the Pentagon told Florida Today . "The program is conducting a regularly scheduled exercise this week."

A miniature, reusable space plane just thirty feet long, the X-37B has been in space for 636 days, a long voyage that is quickly becoming typical for the craft. What we still don't know is, what exactly it is doing up there?

There are two X-37B spaceplanes, and they are launched from U.S. Air Force Atlas 5 launch vehicles . Similar to the old Space Shuttle in appearance, the X-37B is just one quarter the size. Unmanned, it lacks a crew and life support systems. A cockpit and windows aren't necessary, either, giving the craft a bullet-with-wings profile. Like the space shuttle, the X-37B has double doors that conceal a payload bay, in this case one the size of a pickup truck bed.

While the original crewed Shuttle could stay aloft for just 17 days, the unmanned X-37B typically stays in orbit for hundreds of days. The current mission, OTV-4, flies high over some interesting places including Iran, North Korea, and China. That's probably not a coincidence, and leads to speculation that the spacecraft is doing so-called intelligence, surveillance, and reconnaissance work, training cameras, radars, and other spying devices on the ground below. The X-37B is probably more flexible than existing spy satellites, as it can return to Earth and have its payload swapped out at any time.

Other possible roles recently floated by experts consulted by Air & Space Magazine include the idea that X-37B is a testing platform for spaceplane technologies including autonomous flight; a proof-of-concept test that might lead to a larger, manned craft; a temporary communications satellite over a particularly important area or testing future satellite technologies.

Given that previous missions have lasted hundreds of days, it seems possible the X-37B is carrying out multiple roles. Previous missions have been 224 days, 469 days, and 674 days long, so this isn't even the longest mission by far. The Air Force isn't in the business of setting records. OTV-4 will come home whenever its mission is over...whatever that is.

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