The X-37B space plane, an unmanned vehicle operated by the U.S. Air Force, launched into orbit on March 5, 2011 and was spotted by skywatchers all throughout March 2011. Officially known as the Orbital Test Vehicle 2 (OTV-2), this spacecraft's mission is a secret even though its orbital pattern is not, thanks to avid satellite watchers like Ted Molczan, who developed a basic orbital plan derived from OTV-1's flight last year.
The X-37B was built by Boeing (it looks like the NASA space shuttle, but it's only a quarter of its length) and started out as a NASA project in 1999. The X-37B was then transferred to the U.S. Department of Defense and the Air Force by 2006, where it gained its secretive nature and status. See images and specifications of the X-37B in a gallery by Fox News.
The easiest way to find the X-37B in the night sky is to visit N2YO.com's tracking page. While it's not as detailed or as accurate as some of the unclassified satellites the site is tracking, the OTV-2's location is fairly well approximated as it travels around the planet. (There are other sites that offer similar information as well, but some require registration.) You can also use a mobile app such as Satellite Flybys to try to spot the plane. OTV-2 is fairly bright in the night sky -- it's approximately +3.0 magnitude, or as bright as Eta Serpentis within the Serpens constellation.
Have you spotted OTV-2 or any other satellite or other man-made object in the sky? If so, share your story in the comments.
Photo credit of X-37B launch: Boeing