The U.S. Air Force sent its unmanned X-37B spacecraft into orbit for the fourth time on Wednesday
The X-37B is one of two spacecraft, built for the Air Force by Boeing, first launched in 2010. Its mission? That’s a secret.
The two planes have been on three journeys to date: one from April to December of 2010, then from March 2011 to June 2012, and finally a third flight from December 2012 all the way until this past October.
The Air Force has remained mum despite long-running speculation about the purpose of these two spacecraft, the New York Times reports, and in 2010 the Pentagon denied that the planes have anything to do with weapons. Wednesday’s launch of one of the spacecraft (it is also unknown which of the two launched) happened at Cape Canaveral, Fla.
The X-37B spaceplane was carried into the sky by an American Atlas V rocket. Watch a video of the launch below:
This article originally appeared on Fortune.com
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