One early guess as to the function of the X-37B? It might ferry future troops into battle, taking them very high and then sending them back to Earth very fast. The fast transport mission doesn't match the most striking feature of the X-37B's performance, however, which is just how long it can stay in space. The space plane's first major trip into orbit lasted for seven months, and the plane's second kept it in space for 15 months. When it returns to Earth today, its third long trip into space, it will have been 22 months since it left in December 2012.