A team of amateur sky watchers has pierced the veil of secrecy surrounding the debut flight of the nation’s first robotic spaceplane, finding clues that suggest the military craft is engaged in the development of spy satellites rather than space weapons, which some experts have suspected but the Pentagon strongly denies.

Last month, the unmanned successor to the space shuttle blasted off from Florida on its debut mission but attracted little public notice because no one knew where it was going or what it was doing. The spaceship, known as the X-37B, was shrouded in operational secrecy, even as civilian specialists reported that it might go on mysterious errands for as long as nine months before zooming back to earth and touching down on a California runway.

In interviews and statements, Pentagon leaders strongly denied that the winged plane had anything to do with space weapons, even while conceding that its ultimate goal was to aid terrestrial war fighters with a variety of ancillary missions.

The secretive effort seeks “no offensive capabilities,” Gary E. Payton, under secretary of the Air Force for space programs, emphasized on Friday. “The program supports technology risk reduction, experimentation and operational concept development.”