If you were watching the night sky Tuesday night, perhaps because of the Perseids meteor shower that peaked around then, you might have seen something a bit stranger.

It appeared first as a glowing disc, which soon began to spiral, leaving a trail of milky-white light. It made its way across the northern sky over the span of about 10 minutes before slowly dissipating into the darkness.

The phenomenon, which was visible in parts of the East Coast, was caused by an exhaust plume from a rocket that launched just before 9 p.m. Tuesday from Cape Canaveral Space Force Station. An explanation of the phenomenon was posted on the Adirondack Sky Center and Observatory Facebook page, and news outlets in locations including New Jersey and Pennsylvania reported the unusual sight.

According to space and astronomy news site Space.com, the Vulcan rocket launched by the United Launch Alliance was carrying the U.S. military’s first experimental navigation satellite launched in 48 years.

Space.com reports that the satellite will be testing experimental technologies that are meant to resist jamming and spoofing, and it can also be reprogrammed in orbit. Vulcan Centaur is one of two launch providers now certified to launch military and spy satellites, along with SpaceX.