The STS-48 "Zig-Zag UFO" video, Space Shuttle Discovery

Space Shuttle Discovery lifted off from Kennedy Space Center on 12 September 1991. The mission was STS-48. Aboard were John O. Creighton, Kenneth S. Reightler Jr., Charles D. Gemar, James Buchli and Mark N. Brown. Their main task was to deploy the Upper Atmosphere Research Satellite.

Nobody aboard reported seeing anything. The story belongs to a camera. It sat at the back of the cargo bay, pointed at the Earth's horizon. Its pictures went down live on NASA Select, the agency's television feed. On the ground, people were watching. Some were taping.

15 September 1991. Between 20:30 and 20:45 GMT.

Glowing points of light float along against the Earth's limb and airglow. Then a flash appears in the lower left of the screen. The objects change direction and accelerate away sharply, as if in response to the flash. Shortly after, a streak of light moves through the region the flash had vacated. Another streak moves through the right of the screen.

Maryland UFO researcher Donald Ratch recorded the sequence off the feed. Vincent DiPietro, an engineer at NASA's Goddard Space Flight Center, is said to have noticed it live. Richard C. Hoagland promoted the tape and released a video about it in 1992. He saw a "Star Wars" weapons test, with "dummy targets" and "electromagnetic rail guns".

The most detailed case for something unusual came from Jack Kasher, a physics and astronomy professor at the University of Nebraska. His paper appeared in the MUFON 1994 International UFO Symposium Proceedings. Kasher said the main object stopped horizontally for about half a second before it accelerated. Traced backward, the paths of two objects after the flash did not meet at any single thruster. For the ice theory to hold, he calculated, the particle would have to be about 22.2 miles away.

Mark Carlotto followed with a frame overlay analysis in 1999, and Lan Fleming with another in 2003. Fleming concluded the motions were "consistent with the flight paths of large self-propelled objects."

NASA's answer was short. Ice particles reacting to engine jets. The fullest version came from James Oberg, who worked at NASA's Johnson Space Center, including in Mission Control. Writing in 1992 in a personal capacity, he noted that the shuttle has more than 50 sources of ice. Propellant seeping from feed lines freezes and flakes off when the jets fire, in "80-millisecond pulses" under autopilot. In his reading, the flash was a jet firing. Every nearby particle was pushed away from it.

Astronaut Ron Parise reached the same conclusion: an ice particle deflected by a thruster firing. So did Philip Plait in his 2002 book Bad Astronomy. Oberg's name for this kind of debris was "space dandruff".

Nobody disputes that the footage is genuine. The argument is about what it shows. The case appeared in an episode of UFO Hunters and still runs through Metabunk, where Oberg joined the discussion in 2015. The UFO Database rates it "Inconclusive". One blog claims the thruster firing logs and first-generation tapes were never publicly released. So the flash, the turn and the streak remain on screen. Waiting for the next frame by frame reading.