Unknown Object in Two NASA STS-103 Mission Photos
The unknown object is self-luminous and seems to be oval or disk shaped.
Official NASA photo (low resolution):
Streetcap1 managed to get hold of the high resolution UFO photo before NASA pulled it:
The NASA STS-103 Space Shuttle Discovery mission:
Launch date: 19 December 1999 -
Landing date: 27 December 1999.