A space jellyfish (also jellyfish UFO or rocket jellyfish) is a rocket launch-related phenomenon caused by sunlight reflecting off the high-altitude rocket plume gases emitted by a launching rocket during morning or evening twilight. The observer is in darkness, while the exhaust plumes at high altitudes are still in direct sunlight. This luminous apparition is reminiscent of a jellyfish. Sightings of the phenomenon have led to panic, fear of nuclear missile strike, and reports of unidentified flying objects.
A similar effect is the twilight phenomenon.