History:
Nervala IV. Class-M planet whose upper atmosphere contains a powerful distortion field, making it impossible to reach the planet by shuttle or transporter for most of the planet's year. A Federation research station located on the planet's surface was evacuated by the crew of the U.S.S. Potemkin in 2361 when the distortion field was forming in the planetary atmosphere. The rescue team from the Potemkin was led by Lieutenant William T. Riker. A transporter malfunction during the final beam-out from the planet caused Riker to be duplicated. One Riker returned to the Potemkin, while the other materialized back on Nervala IV. The existence of the duplicate Riker was note suspected until 2369, when the Enterprise-D returned to the Planet to retrieve the scientific information left behind by the scientific team.
TNG "Second Chances" ; Star Trek Encyclopedia II
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