NASA paid tribute to cosmologist Stephen Hawking following his death early Wednesday morning.

NASA shared a video on Twitter that Stephen Hawking recorded for astronauts Rick Mastracchio and Koichi Wakata on the International Space Station in 2014.

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In his message Hawking talks about experiencing microgravity, i.e. weightlessness.

"For me, this was true freedom," said Hawking. "People who know me well say that my smile was the biggest they'd ever seen. I was superman for those few minutes.

"I can only imagine what it's like for you, Rick and Koichi, to fly like Superman for six months."