Many people thought it could be aliens or an attack from North Korea, so Elon played on those assumptions in his tweet.
Before the Falcon 9 rocket was launched from Vandenberg Air Force Base in Lompoc, Musk had retweeted the SpaceX account with a link to its livestream.
But his early warning didn't stop people from wondering what the bright, oval shaped object that flew through the sky was around 5:30 p.m.
SpaceX said this is the fourth flight for long-time customer Iridium. Both companies are under contract to launch a total of 75 satellites to provide communications services for regions around the world.