Richard Branson tucked a space rocket under the wing of a 747

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The venerable Boeing 747-400 may be best known as a passenger plane, but thanks to entrepreneur Richard Branson, it's now a key part of the Virgin Orbit private space program. 

A 747 named "Cosmic Girl" completed an unusual test flight Sunday in Southern California by going for a cruise with a 70-foot-long (21-meter) rocket attached under its wing. 

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Virgin Orbit is a Virgin Galactic spinoff that's developing the LauncherOne system to launch rockets from a 747 rather than from the ground. The system is designed to get small satellites into space quickly and cheaply.

"It marks the first time a 747 has carried a rocket, let alone a space rocket, in its 70-year history and is a big step forward for the company as we look forward to reaching orbit in early 2019," Branson wrote in a blog post.

Virgin Orbit's 747 was once a passenger jet, but it's been customized to act as a flying launch pad for the company's carbon-fiber rocket. 

While Virgin Orbit is making strides toward getting satellites into orbit, Virgin Galactic is continuing on its quest to launch tourists into space. 

Galactic completed a supersonic test flight in May, with Branson saying in early October he expects the company to be in space "within weeks." 

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