Virgin Galactic founder Richard Branson will visit the edge of space later today, July 11, as he and three other Galactic employees will take part in a test flight.

Called Unity 22, this will be Virgin Galactic's fourth test flight carrying humans into space, and will be the largest crew yet. The flight marks an important milestone on Virgin Galactic's two-decade long journey to offer space tourism. The four passengers will be flown to the edge of space by two pilots.

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Founded in 2004, Virgin Galactic has sold in the region of 600 tickets to well-heeled individuals who want to get a taste of what it's like to travel into space.

All being well, the journey will see passengers take off in the company's WhiteKnight aircraft, upon which sits a rocket-powered spaceship called VSS Unity. Around 40 minutes after takeoff, Unity is dropped from the WhiteKnight aircraft, its rockets ignite, and the craft is blasted to the edge of space, where passengers will experience several minutes of weightlessness, 55 miles above Earth.

VSS Unity then comes back to Earth, landing conventionally on a runway at Virgin Galactic's Spaceport America in New Mexico. Tickets cost in the region of $250,000 per person.

This is not the same type of space tourism fellow billionaires Elon Musk and Jeff Bezos are working towards, where the plan is for passengers to eventually travel more deeply into space, but Branson's early start means his company is closest to achieving its goal.

Despite this being its fourth manned test flight, Virgin Galactic plans to make Branson's trip into space quite the spectacle. A show presented by Stephen Colbert will be streamed live online, and Branson has promised to "announce something very exciting" on his return to Earth.

The flight can be watched live on Virgin Galactic's social media channels – YouTube, Facebook and Twitter – started from 9am ET / 12pm PT today, July 11.