In what could be termed as a defining moment in the history of space, Virgin Galactic is all set to launch its billionaire CEO Richard Branson and three company employees to the edge of space on Sunday (6.30 pm India time). If successful, the mission, called Unity 22, will mark Virgin Galactic's fourth flight to space carrying humans, with its largest crew yet.

Popular American TV host Stephen Colbert was set to host the mission's livestream while singer-songwriter Khalid was set to perform a new single live onstage following the spaceplane's landing.

Branson said he will "announce something very exciting" after his spaceflight. Four people, including Branson, will test the astronaut cabin experience with two pilots in the cockpit. Virgin Galactic's twin-fuselage WhiteKnight carrier aircraft will carry a rocket-powered spaceship called VSS Unity, with Branson and others on board.

"About 40 minutes after takeoff, Unity will drop from the middle of the mothership and ignite its rocket engine moments later to send Branson and the crew to the edge of space, about 89 kms high, for a few minutes of weightlessness. Unity will return to a landing strip at Spaceport America. Virgin Galactic has already sold nearly 600 tickets, with each ticket costing nearly $250,000.

After Branson, Amazon Founder Jeff Bezos is expected to take off in his own space tourism Blue Origin rocket on July 20. The Karman line, 100 kms above the ground, is the boundary of space. Blue Origin's New Shepard rocket aims to launch its crew capsule just beyond the Karman line for a few minutes of weightlessness, while Virgin Galactic's SpaceShipTwo spaceplane flies 89 kms high — just over the boundary of space defined by the US government.