British billionaire Richard Branson has prepared to climb into his Virgin Galactic passenger rocket plane and soar more than 80 kilometres above the New Mexico desert in the vehicle's first fully crewed test flight to the edge of space.
Branson, one of six Virgin Galactic employees strapping in for the ride, has touted the flight as a precursor to a new era of space tourism, with the company he founded poised to begin commercial operations next year.
Sunday's launch of the VSS Unity rocket plane will mark the company's 22nd test flight of its SpaceShipTwo system, and its fourth crewed mission beyond Earth's atmosphere.
It's also the first to carry a full complement of space travellers - two pilots and four "mission specialists", Branson among them.
The planned takeoff from New Mexico's state-owned Spaceport of America, located near the New Mexico town of Truth or Consequences, was pushed back by 90 minutes due to overnight thunderstorms that kept Virgin Galactic from rolling out its rocket plane on time, the company said.
The gleaming white spaceplane was now due to be borne aloft at around 10.30am (12.30am Monday AEST) attached to the underside of a specially designed twin-fuselage carrier jet VMS Eve - named for Branson's late mother.
Separating from the mothership at an altitude of 50,000 feet, Unity's rocket engine will ignite to send the spaceplane streaking straight upward to the blackness of space some 88km high, where the crew will experience about four minutes of microgravity.
With the engine shut down near the peak of its climb, the craft will then be shifted into re-entry mode before gliding back to a runway landing at the spaceport. The entire flight, from takeoff to touchdown, should take about 90 minutes.
Assuming the mission goes well, Virgin has plans for two further test flights of the spaceplane in the months ahead before beginning regular commercial operation in 2022.
This is no discount travel service. But demand is apparently strong, with several hundred wealthy would-be citizen astronauts already having booked reservations, priced at around $US250,000 ($A330,000) per ticket.
The Swiss-based investment bank UBS has estimated the potential value of the space tourism market reaching $US3 billion ($A4 billion) annually by 2030.
Proving rocket travel safe for the general public is key, given the inherent dangers of spaceflight.
An earlier prototype of the Virgin Galactic rocket plane crashed during a test flight over California's Mojave Desert in 2014, killing one pilot and seriously injuring another.
His ride-along also upstages rival astro-tourism venture Blue Origin and its founder, Amazon mogul Jeff Bezos, in what has been popularised as the "billionaire space race".
Bezos has been planning to fly aboard his own suborbital rocketship, the New Shepard, later this month.
Branson, a week away from his 71st birthday, has insisted he and Bezos are friendly rivals and not engaged in a personal contest to beat one another into space.
Bezos posted a message on Instagram on Saturday wishing Branson and his team good luck and "a successful and safe flight", but nonetheless there has been some public rancour between the two.
Blue Origin has disparaged Virgin Galactic as falling short of a true spaceflight experience, saying that unlike Unity, Bezos's New Shepard tops the 100km-mark, called the Karman line, set by an international aeronautics body as defining the boundary between Earth's atmosphere and space.
However, US space agency NASA and the US Air Force both define an astronaut as anyone who has flown higher than 80km.