SpaceX Polaris Dawn LIVE: Billionaire Jared Isaacman and astronaut crew embark on 'dangerous' world first private spacewalk

The astronauts have already put on their space suits and have started 'pre-breathing' .

A host on the SpaceX live broadcast said: 'They've been undergoing a process called pre-breathing, where the atmosphere pressure inside Dragon is slowly decreased and oxygen is increased, to prepare their bodies for the [spacewalk] suit environment.'

He added that they began putting on their space suits 'some time ago'.

Spaceflight safety experts have warned the crew faces potentially deadly risks of radiation exposure and rapid changes in pressure that can cause life-threatening sickness.

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The billionaire Jared Isaacman, 41, is most well known as the founder and CEO of the American payment processing company Shift4.

Isaacman founded the company in 1999 out of his parent's basement when he was just 16 years old.

After dropping out of high school to work as a cybersecurity consultant, Isaacman used a $10,000 loan from his grandad to convince a bank to certify him to sell credit card terminals.

That business ballooned and now handles transactions for a third of America's restaurants and hotels - processing more than $200 billion in payments every year.

Previously only state-backed space agencies have sent astronauts on spacewalks due to the extremely high risks involved.

Floating beyond the protective walls of a spacecraft is considered to be one of the most dangerous tasks an astronaut can undertake.

In this case, that risk will only be heightened by the fact that none of the crew except for Isaacman have ever been in space before.

Additionally, since the Dragon spacecraft does not have an airlock, the entire craft will need to be depressurized at once.

That will expose all four crew members to the harsh conditions of low orbit and will break the current record for most people simultaneously in the vacuum of space.

The Polaris Dawn mission launched from the Kennedy Space Centre in Florida on September 10 at 5:23 am ET after days of delay due to weather and a helium leak.

Yesterday, the SpaceX Dragon Crew spacecraft carried its crew to an altitude of 870 miles (1,400 km) above Earth - the highest any human has been since NASA's Apollo missions.

The crew have already made a pass through Earth's Van Allen Belt - a region of extremely high radiation.

However, the mission's most dangerous moment will come today as the crew embark on the first private spacewalk.

10.58am UK time: The mission begins

11.28am: Hatch will be unlocked and opened

11.31am: Mission commander Jared Isaacman will exit and begin suit tests

11.44am: Isaacman goes back to his seat

11.49am: Mission specialist Sarah Gillis exits

11.57am: Gillis returns to her seat

11.58am: Hatch is shut and repressurisation begins

12.48pm: Repressurisation finishes

Onboard are the billionaire Jared Isaacman, former airforce commander Scott 'Kidd' Poteet, and SpaceX employees Sarah Gillis and Anna Menon.

Apart from Isaacman, none of the all-civilian crew attempting this dangerous mission have any experience in space.

The first ever privately funded spacewalk is due to start just before 11am (5.58 E.T.) UK time.

It had been due to start at 7am but this was pushed back without any reason given for the delay.