*

First private spacewalk will use new SpaceX spacesuits

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Crew Dragon to travel farthest from Earth since Apollo

program

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Crew to conduct scientific experiments on cosmic radiation

and

space vacuum effects

(Recasts first six paragraphs with launch; new headline)

By Joey Roulette and Gerry Doyle

WASHINGTON, Sept 10 (Reuters) -

Four private astronauts blasted into space early on Tuesday

in a modified SpaceX Crew Dragon capsule, kicking off the

company's five-day Polaris Dawn mission, which aims to test new

spacesuit designs and conduct the first private spacewalk.

The crew, a billionaire entrepreneur, a retired military

fighter pilot and two SpaceX employees, lifted off from NASA's

Kennedy Space Center in Florida about 5:23 a.m. EST (0923 GMT),

with flight controllers noting that the Falcon 9 booster had

cleared the launch tower.

It is Crew Dragon's fifth - and riskiest - private

mission so far. After reaching space a few minutes after launch,

the spacecraft will settle into an oval-shaped orbit, passing

as close to Earth as 190 km (118 miles) and as far as 1,400 km

(870 miles), the farthest any humans will have ventured since

the end of the U.S. Apollo moon program in 1972.

An attempt to launch last month was

postponed

hours before liftoff over a small

helium leak

in ground equipment on SpaceX's launchpad. SpaceX fixed the

leak, but the company's Falcon 9

was then grounded

by U.S. regulators over a booster recovery failure during

an unrelated mission, further delaying the Polaris launch.

The launch on Tuesday was delayed about two hours because of

unfavorable weather.

Only highly trained, well-funded government astronauts have

done spacewalks in the past. There have been roughly 270 on the

International Space Station (ISS) since its creation in 2000,

and 16 by Chinese astronauts on Beijing's Tiangong space

station.

The Polaris Dawn spacewalk is planned for the mission's

third day at 700 km in altitude and will last about 20 minutes.

SpaceX's Crew Dragon craft will slowly depressurize its entire

cabin - it has no airlock like the ISS - and all four astronauts

will rely on their slimmed-down, SpaceX-built spacesuits for

oxygen.

The first U.S. spacewalk was in 1965, aboard a Gemini

capsule, and used a similar procedure to the one planned for

Polaris Dawn: the capsule was depressurised, the hatch opened,

and a spacesuited astronaut ventured outside on a tether.

Jared Isaacman, 41, a pilot and the billionaire founder of

electronic payment company Shift4, is bankrolling the Polaris

mission, as he did for his Inspiration4 flight with SpaceX in

2021. He has declined to say how much he is paying for the

missions, but they are likely to cost hundreds of millions of

dollars.

Joining him is mission pilot Scott Poteet, 50, a retired

U.S. Air Force lieutenant colonel; and SpaceX employees Sarah

Gillis, 30, and Anna Menon, 38, both senior engineers at the

company.

For the spacewalk, Isaacman and Gillis will exit the

spacecraft tethered by an oxygen line while Poteet and Menon

stay in the cabin.

The mission is the first in Isaacman's private Polaris

program that includes a follow-on Crew Dragon mission in the

future, followed by a flight on SpaceX's Starship, a giant

rocket the company has spent billions of dollars developing as a

flagship moon and Mars vehicle.

The four-person crew are effectively test subjects for an

array of scientific experiments that will aim to shed light on

how cosmic radiation and the vacuum of space affect the human

body, adding to decades of studies on astronauts living aboard

the ISS.

Since the retirement of the Space Shuttle in 2011, NASA has

relied heavily on the company and its Crew Dragon, which has

flown nine astronaut missions to and from the ISS for the agency

as the only U.S. crew-grade vehicle in operation.

The company has previously flown four private missions:

Isaacman's Inspiration4, and three private astronaut flights

arranged by Houston-based mission broker Axiom Space.

Boeing ( BA ) is struggling to develop a similar spacecraft,

Starliner, that could rival Crew Dragon. But Starliner's latest

NASA test mission that began in June - its first time flying a

crew - left its astronauts on the ISS last week because of

issues with its propulsion system.