Adds details from paragraph 6 onward on crew's spacecraft exit

By Joey Roulette

WASHINGTON, April 4 (Reuters) - Four private astronauts returned to Earth in a SpaceX capsule on Friday after roughly four days orbiting the planet in a novel polar trajectory, walking out of their spacecraft with little assistance to cap off the Elon Musk-led company's sixth fully private space mission.

Since launching Monday night from Florida, the four-person crew, led by and paid for by Maltese investor Chun Wang, traveled in a circular orbit around Earth from pole to pole, passing over the icy masses every 40 or so minutes in a particular orbit that no humans have flown before.

During the mission, they conducted 22 research experiments largely focused on how the human body changes in microgravity.

The four-person crew included three of Wang's friends and associates: Norwegian film director Jannicke Mikkelsen, German robotics researcher and polar scientist Rabea Rogge, and Australian adventurer Eric Philips.

Their Crew Dragon capsule had tightened its orbit around Earth Friday morning and splashed down hours later off the coast of California around noon EDT (1600 GMT), before the gumdrop-shaped spacecraft was hoisted out of the water by a SpaceX vessel and scooted under a shaded platform onboard.

As a final experiment, the crew exited Dragon without the delicate assistance from medical and support teams that astronauts usually receive upon returning to Earth. No stretchers rolled them out of the capsule, to demonstrate how well astronauts could walk off a spacecraft on the moon or Mars.

Spaceflight, particularly on missions much longer than the Fram2 flight, is known to reduce bone density and muscle mass, among other bodily effects that have been studied for decades by NASA with its own astronauts on the International Space Station.

Each crew member on Friday slowly crawled out of Dragon one by one, their flexibility seemingly constrained only by their flight suits, before standing upright with smiles.

"All four framonauts have safely exited Dragon unassisted," SpaceX said, referring to the crew.

SpaceX and its Dragon craft have dominated the nascent market for private orbital spaceflight, an area in which a key source of demand originally came from a small field of wealthy tourists. Dragon is the world's only privately built capsule routinely flying missions in orbit. Rival Boeing's BA.N Starliner capsule has been held up in development.

In recent years, with Dragon flights costing roughly $55 million per seat, the spaceflight market - involving companies such as Axiom Space that contract Crew Dragon missions - has fixated more on astronauts from governments willing to pay the sum mainly for national prestige and bolstering domestic spaceflight experience.

(Reporting by Joey Roulette; Editing by David Gregorio and Diane Craft)

((Joey.Roulette@thomsonreuters.com; 7034696632;))