Entrepreneur Elon Musk has released the first photo of his SpaceX spacesuit, making the point it's not just a mock-up: "This actually works".

The picture shows a white suit with black piping and a dark-tinted visor on an astronaut apparently strapped in and ready to take to the skies.

"First picture of SpaceX spacesuit," Mr Musk wrote.

"More in days to follow. Worth noting that this actually works [not a mock-up]. Already tested to double vacuum pressure.

"Was incredibly hard to balance esthetics [sic] and function. Easy to do either separately."

The Instagram announcement has prompted a flurry of speculation about the suit's purpose and specifications.

"Double vacuum pressure" was one of the terms that prompted head-scratching — a vacuum is an absence of matter, so a double vacuum doesn't make sense, people argued — but several users on science forums said it would refer to a difference in pressure inside versus outside the suit.

"'Double vacuum pressure' just means they tested a 2 atm [atmosphere] pressure difference between the suit and the environment," Redditor RyanW1019 explained.

"This allows them to be extra sure that the suit won't rupture if it is exposed to vacuum."

The post didn't explicitly specify whether this suit is designed for in-flight travel or spacewalks, but it appears to be meant for use inside the spacecraft.

Spacesuit's comic book connections

This NASA suit for astronauts on the Boeing Starliner craft was unveiled in January. ( Supplied: NASA )

The new SpaceXsuit is sleek and monochromatic, and looks a bit more like something you might see in a sci-fi movie than the spacesuits that have come out of NASA recently, like the blue Boeing Starliner suit unveiled in January.

This could be because movie costume designer Jose Fernandez, who is behind the outfits worn by Batman, Wonder Woman and Spiderman, has been part of SpaceX's suit-design process.

"[Mr Musk] wanted it to look stylish. It had to be practical but also needed to look great," Mr Fernandez said in a 2016 interview with Bleep Magazine.

"When people put this spacesuit on, he wants them to look better than they did without it, like a tux. You look heroic in it."

SpaceX has been running supply missions to the International Space Station — the most recent of which carried experiments coded by Australian high school students — as it refines its technology.

Its first successful Falcon 9 rocket was launched in January after a fireball engulfed a similar rocket about four months earlier.

The organisation's ultimate goal is to enable people to live on other planets.