Every spacesuit NASA astronauts have worn — and the new models that may revolutionize how they explore the solar system

May 4 is International Space Day: the world's small homage to and celebration of the final frontier.

But that frontier is wildly inaccessible, and downright deadly, to any plucky human without a good spacesuit.

For nearly six decades, NASA and private companies have met the call to protect high-flying astronauts as they risk their lives in the name of space exploration.

From the silvery suits of the 1950s and 1960s Mercury program to future commercial and government designs, here's how astronauts' spacesuits have evolved over six decades.

If all goes well, the next generation of spacesuits just might revolutionize human exploration of other worlds.

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Mercury Suit (1961-1963)

Project Mercury marked the first time US citizens ventured into orbit around Earth.

To protect the first astronauts from sudden pressure loss, NASA modified high-altitude jet-aircraft pressure suits from the US Navy. Each space suit had a layer of neoprene-coated nylon on the inside and aluminized nylon on the outside (to keep the suit's inner temperature as stable as possible).

Six astronauts flew into space wearing the suit before NASA retired it from service.

Gemini Suit (1965-1966)

Gemini was NASA's second space program — and one with more ambitious goals. The Gemini capsule carried a two-astronaut crew into space, and had one (uncomfortable) mission that lasted two weeks.

The David Clark Company designed Gemini suits to be flexible when pressurized, and took extra steps to make them more comfortable than Mercury suits. For example, they could be connected to a portable air conditioner to keep the astronauts cool until they could hook up to the spacecraft's lines. These suits weighed 16-34 lbs.

Gemini Spacewalk Suit (1965-1966)

One type of Gemini suit, called G4C, was designed with NASA's first spacewalks in mind. Astronauts would open the hatch during these ventures and leave the safety of their vehicle to work in the vacuum of space.

To withstand the harsh space environment, the suit connected the astronauts to the spacecraft via a hose, which supplied them with oxygen. In case there was a problem, though, some variants of the suit provided up to 30 minutes of backup life support. The heaviest variant weighed about 34 lbs.