Astronaut Nicole Stott says folks who are cynical over seeing a billionaire complete the first-ever civilian spacewalk need to cool their jets ... because it's actually to everyone's benefit.

Nicole, who did her own spacewalk in 2009 at the International Space Station, tells TMZ ... billionaire Jared Isaacman bankrolling a historic spacewalk is good for humankind in the long run, and it will help us eventually reach Mars.

The way Nicole sees it ... private space exploration is vital toward our goals of going further into space than ever before, and while the average person can't pay for a trip into orbit right now, it likely will be affordable in the future.

Nicole points out ... about 50 years ago, the average person could not afford a plane ticket ... and she believes space travel will evolve to become more commonplace moving forward.

As we reported ... Isaacman made history Thursday with his own Neil Armstrong moment, becoming the first civilian to experience unconfined space as part of the Polaris Dawn mission.

Billionaires are clearly obsessed with the final frontier -- Richard Branson and Jeff Bezos blasted into space in July 2021 -- and Nicole says the public should be interested in space exploration ... for entertainment as well as science. She says it will help us learn how we can improve society and our planet.

Isaacman was tethered to the SpaceX craft during his spacewalk, and Nicole says the whole thing was safe. She says there is no more danger to a civilian than for a NASA astronaut, as private expeditions include the same training as NASA missions.

Nicole was an astronaut on both the ISS and the Space Shuttle, so she knows her stuff ... she even wrote a book about her experiences, "Back to Earth: What Life in Space Taught Me About Our Home Planet -- And Our Mission to Protect It."

Nicole's also an aquanaut -- someone who explores or lives long-term underwater -- and she's over in London right now preparing for a big event tonight ... "Uniting Sea and Space: Global Nonprofits Take Over Piccadilly Lights to Advocate for Ocean Conservation."

Bottom line for Nicole ... ease up on the cynicism, and look to the future.