As an astronaut, Chris Hadfield flew on three space missions and spent nearly half a year in orbit.

People could have landed on Mars "decades ago" if we'd really wanted to, Hadfield says, but such missions may have been deadly because of the state of rocket and spaceship technologies.

Even if NASA, SpaceX, and Blue Origin soon build capable rocket ships for Mars, Hadfield isn't sure they'd be safe enough.

Chris Hadfield is best known for recording David Bowie's song "Space Oddity" in zero gravity. But his real accomplishment is an incredible amount of space travel.

From 1995 to 2013, Hadfield flew inside two NASA space shuttles and a Russian Soyuz spacecraft and lived aboard the International Space Station. In all, he spent 166 days in orbit.

Hadfield has since retired as an astronaut, but he recently shared some of his spaceflight knowledge as part of a new web course on the online education platform MasterClass.

We followed up on Hadfield's lessons by asking whether he's hopeful that NASA, SpaceX, Blue Origin, or other players in the new space race might send people to Mars (and begin colonizing it) within the next decade or so.

"We could send people to Mars decades ago," Hadfield told Business Insider. "The technology that took us to the moon and back when I was just a kid — that technology can take us to Mars."

A Saturn V rocket launches an Apollo mission toward space. NASA Indeed, scientists like Wernher von Braun, who was the chief architect of NASA's Saturn V moon rocket, worked to plan a crewed Mars mission as early as 1952. This was more than 15 years before the first Apollo mission.

But Hadfield noted that having the ability to go doesn't mean it'd be easy, safe, or worth the risk of human life — even considering new spacecraft.

"The majority of the astronauts that we send on those missions wouldn't make it," Hadfield said. "They'd die."

Here's why Hadfield is cautious about our immediate prospects for landing humans on Mars and what gives him hope about the future of space exploration.

A long and risky interplanetary boat trip

The risks of visiting Mars aren't too different from those NASA faced in the 1960s and early 1970s. Back then, astronauts stared down death at seemingly every turn.

In addition to the Apollo 1 fire, which killed three astronauts on the ground during a training exercise, NASA nearly lost the crew of Apollo 13 during its mission. Even Apollo 11 had a close call: It nearly ran out of fuel before landing on the lunar surface (in a spaceship with aluminum walls as thin as a few sheets of paper).

Longer-term problems later appeared, too, such as health risks from exposure to deep-space radiation. According to a 2016 study published in Nature, the flight to and from the moon exposed 24 astronauts who made the trip to a much greater risk of deadly heart disease.

All that happened when NASA was just trying to send people 239,000 miles away from Earth for about a week.

"Mars is further away that most people think," Hadfield said. The red planet is about 660 times as far as the moon, with a round-trip journey could take 500 days or even three years in a tiny tube.

A simulation of Mars being blasted by solar radiation. NASA Scientific Visualization Studio/MAVEN Science Team

Therein lies Hadfield's issue: A long slog to Mars increases the risks of explosions, radiation, starvation, and other problems. Technologies that could mitigate these issues — such as lightweight yet effective shielding, hibernation capsules, and bioregenerative life-support systems — don't yet exist.

He likened the latest Mars exploration plans to early ocean voyages by ship.

"Magellan, when he launched in 1519, they launched with five ships and 250 people to try and just go around the world once, and almost everybody died," he said. "They only came back with like 15 or 18 people and one out of the five ships."

One big problem is that today's rockets still burn chemical fuels (plus oxygen) to lift off Earth and travel through space.

"Burning chemical rockets is the equivalent of using a sailboat or a pedal boat to try and travel around the world," Hadfield said.

Chemical propellants aren't as efficient as anyone would like. When designing a spacecraft, engineers have to sacrifice radiation protection, supplies, tools, and living space to ensure there's enough fuel to make the trip.

That's why Hadfield thinks NASA's Space Launch System, SpaceX's Big Falcon Rocket, or Blue Origin's New Glenn rocket won't disrupt space travel to the extent their creators often suggest: They all plan to burn fuel for propulsion.

"My guess is we will never go to Mars with the engines that exist on any of those three rockets unless we truly have to," he said. "I don't think those are a practical way to send people to Mars because they're dangerous and it takes too long, and it therefore exposes us to a risk for a long time."

'Someone has to invent something we haven't thought of yet'

Hadfield still wants people to try to land on Mars and even colonize it. It's just that he thinks we should be clear-eyed about the grave risks involved in any push to get to the red planet using today's technologies.

"You can look at American history, with Jamestown and all of the settlements that failed — how hard it was for the first people to try and transition from just a quick look to actually trying to set up life in a new and, by definition, hostile location," Hadfield said. "But if you don't give yourself a grandiose goal, then it will never happen. And so I'm all for it."

Hadfield said he thought someone could eventually find a way to get to Mars safely, given the spaceflight advancements he'd witnessed and made a career out of.

"Spaceflight was impossible when I was born," he said. "And I'm still alive. It's not very long ago. We've gone from impossible to Peggy Whitson — 665 days in space — and six people living on a spaceship for the last 17 1/2 years, continuously. We left Earth. We settled space, just at the turn of the century. So we've made extremely fast strides."

Elon Musk and a SpaceX illustration of its Big Falcon Rocket on Mars. SpaceX; Kevork Djansezian/Getty; Shayanne Gal/Business Insider

But if NASA or private companies are serious about visiting and eventually colonizing Mars, as SpaceX's founder, Elon Musk, hopes to do, Hadfield said there should be more investment in basic research.

Hadfield said he didn't know what future technologies might look like, but he noted the recent advancements in ion propulsion and NASA's resurgent interest in nuclear reactors. There could even one day be breakthroughs in investigations of dark matter and dark energy that would help this effort.

"Someone has to invent something we haven't thought of yet," Hadfield said. "Maybe the work that's going on with the Alpha Magnetic Spectrometer on the space station and in the particle accelerator in CERN and other places ... is going to show us how we can harness gravity."

"It sounds outlandish," he added, "but we figured out how to harness electricity and what electrons do, and that seemed crazy, and it's revolutionized life and travel. So who knows?"

Dana Varinsky contributed to this report.