Elon Musk is an ambitious visionary, a disturbingly hard worker, and a master of hype. But is he a realist?

On Tuesday, the entrepreneur laid out a wildly aggressive vision for beginning a colony on Mars. The plan entails getting the first humans on the planet in 2023, then ferrying 100 people at a time in reusable rockets and running the flights with regular frequency.

Ever since Musk announced that he'd be presenting his road map for occupying the Red Planet, it was clear there was going to be some suspension of disbelief necessary. The only thing the U.S. has gotten to Mars's surface is a robot, and no humans have been to the moon--which is about 1/170th the distance away--since 1972. So how far-reaching is the plan, really?

"I think he's not going to be able to deliver on those promises," says Tom Jones, a former astronaut who made four space shuttle trips between 1994 and 2001. "The technology is just not going to be there in this 10-year time frame he's talking about."

Musk is known to oversell: A Wall Street Journal report in August revealed that Tesla has missed more than 20 deadlines in the last five years.

Where Jones sees the biggest issue is in landing the massive ship proposed by Musk--bigger than any that's ever been created--on Mars's surface. Mars has a thin atmosphere, which means a spacecraft wouldn't naturally slow down much upon approaching its surface. The rockets required to create the resistance necessary for a safe landing would have to be far more powerful than anything that's been developed before.

"The biggest thing we've landed on Mars is the one-ton Curiosity rover in 2012," Jones says, "and that was at the limit of what we could do technologically." As such, he estimates it would take at least 25 years to develop the technology to build and land a ship of the size that Musk is proposing, which would place a manned trip to Mars sometime in the 2040s.

And as for that ship: To safely fit 100 people and protect them from dangerous solar radiation outside, it would have to be twice the size of the biggest spaceship ever built--the Saturn V, which transported astronauts to the moon. It would also require tens of millions of pounds of thrust (Saturn V created 7.5 million) and around 30 to 40 rockets in its first stage (Saturn V had five). That's not to say these things are impossible--they're not. But they certainly aren't imminent.

Musk clearly knows the odds are against him. It's an angle he plays up often, as he did early in his speech on Tuesday when discussing SpaceX's 2002 origins. "I thought maybe we had a 10 percent chance of doing anything--of even getting a rocket into orbit, let alone getting beyond that and taking Mars seriously," he said.

That's a classic Musk tactic: predicting future success by referring back to a previous successful venture and stating that the odds were slim when it first began. He did it in his recent Tesla master plan, referring to the low chance of success he knew he had when starting Tesla by citing the fact that Ford is the only U.S. car company to have avoided bankruptcy--knowledge he didn't have when he launched Tesla in 2003, six years before GM and Chrysler filed for Chapter 11 bankruptcy.

But regardless of whether he's overstating his past success and his future chances, Musk is arguably serving a greater purpose by creating these lofty goals and tight timelines: He's driving innovation, both from his own company and the rest of the world.

"I think it's less important to him that he actually get to Mars personally than that he lay down some steps that will help somebody else get there," Jones says. He predicts that Musk will settle for a less ambitious result, like getting a robotic spacecraft to land on Mars, within the decade timeframe.

Jones also says Musk could turn his short-term attention to industrializing the area between Earth and the moon as a way of eventually helping humans reach Mars. Lifting off from Earth requires far more thrust and a larger spacecraft than initiating from space, so building the necessary infrastructure outside the atmosphere could have big benefits. It's a plan Blue Origin founder Jeff Bezos has touched on in discussing moving big industry into space. And other companies, like Planetary Resources--for which Jones is an adviser--intend to mine asteroids for resources that can then be turned into propellant for ships already in orbit.

Whatever the plan that eventually gets us to Mars, one thing is clear: Musk wants to play a big role in it. That, he said Tuesday, is much of the reason he founded SpaceX: After humans successfully landed on the moon in the late 60s and early 70s, progress stalled out. I came to the conclusion that if there wasn't some new entrant into the space arena with a strong ideological motivation, then it didn't seem like we were on a trajectory to ever be a space-faring civilization," he said.

That hits home for Jones. Even if he might not agree with how Musk plans to execute, he admires the motivations.

"He's genuinely interested in seeing humans get to Mars," Jones says. "I think he wants to see it within his lifetime, which is something I'd like to see happen, too. To push the idea that we can overcome those obstacles and get humans to Mars is a good thing."