Elon Musk has announced his latest ambitious timeline for sending humans to Mars, but some think his head is in the clouds.

Elon Musk dreams of colonizing Mars in his lifetime, but the clock is ticking—and he knows it.

Such is evident from a recent string of X posts about the precise timing required for interplanetary travel and the pesky regulations standing in his way.

The rising anxiety adds up to what the Wall Street Journal called a “classic midlife crisis” for Musk.

Musk is insistent that his goal is of the utmost importance for all on Earth. “Becoming multiplanetary is critical to ensuring the long-term survival of humanity and all life as we know it,” he posted to X earlier this week.

A window for travel from Earth to Mars opens up only around every 26 months, and Musk won’t be ready for the soonest alignment, which is in October of this year.

“The first Starships to Mars will launch in 2 years when the next Earth-Mars transfer window opens,” he said earlier this month. “These will be uncrewed to test the reliability of landing intact on Mars. If those landings go well, then the first crewed flights to Mars will be in 4 years.”

But some experts say his aggressive projected timeline for building a city on Mars just isn’t quite realistic.

Christopher Combs, an aerodynamics professor at the University of Texas at San Antonio, told the Journal that, more probably, it could be 15 to 20 years before a rocket carrying humans reaches Mars.

The trouble is the infrequent travel windows and the many technical problems—for one, how to return from Mars—that still remain.

Peter Hague, who writes a newsletter on space travel, also shared a skeptical stance on Musk’s timeline, although he held firm that manned travel to Mars isn’t so far off. In an X post, he suggested that humans could reach the red planet by 2033, if not 2031.

“This is happening and you’ll get to see it,” he added.

Musk has acknowledged that his ambitious timeline could again slip if his team faces challenges.

“Attempting to land giant spaceships on Mars will happen in that timeframe, but humans are only going after the landings are proven to be reliable,” he posted earlier this month. “4 years is best case for humans, might be 6, hopefully not 8.”