Reports of a new space race tell of NASA’s plan to send astronauts to Mars in the 2030s and SpaceX Chief Executive Elon Musk’s ambition to get private explorers there first. This race, however, can’t be won with either’s current technology, regardless of their spending or commitment.

The barrier is human biology. Even a short, sortie mission to Mars and back would be extremely hazardous to human health. A Mars colony is out of the question. Living long-term on its surface is beyond the capacity of our bodies to survive.

NASA doesn’t talk about this much. Starry-eyed with space enthusiasm, most science reporters haven’t covered this aspect of the story either.

The worst monsters in the moat between the Earth and Mars are Galactic Cosmic Rays, or GCRs, which are particles flung across space by exploding stars. The most destructive of these particles are iron nuclei traveling at close to the speed of light and carrying the energy of a major league fastball.


A 2014 National Academy of Sciences report ... listed nine health risks for a Mars mission that are at an unacceptable level.

Astronauts already encounter GCR particles, seeing telltale flashes of light when they pass through their optic nerves. These radiation doses are tolerable during short stays on the International Space Station, which is partially sheltered from GCRs by the nearby Earth. On a long mission away from Earth, however, shielding humans from GCRs is practically impossible. Cancer and other medical risks rise to unacceptable levels.

Pure physics rules out shielding. The most effective element to stop GCRs is hydrogen, making water an ideal shielding material. Indeed, water in our atmosphere protects us on Earth. But it takes two meters of water to filter out about half the radiation, and a cubic meter of water weighs 2,205 pounds. Carrying enough water to insulate a spacecraft is far beyond current capabilities.

On Earth, GCR radiation exists only in particle accelerators, making its health impact difficult to study. Twenty years ago, a National Research Council report called for an aggressive program to understand the problem and find countermeasures. That never happened.


Several years ago, Dr. Frank Cucinotta, then head of radiation health at NASA, highlighted the GCR problem. His estimates showed that the risk of cancer was greater even than that of a spaceflight accident. He said his superiors resisted his message with such severe negativity that he left the agency. A NASA official denied Cucinotta was forced out.

Cucinotta’s point of view was vindicated, however, by the findings of a 2014 National Academy of Sciences report. It listed nine health risks for a Mars mission that are at an unacceptable level and six more for which the severity is unknown. Issues include heart damage from radiation, food and medicine stability, and astronauts’ psychological health.

With subsequent research, the radiation concerns have only grown. Scientists at UC Irvine, with Cucinotta (now at the University of Nevada, Las Vegas), reported last year that mice exposed to GCR particles at levels similar to those in outer space sustained brain damage and cognitive losses even without causing cancer. Some of those in NASA who initially disagreed with Cucinotta increasingly see the cognitive issues as a critical problem.


Doctor-astronauts working on the International Space Station in 2009 also discovered that living in weightlessness can damage the optic nerve. In the absence of gravity, cerebral-spinal fluid stops circulating. On voyages of more than a year, that could lead to blindness or contribute to dementia.

The simplest solution would be to travel faster so the ravages of space have less time to do damage. Space propulsion would need a giant technical leap to make a Mars round-trip in a safe period of about 150 days rather than NASA’s current estimates of more than a year.

But even if you could get humans to Mars quickly enough, staying is a problem. With the scant Martian atmosphere, human beings would have to live underground for long-term radiation protection, a daunting challenge and an unappealing prospect. Musk imagines a space colony as a refuge from calamity on Earth. But we can live underground here. What conditions could develop that would make it easier to do so on Mars?

That’s not to say space colonization won’t happen. It just won’t be on Mars. A more promising spot for a permanent, self-sustaining settlement is much farther away: Titan. This moon of Saturn has a thick nitrogen atmosphere and unlimited supplies of hydrocarbons and water. It is the most Earth-like object in the solar system, where humans could live without radiation shielding or even pressure suits — although they would need respirators and heated clothing.


Today, it takes seven years to get to Titan, still much too long. To travel there, or to get anywhere beyond the moon, we must find ways for humans to survive the journey.

Charles Wohlforth, a science writer, and Amanda Hendrix, a planetary scientist, are coauthors of “Beyond Earth: Our Path to a New Home in the Planets.”

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