Ask a planetary scientist what worries them most about sending humans to Mars and they will not hesitate. They have a list. It is specific, well-researched, and sobering in a way that the press releases and mission timelines rarely are.
The list does not say Mars is impossible. It says Mars will take something from whoever goes there, and the only question is how much.
This is the conversation happening in laboratories and mission planning documents right now, as SpaceX targets an uncrewed Starship landing in the 2026–27 launch window and a crewed mission somewhere in the early 2030s. The science of what those crews will face has been accumulating for decades. The picture it presents is not one of manageable inconveniences. It is one of compounding, partially irreversible biological damage, psychological deterioration, and a return journey from which some people may not fully recover.
Every serious person in this field knows this. And we are going anyway.
What radiation actually does to the human body over three years
A round-trip Mars mission is expected to last approximately three years. For that entire duration, the crew will be exposed to galactic cosmic rays — high-energy particles originating outside the solar system — at a rate roughly three times higher than in low Earth orbit, where the planet’s magnetosphere provides partial shielding. Mars has no global magnetic field. Its thin atmosphere offers minimal protection. There is nowhere to hide.
Research published in PLOS One using NASA’s own risk models estimated that central projections for radiation-induced mortality from a Mars mission could exceed 5 percent, with upper confidence intervals approaching 10 percent. Morbidity estimates — meaning serious health consequences that do not necessarily result in death — reach up to 20 percent at the upper range. These figures include cancer risk and cardiovascular damage. They do not fully account for what the same research describes as qualitative differences in how cosmic radiation damages biological tissue compared to the terrestrial radiation those models are based on. The authors note that those differences may significantly increase the estimates, and will require new knowledge to evaluate. It is worth noting that this research dates from 2013, and radiation risk modelling for Mars remains an active and evolving field. Thus, these figures represent one point in a range of estimates rather than a settled scientific consensus.
There is also the problem of solar energetic particle events — sudden, unpredictable bursts of radiation from the sun that can reach acute exposure levels. A 2025 study in Space Weather developed a nowcasting system for these events using data from the Curiosity rover’s onboard radiation detector. The best warning time the system can provide is approximately 30 minutes. In that window, the crew would need to reach a shielded shelter and remain there for the duration of the event. Whether adequate shelter can be built, and how long events might last, remain open engineering and scientific problems.
What happens to bones and muscles in the absence of gravity
The transit to Mars takes approximately seven months in each direction. The surface of Mars has gravity of approximately one-third that of Earth. Whether that is enough to prevent the physiological deterioration that microgravity causes in low Earth orbit remains genuinely unknown.
Modelling published in PLOS One predicts that bone mineral density loss over the course of a Mars mission will exceed safe clinical thresholds for the majority of crew members on a shorter opposition-class trajectory, and for essentially all crew members on the longer conjunction-class trajectory. The specific prediction is that 100 percent of astronauts on a conjunction-class mission — lasting 1,000 to 1,200 days — will develop osteopenia. One-third are predicted to be at risk for osteoporosis. These are not hypothetical worst-case scenarios. They are central estimates derived from measured data on astronauts who have completed long-duration ISS missions.
Bone mineral density in weight-bearing bones decreases at approximately 1 to 1.5 percent per month in microgravity. The body also loses muscle mass at a rate that exercise countermeasures can slow but not fully prevent. A crew arriving on Mars after a seven-month transit will land on a planet with one-third Earth gravity, in bodies that have already spent months weakening, and will need to work.
What isolation does to the mind over 500 days
The Mars-500 study, conducted at a facility in Moscow, confined six men in a sealed habitat for 520 days to simulate the psychological conditions of a Mars mission. It remains the longest high-fidelity simulation of an interplanetary mission ever conducted.
What the researchers found was not dramatic breakdown. It was quieter and, in some ways, harder to address. Crew members showed substantial individual differences in how they adapted — or failed to adapt — to prolonged isolation and confinement. Several developed chronic sleep disturbances. Physical activity declined significantly over the course of the mission. One participant spent 97 percent of the mission lying down or sitting. Mood deteriorated. The markers of psychological disengagement accumulated slowly, unevenly, and without obvious crisis points that would have triggered intervention.
A real Mars mission will add communication delays of up to 24 minutes each way, making real-time support from Earth impossible. It will add genuine mortal risk, which the Moscow simulation did not replicate. It will add the knowledge that there is no early return, no rescue, and no margin.
A 2025 biopsychosocial review of long-duration spaceflight risks summarises the combined picture: fluid shifts, bone density loss, muscle atrophy, cardiovascular deconditioning, circadian disruption, impaired cognition, interpersonal tension, and miscommunication. Not as separate problems to be solved in sequence. As a system of compounding pressures, each of which makes the others harder to manage.
Why we are going anyway
None of this is a secret. NASA’s Human Research Program has published detailed documentation of all five primary hazard categories for Mars missions. The scientific literature on radiation risk, bone loss, and psychological deterioration is extensive, peer-reviewed, and accessible. The people planning these missions have read it.
One analysis published in Nature concluded that a crewed Mars mission using the current Starship architecture faces fundamental engineering constraints that have not been resolved, including a severe delta-v deficit that would prevent a return flight under current mass projections. SpaceX has not publicly addressed this analysis in detail.
Elon Musk has said he personally wants to die on Mars, “just not on impact.” He has shifted the projected timeline for crewed missions repeatedly — from 2022 to 2024 to 2026 to 2029 and now somewhere in the early 2030s. Most independent aerospace analysts place a realistic crewed landing in the mid-2030s at the earliest. A 2024 study noted SpaceX’s architecture still requires orbital refueling at a scale never attempted, in-situ propellant production that does not yet exist on Mars, and life support systems for a multi-year surface stay that remain undesigned.
And yet the mission will happen. Not because the risks have been resolved, but because the logic driving it does not require them to be.
What kind of decision this actually is
The history of human exploration is a history of knowing the odds and going anyway. The crews of Apollo 1, Challenger, and Columbia did not die because their agencies were reckless about risk. They died because acceptable risk in high-stakes exploration is defined differently than in ordinary life, and the definition is arrived at through a combination of institutional calculation, individual will, and cultural narrative that is difficult to examine from inside.
Mars will be no different, except in scale. The risks are larger, the distances are greater, the margin for error is thinner, and the return options are fewer. Anyone who volunteers for a Mars mission will do so knowing, in precise quantitative terms, what the probability distributions on their health outcomes look like.
The question worth asking is not whether we should go. That decision is already being made. The question is what it means that we are building a framework in which people will voluntarily accept a significant probability of cancer, irreversible bone loss, psychological deterioration, and possible death, in exchange for the chance to stand on another planet.
There is something in that willingness that is worth understanding clearly, rather than simply celebrating. It is not recklessness. It may not even be courage in any conventional sense. It is something more like the recognition that some things matter enough to absorb a cost that cannot be fully calculated in advance.
Mars will take something from whoever goes there. The serious question is whether what comes back will be worth it — and for whom.
