Why Mars Will Kill You Before You Even Touch the Ground - Leonard Susskind Explains

Added: 2026-05-09

[00:00:00]Mars is 140 million miles away on a good day. Most people hear that number and think far. What they should think is dead because the distance isn't what kills you. What kills you is what the distance means. And those are two completely different conversations. And almost nobody in the popular press is having the right one. Stand up in front of a room and say, "We're going to Mars." And watch what happens to people's faces. They light up. They get the look, the frontier look, the same look that got people onto uh ships in the 15th century that were completely unsuited for the ocean, which at least had air. There is something deeply wired into human beings that converts the words we're going there into and it will be hard, but we will manage. I understand that impulse. Intellectually, I even respect it. But physics doesn't care about your impulse. Physics has a list and the list is long and it starts before you've even left the ship. I want to build this from the ground up because that is the only honest way to do it. If I skip steps, I'm selling you something.

[00:01:08]And I'm not in the business of selling.

[00:01:10]I am in the business of telling you what is actually true at whatever cost to the romance of the thing. Start with what Mars is. Not the mythology, not the red paintings, the physical object. Mars is a rock roughly half the diameter of Earth orbiting the sun at between 55 and 400 million km, depending on where both planets are in their respective orbits.

[00:01:35]It has a thin atmosphere, and when I say thin, I mean 1% of Earth's atmospheric pressure at sea level. 1%. If you stood on the surface of Mars without a suit, your blood would begin to boil within seconds. Not because Mars is hot. It isn't. Surface temperatures average around -60° C, but because at 1% atmospheric pressure, the boiling point of water drops below body temperature.

[00:02:03]Your blood is water. That is not a metaphor. That is thermodynamics.

[00:02:09]So the suit problem is real and it's hard and engineers are working on it and fine. But I didn't start with the suit.

[00:02:17]I started with the distance. And the reason I started with the distance is that the distance is where the first kill mechanism lives. And it is a kill mechanism that most people when they think about interplanetary travel manage to overlook almost completely.

[00:02:35]Radiation. Not in the science fiction sense. not glowing green and growing extra limbs. In the physics sense, the kind that doesn't denounce itself, doesn't feel like anything while it's happening, and quietly dismantles the molecular machinery of your cells with the precision of a very patient, very thorough executioner. On Earth, you don't think about cosmic radiation. And the reason you don't think about it is that Earth has thought about it for you.

[00:03:02]Earth has a magnetic field. The magneettosphere generated by the churning of liquid iron in the planet's outer core. And this field extends tens of thousands of kilometers into space in every direction. It is a shield, a genuinely extraordinary shield. Charged particles streaming from the sun, the solar wind arrive at Earth with real energy and are deflected. Not all of them. The Van Allen belts catch some.

[00:03:30]The atmosphere absorbs others. But the essential point is that the surface of the Earth and everything living on it sits inside a multi-layered defense system that took billions of years to build and that you personally have never had to think about for a single second of your life. Mars has almost none of this. Mars' magnetic field is remnant, patchy, effectively negligible as a planetary shield. Its atmosphere, as I said, is 1% of Earth's. What this means is that the surface of Mars is exposed to the full brunt of solar energetic particles and galactic cosmic rays in a way that Earth's surface simply is not.

[00:04:12]The radiation dose on the surface of Mars is estimated at roughly 300 milliseverts per year. To put that in context, the occupational limit for radiation workers in the United States, the people who work in nuclear plants, the people we considered to be in a radiation elevated profession is 50 millisevers per year. Mars hits six times that every year without a single extraordinary event. And here is where it starts to get genuinely ugly because I haven't told you about the transit yet. The trip to Mars, depending on the orbital window and the propulsion system, takes somewhere between six and nine months. During that transit, your spacecraft is not sitting inside Earth's magneettosphere. It is in interplanetary space, which is an environment that makes the surface of Mars look almost cozy by comparison. The estimated radiation dose during the transit alone one way is in the range of 300 to 600 millisevers.

[00:05:13]You haven't landed yet. You haven't touched the ground yet. You are still in your spacecraft eating your freeze-dried food and doing your exercises and watching the Earth shrink to a bright dot in the window. And you have already received a radiation dose that a health physicist on Earth would look at and say, "We have a serious problem." Now, I want to stop here for a moment because I know what some of you are thinking. And what you're thinking is shields.

[00:05:43]shielding. You put enough material between the astronaut and space and you solve the problem. And I want to engage with this seriously because it deserves a serious answer. And the serious answer is it is much harder than it sounds for reasons that are physically fascinating and practically brutal. There are two kinds of radiation you are dealing with in deep space. Solar energetic particles, protons and electrons accelerated by solar events, particularly solar flares and coronal mass ejections and galactic cosmic rays which are high energy atomic nuclei traveling at relativistic speeds some fraction of the speed of light originating from supernova and other violent processes elsewhere in the galaxy. These are two completely different problems with partially incompatible solutions. Solar energetic particles are relatively low energy by cosmic ray standards and they can be substantially blocked by shielding.

[00:06:46]Polyethylene water hydrogen-rich materials work best because protons interact electromagnetically with hydrogen nuclei and lose energy. Fine.

[00:06:55]Build a storm shelter in the spacecraft with dense walls. Have the crew retreat there during solar events and you can manage the solar particle problem. It is not trivial engineering but it is engineering. It is in principle solvable. Galactic cosmic rays are a different animal entirely. These are particles moving so fast that when they hit a material, any material, they don't just stop, they shower. They produce what physicists call secondary radiation. A cascade of secondary particles generated by the interaction of the primary particle with the shield material. And the nightmare scenario, the thing that keeps radiation shielding engineers genuinely awake at night is that for galactic cosmic rays, adding more shielding can actually increase your radiation dose at some thicknesses.

[00:07:48]Because you are generating more secondary particles than you are stopping primary ones, the shield becomes a source. This is not a hypothetical. This is measured physics.

[00:07:59]So there is no clean engineering solution to the galactic cosmic ray problem. Not with passive shielding.

[00:08:06]Active magnetic shielding essentially building a miniature magnetosphere around the spacecraft has been studied and the mass and power requirements are staggering. We are talking about a technology that does not exist in operational form and whose development timeline is measured not in years but in decades if it is possible at all at the required scale. I spent time early in my career thinking about uh particle interactions in ways that touched on this. Not the shielding problem directly but the fundamental physics of high energy particle collisions. The way energy deposits and uh redistributes in matter and I will tell you something the numbers in the cosmic ray problem are not close. There is no variant of push a little harder on the engineering that solves this. The physics is genuinely adversarial. But let me keep building because radiation is one item on the list and I promised you a list and we have barely started. Gravity or rather the absence of it. During the 6 to9 months of transit to Mars, the crew is in microgravity, weightlessness. And weightlessness sounds wonderful. It looks wonderful in the footage from the International Space Station. people floating, doing flips, playing with water droplets. What the footage doesn't show is what is happening inside their bodies. The human body evolved over hundreds of millions of years inside a gravitational field of 1g. Every system in your body, every system is calibrated for that field. Your bones maintain their density because the mechanical load of bearing your weight signals the osteocytes, the bone maintenance cells to keep building. Remove that load and the signal stops. Bones begin to lose density at a rate of roughly 1 to 2% per month in microgravity, not 1 to 2% total per month. Over a six to ninemonth transit, an astronaut can lose 10 to 20% of their bone density in the loadbearing bones, spine, hips, femurss, the exact bones you need when you arrive on Mars and try to stand up and move around.

[00:10:21]Your muscles do the same thing for the same reason. Muscle mass decreases in microgravity because the muscles are not required to work against gravity. The counter measure is exercise. Astronauts on the International Space Station are required to exercise 2 hours per day specifically to combat this. And the exercise regimen is partially effective.

[00:10:42]Partially, not completely. Your cardiovascular system remodels in microgravity.

[00:10:49]Fluid shifts toward the head. You've seen this in astronaut photographs, the slightly puffy faces. The heart reading this as an increase in blood volume adjusts by reducing its output. The cardiovascular system becomes less efficient at the task it will need to perform when the astronaut arrives in gravity again. The heart muscle itself can atrophy. Your vestibular system, the system responsible for balance, becomes confused and eventually recalibrates to a weightless environment. When you arrive at Mars and step out into 1/3g, your balance system is operating on calibration data that is completely wrong. You are a person who has spent 9 months in zero gravity trying to walk on an alien planet. The fall risk alone is significant and falls in a pressure suit on a planet with no hospital are not trivial events. And here is something that has emerged from the long duration spaceflight data in recent years that is frankly alarming and that I do not hear discussed nearly enough in the public conversation about Mars. Intraranial pressure in microgravity, the fluid shift I mentioned pushes cerebral spinal fluid upward. The optic nerve sits at the back of the eyeball and connects through a sheath that is continuous with the brain's fluid system. The increased pressure deforms the optic nerve sheath, flattens the back of the eyeball, and causes what is called spaceflight associated neuroccular syndrome.

[00:12:24]Multiple longduration astronauts have returned from the International Space Station with measurably degraded vision, some of it permanent. The mechanism is still being studied. The counter measures are not established. I want to pause here because I can feel the room processing this and I know what the optimistic response is. The optimistic response is these problems will be solved. Medicine advances, engineering advances, and I want to be honest about my reaction to that because my reaction is not pure dismissal and it is not pure agreement. Some of these problems will be partially mitigated. Exercise counter measures reduce but do not eliminate bone and muscle loss. Pharmaceutical interventions are being studied for bone density.

[00:13:10]Artificial gravity rotating the spacecraft or a section of it would largely solve the physiological gravity problem but adds enormous engineering complexity and has never been done on a crude deep space vehicle. These are genuine research programs with genuine progress. I am not saying they are hopeless. What I am saying is that the word solved is doing a lot of fraudulent work in most public discussions of Mars travel. There is a very large distance between we have research programs addressing this and this problem is solved well enough to send humans safely and the people who speak about Mars colonization in breathless press releases are with very few exceptions not being honest about that distance keep building psychological isolation.

[00:14:01]This one doesn't have a clean physics formula attached to it but it is real.

[00:14:06]It is documented and I think physicists have an obligation not to pretend that human beings are point masses with no interior life. The Mars transit is 6 to9 months of uh confinement in a space roughly the size of a large apartment with the same four to six people with a communication delay to Earth that starts at a few minutes and grows to as much as 20 minutes one way as the planets separate. 20 minutes one way, 40 minutes round trip. If something goes wrong medically, mechanically, interpersonally, there is no realtime conversation with mission control. There is no rescue. There is no abort option in any meaningful sense. Once you are past a certain point in the trajectory, the psychological profile of the people who can function under those conditions in that isolation under that radiation with that physiological degradation knowing that if anything goes seriously wrong they are going to die is extremely narrow. The research on analog environments, Antarctic winter rover crews, submarine crews, long duration ISS astronauts consistently shows that psychological stress, interpersonal conflict, and cognitive degradation are not edge cases. They are expected outcomes of prolonged confinement and isolation. Selection and training reduce the risk. They do not eliminate it. I have a colleague, I will not name him because he would be annoyed, who spent several years consulting on longduration human spaceflight psychology. And he said something to me that stuck. He said, "The biggest problem with the Mars mission psychological research is that we have never actually put humans in a situation where the isolation was real rather than simulated. In every analog environment, in every simulation, the participants know on some level that they can leave. That knowledge changes everything. It changes the threat assessment, the cortisol response, the decision-making under pressure. He said, "We are flying somewhat blind on the genuine psychological profile of the deep space transit because we have no data from people who genuinely cannot leave." That keeps me up a little. Now I want to go somewhere more fundamental because everything I have described so far is in a sense engineering and medicine problems that live in the domain of things that are hard but that humans have some chance of addressing through technology and research. Uh I want to go to the level beneath that the physics level. I want to talk about what it means that Mars is 140 million miles away. Not just as a travel time problem, but as a fundamental constraint on the relationship between cause and effect.

[00:17:02]The speed of light is approximately 300,000 km/s.

[00:17:07]Nothing with mass moves faster. This is not a current technological limitation.

[00:17:12]And this is a feature of spacetime itself baked into the geometry of the universe at a level that no engineering advance will ever change. The distance from Earth to Mars translates at the speed of light to a one-way communication delay of between 3 and 22 minutes depending on orbital positions.

[00:17:32]I already mentioned this, but I want to make you feel what it means, not just hear the number. In medicine, 3 minutes is an eternity. If someone on that spacecraft has a stroke, a clot in a cerebral artery, oxygen cut off to a region of the brain, the treatment window is hours at best, but the first interventions need to happen in minutes.

[00:17:53]The crew has to diagnose, decide, and act entirely on their own. Mission control cannot help in real time. The physician on the crew, assuming there is one, assuming they are conscious, assuming they are not the one having the stroke, has to function as a oneperson hospital with whatever medical resources fit on the spacecraft. And this scales to every failure mode. A pressure breach, a fire, a propulsion system anomaly, a psychological crisis. Every single emergency on that vehicle has to be handled by the crew in real time with whatever knowledge and equipment they brought with them because there is literally no other option. The speed of light has removed it. This is not a new insight. Mission planners know this. But there is a gap between knowing it intellectually and building a system that is genuinely resilient to it. The International Space Station is 400 km above Earth. The communication delay is negligible. Rescue is possible within days. The entire operational culture of human space flight to date has been built around proximity to Earth. The ability to get home quickly, to communicate instantly, to receive physical supplies. Mars is not that.

[00:19:10]Mars is a regime change. It is not a harder version of what we have done. It is categorically different. I want to tell you about something that happened in the Biosphere 2 project in the early 1990s because it is directly relevant and because I find it genuinely instructive rather than simply amusing.

[00:19:30]Biosphere 2 was an attempt to build a closed ecological system in the Arizona desert, a sealed structure that would support eight people for 2 years using only the resources inside it. The science behind it was serious. The people involved were serious. The engineering was for its time impressive.

[00:19:51]Within months, the oxygen level inside the structure was dropping faster than expected because the soil microbes were consuming it and the carbon dioxide was rising. The crew was operating at the oxygen equivalent of living at high altitude and growing progressively impaired. They were hungry because the agricultural system was not producing enough calories. They were psychologically stressed and interpersonally fractured. After a year, oxygen was secretly pumped in from outside, which was not in the original protocol. The project was in the honest assessment of the scientific community, a partial failure, not a catastrophic failure, but a demonstration that closed life support systems at the scale required for human survival are far more complex and fragile than their designers anticipated. I am not telling the story to mock the people who tried it. They were attempting something genuinely hard and they learned real things from failing at it. I am telling the story because Biosphere 2 was in Arizona in the desert but on Earth at 1G with immediate medical access with an atmosphere outside the walls with the ability to open a door and walk out and it still nearly killed them. The closed life support problem on a Mars mission has no door to open. The life support problem on a Mars transit spacecraft is one of the most underappreciated challenges in the public discussion. You are building a system that must recycle air, water, and waste with essentially zero loss over 9 months in a microgravity environment with hardware that cannot be replaced if it fails.

[00:21:32]operated by uh people who are not life support engineers and who are simultaneously dealing with all of the other stresses I have described. The current best technology for atmospheric revitalization on the ISS works, but it requires regular maintenance and replacement parts delivered by resupply missions that happen multiple times per year. Mars doesn't have resupply missions, and I haven't even gotten to Mars yet. We are still on the ship. When you arrive at Mars, you have survived the radiation of the transit partially and the physiological degradation of 9 months in microgravity and the psychological pressure of 9 months of confinement. And you are now going to attempt to land on a planet with an atmosphere that is simultaneously thick enough to create dangerous heating during entry and thin enough to provide almost no useful breaking. This is called the seven minutes of terror in popular accounts of Mars landing. And for once, the popular account is not exaggerating. Earth entry and descent works because Earth's atmosphere is dense. A blunt body entering at orbital velocity generates a shock wave that rapidly decelerates it, and parachutes in the thick lower atmosphere finish the job. Mars's atmosphere at 1% of Earth's pressure generates a shock wave. Yes.

[00:22:52]But by the time you have decelerated to the point where parachutes deploy, you are still moving too fast for a safe landing and too low to decelerate further with traditional retro rockets without enormous fuel mass. The heaviest thing ever successfully landed on Mars is the Perseverance rover at about 1 metric ton using a combination of heat shield, supersonic parachutes, and a rocket powered sky crane. A crude Mars mission vehicle will mass somewhere between 40 and 100 metric tons. We do not have a demonstrated technology that lands that mass on Mars. This is not a small gap. The Mars entry, descent, and landing problem for humanass payloads is an open engineering problem and any honest engineering assessment will tell you that. So take stock. By the time you stand on the surface of Mars, assuming you survive the transit radiation, assuming you survive the physiological degradation, assuming you survive the psychological isolation, assuming the life support doesn't fail, assuming the entry and descent works, you are a person with measurably reduced bone density, reduced muscle mass, a potentially compromised cardiovascular system, a vestibular system calibrated for zero gravity, now oper operating in 1/3 G, possible vision degradation from intraraanial pressure, and a radiation exposure history that a doctor on Earth would look at with serious concern, and you are standing on a surface with no breathable atmosphere, temperatures that swing between -80 and 20° C depending on location and season, a thin layer of perchlorate contaminated soil, perchlorates being toxic to humans at sufficient concentrations and a radiation environment that will continue to accumulate dose on your body every single day. You are there and you cannot leave whenever you want. The orbital mechanics of Mars missions mean that the Earth and Mars are only in the right alignment for a return trip every approximately 2 years. Miss the window and you wait on Mars in your habitat for two years. I want to say something here that I think is important and that I want to be careful about because I do not want to be misunderstood. I am not arguing that we should not go to Mars. I am not a defeist about human space flight. I have spent my life studying the universe and I believe with everything in me that expanding the domain of human presence and human knowledge is worth doing, is worth risk, is worth sacrifice. The people who will eventually go to Mars if we do this honestly and carefully will be among the most extraordinary human beings who have ever lived. I mean that what I am arguing what I am insisting on actually with all the insistence I have left in me is that the public conversation about Mars is not honest. The cheerful press releases, the renderings of sleek habitats and smiling astronauts in clean suits, the language of challenging but achievable, and the next step for humanity. This language is not lying exactly, but it is performing a selective edit on physical reality that I find professionally somewhere between frustrating and unconscionable. The people planning to go to Mars deserve to know exactly what uh is going to try to kill them and exactly how well we understand how to stop it. Not a curated version. The actual version and the actual version is the radiation problem is serious and not solved. The physiological problem is serious and partially mitigated. The life support problem is serious and not demonstrated at the required scale. The entry, descent, and landing problem is serious and not solved for human class payloads.

[00:26:58]The psychological isolation problem is serious and not well characterized.

[00:27:04]Every one of these problems sits between you and a successful Mars mission. And the distance between we have research programs and this is ready is not a few years of additional engineering. It is much larger than that. There was a version of this conversation I had years ago with a physicist friend, someone who worked at JPL for a long time on Mars mission planning. And he said something that I thought was absolutely right. He said, "The problem with the public Mars conversation is that it conflates enthusiasm with analysis." And enthusiasm is wonderful and necessary.

[00:27:41]Enthusiasm is what drives the funding, drives the careers, drives the young people into aerospace engineering programs. But enthusiasm without analysis is not a space program. It is theater. And theater gets people killed.

[00:27:56]I want to close on something that connects this to a bigger question because I think the Mars problem is actually a window into something deeper, something about what the universe is and what it is willing to give us. When I was young and I was working on the problems that eventually led to what is now called the holographic principle, I kept running into a phenomenon that I came to think of as the universe's adversarial architecture. The universe is not designed to be hospitable. It is not optimized for our survival or our exploration. It is what it is. And what it is includes black holes that destroy information, spaceime that curves in ways that trap you, radiation environments that dismantle the molecules you are built from, and distances that take so long to cross that the very act of crossing them damages you. The speed of light is not a challenge that technology will eventually overcome. The inverse square law of radiation intensity is not a challenge that better shielding will fully neutralize. The biology of the human body tuned by hundreds of millions of years of evolution for a specific gravitational and radiation environment does not have a software update that rewrites it for a different planet.

[00:29:11]These are structural features of the universe and they apply to Mars with a uniformity and impartiality that should command genuine respect. What I find fascinating and genuinely productively unsettling is the question of whether there is any configuration of human biology and technology that makes sustained human presence beyond Earth's magnetic shield a long-term reality. Not a mission, not a short duration expedition, a civilization. The radiation alone, galactic cosmic rays, which are not correlated with solar cycles and cannot be predicted or waited out, may set a hard ceiling on how long a human being can live on the surface of Mars before cumulative dose becomes a primary driver of cancer risk, cognitive decline. We do not actually know where that ceiling is. The Mars surface dose data we have is from rovers, not humans.

[00:30:07]And the biological dose response at these cumulative levels is not well characterized in the peer-reviewed literature. This is a genuine gap in our knowledge and it is a gap that matters enormously if anyone is serious about Mars as a place for humans to live rather than visit briefly. I do not have a tidy resolution to offer. I spent 50 years building physics and one thing physics has taught me is that the universe does not owe you a tidy resolution. Sometimes the honest answer is we don't know enough yet and the people who tell you otherwise are either mistaken or they have something to sell you. What I want to leave you with is this.

[00:30:50]140 million miles sounds like a distance. What it is really is a summary of everything I just told you. The radiation that accumulates on every kilometer of that distance. The time that passes in microgravity while that distance is being crossed. The communication delay that grows with every kilometer. The orbital mechanics that make coming home not a decision but a calendar entry. The distance is not the danger. The distance is the container for all the dangers. And until we can face every item inside that container with the full seriousness of physics and medicine and engineering, not optimism, not enthusiasm, not marketing, but serious scientific analysis, we are not ready. And right now, we are not ready. The question is whether we have the honesty to say