Posted by Steve Squyres, who led the Spirit and Opportunity rover missions, liked to point out an awkward truth about his own robots: a trained field geologist on foot could probably do in days what his rovers took years to accomplish. That is the scientific argument for the human exploration of Mars in a sentence. People can scramble up an outcrop, notice that one rock looks wrong, crack it open and change the plan on the spot. The catch is that keeping those people alive for a round trip of two or three years, hundreds of millions of kilometers from the nearest hospital, is a problem engineers have been working on since the 1950s and have still not fully solved.
So rather than ask whether we can go, it’s more useful to walk through the trip itself and look at what each stage demands.
The long cruise and the radiation bill
Mars and Earth line up for an efficient transfer roughly every 26 months, and the trip itself takes something like six to nine months with chemical rockets. Because the planets keep moving, a classic “conjunction-class” mission also means waiting on Mars for over a year until the geometry allows a return. Total time away from Earth: around two and a half to three years.
During all of it, the crew is outside Earth’s magnetic field. In 2011 and 2012, the Curiosity rover carried a radiation detector called RAD inside its spacecraft on the way to Mars, effectively riding in the seat an astronaut would occupy. Extrapolated to a round trip, its measurements implied a dose of roughly 0.66 sieverts from the cruise alone, before counting any time on the surface. That is a sizeable fraction of the career limits space agencies currently allow, and much of it comes from galactic cosmic rays, heavy fast particles that are hard to stop with any reasonable thickness of shielding. Solar storms are the other danger: brief and intense, but at least something you can hide from in a well-shielded corner of the ship, often built from water tanks and food stores.
Faster propulsion would help, because less time in transit means less dose. Nuclear thermal rockets, tested on the ground in the 1960s and revived in recent studies, could shave months off the journey. None is flying yet.
Landing a house, not a rover
Here’s a problem that gets less attention than it deserves. Perseverance, the heaviest thing ever landed on Mars, weighs about a tonne. A crewed landing would need to set down something in the range of twenty to forty tonnes or more per vehicle, including the crew cabin, a habitat, power systems and an ascent vehicle to get home.
The Martian atmosphere is the worst of both worlds for this. It’s thick enough to generate fierce heating on entry, so you need a heat shield, but far too thin for parachutes to slow a heavy vehicle enough. Engineers expect to rely on supersonic retropropulsion, firing engines into the oncoming air during descent, which SpaceX has practised with Falcon 9 boosters returning through Earth’s upper atmosphere. Inflatable heat shields are another option: NASA tested a 6-meter version called LOFTID on re-entry from orbit in 2022. Nobody has yet put the pieces together on Mars, at scale, with people aboard.
Air, water and the machines that recycle them
On a Mars mission, everything that can be recycled must be. The International Space Station gives the best preview. In 2023, NASA reported that the station’s water system had reached about 98 percent recovery, pulling usable water back out of humidity, sweat and urine. That number sounds triumphant, and it is. It also comes from hardware that breaks down regularly and gets fixed with spare parts sent up from Earth every few months, a luxury a Mars crew won’t have.
The life-support systems for a Mars ship must therefore be simpler, more reliable and repairable by the crew with what they carry. Every pump, filter and valve is a potential single point of failure two years from home.
Electricity when the dust rolls in
Solar power has served Mars rovers well, until it didn’t. Spirit and Opportunity both ended their missions in part because of dust and storms, and the InSight lander slowly starved in 2022 as its panels went brown. A crewed outpost will need power around the clock, with heaters, life support and fuel production running constantly.
That’s why NASA’s Kilopower project built and tested a small fission reactor prototype, KRUSTY, in Nevada in 2018. It ran successfully through a series of tests, including simulated failures. A handful of reactors of that general class, each roughly the size of a refrigerator in its core, could keep a small base alive through a global dust storm. They still have to be turned into flight hardware.
Suits, rovers and the work itself
The whole point of going is fieldwork, and fieldwork happens outside. Apollo suits were designed for a few days on the Moon. Martian suits have to survive hundreds of excursions, keep fine abrasive dust out of their joints and seals, let a geologist bend, kneel and swing a hammer, and stay serviceable in a habitat airlock.
Range matters too. On Apollo 17, Gene Cernan and Harrison Schmitt drove their Lunar Roving Vehicle across the Taurus-Littrow valley and covered around 35 kilometers in three days. A Mars crew would want much more, ideally in pressurized rovers where they can drive for days in shirt sleeves and step out only at the interesting sites. Every kilometer, though, is a kilometer the crew must be able to walk back if the vehicle fails.
When mission control can’t help
At its closest, Mars is a few light-minutes away. At its farthest, a radio message takes over twenty minutes each way. And for about two weeks every couple of years, when Mars passes behind the Sun from Earth’s point of view, communication can drop out almost entirely.
This changes how a mission works. On the space station, a ground team watches every system in near real time and talks the crew through problems. On Mars, a crew facing a fire, a medical emergency or a failing pump has to act on its own and send the report afterward. Training, procedures and onboard software need to assume that.
The psychology is the subtler half. Russia’s Mars-500 experiment locked six men inside a mock spacecraft in Moscow for 520 days in 2010 and 2011, and several showed disrupted sleep and lethargy as the months went on. NASA’s CHAPEA analog started in 2023 with four volunteers living for a year in a 3D-printed habitat in Houston. Analogs can’t reproduce danger or real isolation, but they show where human limits begin to fray.
Keeping Mars clean, and Earth too
A final, slightly paradoxical issue. The main scientific question on Mars is whether it ever had life, and people are walking clouds of microbes. Planetary protection rules for robots are strict, and with astronauts on the surface, keeping samples uncontaminated becomes much harder. Sensitive places, especially any where liquid water might exist today, could be off limits to boots. The return trip raises the reverse question: how do you handle Martian samples, or returning crew members, so that nothing unwanted comes back? Even NASA’s robotic sample-return plans have struggled with cost and complexity in recent years.
None of this makes the human exploration of Mars impossible. It does explain why a mission keeps sliding toward “the 2030s or later”. The technologies mostly exist in prototype form. Integrating them into one system that a crew can trust with their lives, and testing it thoroughly, is the actual job.
There’s far more to say about each of these stages, and SETIworld keeps adding to it, from radiation science to new findings at Jezero Crater. Come read along and argue about which problem you’d fix first if you ran the human exploration of Mars.
