The first crewed mission to Mars is often imagined as the beginning of humanity's journey to another world.
But getting astronauts there is only half the problem. Can we actually bring them home?
Mars has fascinated humanity for centuries. From the canals imagined by nineteenth-century astronomers to today's robotic explorers, the Red Planet has gradually changed from a distant point of light into a destination.
But there is a question that receives surprisingly less attention than the journey there:
If humans eventually land on Mars, will they actually be able to come back?
A mission to Mars is not simply a longer version of a trip to the Moon. The distances are enormous, launch windows are limited, communication is delayed and returning a crew safely requires spacecraft, fuel, power and life-support systems to work hundreds of millions of kilometres from Earth.
And unlike the Apollo missions, astronauts travelling to Mars could be away from Earth for years.
Mars Is a Long Way From Home
The distance between Earth and Mars is constantly changing because both planets orbit the Sun.
At their closest, the two worlds can be roughly 54.6 million kilometres apart. At other times, they are separated by more than 400 million kilometres.
That creates a fundamental problem.
A spacecraft cannot simply land on Mars, decide to leave the next day and point itself towards Earth.
Earth and Mars have to be in suitable positions in their respective orbits.
The most efficient return opportunities occur during particular orbital alignments. Consequently, astronauts arriving on Mars may have to remain there for many months before the geometry of the planets allows a practical journey home.
A human Mars expedition could therefore involve a journey to Mars, a lengthy stay and another journey home — potentially resulting in a mission lasting several years.
Getting Off Mars Is the First Big Problem
Landing on Mars is difficult. Leaving it is difficult too — although not because of the thin atmosphere.
Mars does have an atmosphere, but it is less than 1% as dense as Earth's. For a rocket taking off, this is actually an advantage because there is far less aerodynamic drag to overcome.
Mars's low gravity also helps enormously. Surface gravity is only about 38% that of Earth, while its escape velocity is approximately 5 kilometres per second, compared with about 11.2 kilometres per second on Earth.
So, in principle, a rocket launched from Mars requires considerably less energy to escape the planet than one launched from Earth.
The real challenge is propellant and reliability.
A crewed Mars mission would need a launch vehicle capable of lifting astronauts from the surface and placing them on a trajectory back towards Earth. That vehicle would have to be available on Mars, fully operational and supplied with enough fuel and oxidiser.
There would be no conventional launch infrastructure nearby, no large engineering team standing by and no possibility of quickly sending replacement parts from Earth.
If the ascent vehicle failed, the consequences could be catastrophic.
This is why some proposed Mars missions involve sending the return vehicle and its propellant to Mars before the astronauts arrive. In some concepts, fuel could even be manufactured on Mars using resources already available there.
The thin atmosphere isn't the obstacle.
Making sure the rocket, its fuel and every critical system are ready when the astronauts need to leave is.
Mars also has only about 38% of Earth's surface gravity.
That actually helps.
A rocket launched from Mars doesn't need as much velocity to escape the planet as one launched from Earth.
The escape velocity from Mars is approximately 5 kilometres per second, compared with about 11.2 kilometres per second from Earth.
That makes a Mars ascent vehicle considerably smaller than the enormous rockets required to escape Earth.
But "easier" does not mean easy. The rocket would need to work perfectly. There would be no conventional launch pad nearby, no large engineering team standing by and no quick rescue mission from Earth.
If something went wrong during launch, the crew could have nowhere to go.
Could Astronauts Make Their Own Fuel?
One of the most important ideas in Mars mission planning is in-situ resource utilisation, or ISRU.
Instead of transporting every kilogram of fuel from Earth, astronauts could potentially manufacture some of what they need after arriving.
One proposed approach involves using Martian carbon dioxide and hydrogen to produce methane and oxygen.
The basic chemistry is well established. The challenge is building and operating the machinery reliably on another planet.
Such a system could potentially produce propellant before the astronauts even arrive.
This creates a fascinating possibility.
Rather than landing on Mars with enough fuel to return home, a robotic precursor mission could arrive years beforehand, manufacture fuel and store it.
The crew would then only attempt the human landing once mission controllers had confirmed that the return propellant was available.
In principle, this could dramatically reduce the amount of material that must be launched from Earth. But it introduces another terrifying dependency:
The astronauts' survival could depend on a fuel factory operating flawlessly on another planet.
What If the Fuel Plant Breaks?
This is where Mars exploration becomes fundamentally different from a normal expedition.
On Earth, a failed machine can often be repaired. On Mars, replacement components could be millions of kilometres away. A spare part might take months to arrive. Some components might be impossible to replace.
A mission would therefore need extraordinary redundancy.
Multiple systems might have to be carried. Critical equipment could require backup power supplies, spare electronics, alternative manufacturing methods and perhaps the ability to cannibalise one machine to repair another.
The philosophy would be simple:
Assume something will break.
The question is whether the crew can survive when it does.
Radiation Is Another Threat
Mars has another major disadvantage. Earth is protected by a substantial atmosphere and a global magnetic field that shield life from much of the dangerous radiation arriving from space.
Mars has neither comparable protection. Its atmosphere is less than 1% as dense as Earth's, providing relatively little shielding.
Astronauts travelling between the planets would spend months exposed to cosmic radiation and solar particle events.
Once on Mars, they would continue receiving elevated radiation doses.
One potential solution is to build habitats beneath the surface or cover structures with layers of Martian soil.
Even a relatively modest amount of material above a habitat could provide additional protection.
Future explorers might therefore spend much of their lives underground.
The first Martian settlement could look less like a science-fiction city and more like a network of buried laboratories, tunnels and pressure chambers.
What Happens if Someone Gets Sick?
This may be one of the least appreciated dangers. Imagine suffering appendicitis on Mars.
On Earth, an ambulance can take you to hospital. Surgeons can operate, specialists can be consulted and sophisticated medical equipment is readily available.
On Mars, there may be no surgeon. There may not even be another human being capable of performing a complex operation. A crew would need extensive medical training and equipment.
Eventually, artificial intelligence and robotic medical systems could potentially assist astronauts, but relying on autonomous medicine millions of kilometres from Earth would still represent an extraordinary challenge.
And because of the communication delay between Earth and Mars, real-time conversations with doctors would not be possible.
Depending on the planets' positions, one-way communication can take roughly 3 to 22 minutes. A medical emergency could therefore become a situation in which the crew has to act before Earth even receives the message.
What If They Don't Want to Leave?
There is another question — one that belongs as much to philosophy as engineering.
Suppose humans establish a functioning settlement on Mars. Suppose people are born there. Suppose someone spends decades living beneath the Martian surface.
Would they still regard Earth as home?
The first explorers would almost certainly have strong connections to Earth. But later generations might develop a distinct Martian identity.
Earth could eventually become the distant ancestral world rather than the place they personally identify with.
This raises a remarkable possibility.
The first permanent human settlement on Mars might begin as a scientific expedition but eventually become something entirely different.
A colony.
The One-Way Mars Mission
Some proposals have historically considered the concept of a one-way human mission to Mars.
The idea is controversial. Instead of designing the mission around returning everyone to Earth, astronauts would establish a permanent presence. They would live out the remainder of their lives on Mars.
From a purely engineering perspective, removing the requirement for a return journey could reduce the mass and complexity of a mission.
But ethically, psychologically and politically, the implications are enormous.
Would volunteers genuinely be able to give informed consent to a mission from which there was no realistic possibility of returning?
And what would happen if a technology developed after their departure suddenly made a return possible?
The distinction between an expedition and permanent settlement would become blurred.
Could They Be Rescued?
A rescue mission sounds reassuring. It probably wouldn't be. If astronauts became stranded on Mars, Earth couldn't simply send another spacecraft immediately. The planets might be in the wrong positions.
A rescue vehicle would itself need to be launched, travel to Mars, land, reach the stranded crew and then somehow transport everyone back.
The entire operation could take years.
That means a Mars mission would have to be designed around self-rescue. The crew would need sufficient supplies, power, shelter, medical capability and spare equipment to survive serious failures.
The safest Mars mission may therefore be one in which the astronauts never depend on Earth rescuing them.
So Will Humans Come Back?
Technically, there is no fundamental law of physics preventing humans from travelling to Mars and returning to Earth.
We already possess much of the underlying technology. We know how to navigate spacecraft between planets. We know how to land robotic spacecraft on Mars. We know how to launch rockets from planetary surfaces.
We understand the chemistry required to manufacture potential rocket propellants. And robotic missions have demonstrated that spacecraft can operate on Mars for years.
The problem is combining all these capabilities into one incredibly complicated human system.
A crewed Mars expedition would have to survive:
- The launch from Earth
- Months in deep space
- Radiation exposure
- A high-risk Mars landing
- Life on another planet
- Dust and equipment degradation
- Medical emergencies
- Psychological stress
- Production or storage of return propellant
- A Mars ascent launch
- Another deep-space journey
- Re-entry into Earth's atmosphere
- Landing back on Earth
Every stage represents another opportunity for something to fail.
The Real Question May Be Different
Perhaps the question isn't: "Can humans return from Mars?"
Perhaps it is: "Can we make returning from Mars reliable enough to become routine?"
That is a much harder problem.
The first astronauts to Mars may be pioneers in the truest sense of the word. They won't simply be travelling somewhere no human has visited before. They will be entering an environment where mistakes can become irreversible.
The first successful Mars mission may therefore depend less on spectacular new technology than on something much less glamorous: redundancy. Spare systems. Backup power. Multiple sources of oxygen. Multiple ways to produce water. Multiple ways to make fuel. Multiple ways to communicate.
And, above all, multiple ways to get home.
Because when humans finally stand on Mars, the most important piece of equipment may not be the rover, the laboratory or even the habitat.
It may be the rocket waiting silently on the surface.
The rocket that brings them home.
The unsettling possibility
There is a darker thought. Humanity's first footprints on Mars may not mark the beginning of a temporary expedition.
They could mark the beginning of a permanent split in human civilisation.
If we eventually establish self-sufficient settlements there, the first humans to leave Mars might not be returning to Earth.
They might be leaving one world for another.
And one day, someone born on Mars could look up at the tiny blue dot of Earth and ask the opposite question:
"If humans ever came from Earth, why did they leave?"
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