The Challenges of Sending Humans to Mars

Humans have lived in orbit for months at a time and walked on the Moon, a three-day trip from Earth.

Mars is a different kind of challenge altogether.

Even at its closest, it is more than a hundred times farther away than the Moon,

and a round trip would keep a crew away from home for years.

Below are the biggest obstacles standing between people and the Red Planet, from radiation and landing to food,

health and isolation, along with the ideas researchers have tested to overcome them.

Together they show why a crewed Mars mission is one of the hardest projects humans have ever considered.

The Distance Problem

Earth and Mars both orbit the Sun at different speeds, so the distance between them keeps changing,

from roughly 55 million kilometers at the very closest to about 400 million kilometers when they sit on opposite sides of the Sun.

Spacecraft cannot simply fly straight there.

They follow long, curved paths that line up with the planets' motion, and the most efficient launch opportunities come around only once every 26 months.

With the chemical rockets used so far, the journey takes roughly six to nine months each way.

Because the planets must realign for the trip home,

many mission designs add up to two to three years away from Earth,

including a long stay on the surface while waiting for the next window.

Distance also affects communication.

Depending on the planets' positions, a radio message takes about 3 to 22 minutes to travel one way,

so a question and its answer can be separated by the better part of an hour.

When the Sun passes between Earth and Mars, communication can be disrupted for about two weeks.

A Mars crew would need to handle emergencies largely on its own.

Radiation Beyond Earth's Shield

On Earth, and even on the International Space Station, the planet's magnetic field deflects much of the radiation streaming through space.

On the way to Mars, a crew would lose that protection.

Two main threats stand out: galactic cosmic rays, extremely energetic particles from distant exploding stars,

and bursts of particles thrown out by the Sun during solar storms.

The Curiosity rover carried a radiation detector during its cruise to Mars in 2011 and 2012.

Its measurements suggested that the round-trip journey alone could expose astronauts to roughly 0.66 sieverts,

several times what astronauts typically absorb during a six-month stay on the space station.

Radiation exposure raises long-term cancer risk and may affect the brain and heart.

Shielding against cosmic rays is difficult because they can pass through thin metal.

Ideas include surrounding the crew's sleeping area with water, food and supplies,

building a small storm shelter for solar events and shortening travel time with faster propulsion.

On Mars itself, the thin atmosphere and lack of a global magnetic field mean habitats would likely need thick shielding,

perhaps from buried structures or piled-up soil.

The Toll of Weightlessness

Months without gravity weaken bones and muscles, shift fluids toward the head and can change vision.

Station crews counter this with hours of daily exercise and the knowledge that recovery teams will help them after landing.

A Mars crew would not have that luxury. After six months or more in weightlessness,

astronauts would step onto a planet with about 38 percent of Earth's gravity and need to work almost immediately,

with no one there to help them out of their seats.

Researchers study exercise programs, nutrition and other countermeasures to keep crews strong enough for that moment.

Landing Something Heavy

Mars has an awkward atmosphere for landing. It is thick enough to heat a spacecraft fiercely on the way in,

but its surface pressure is less than one percent of Earth's,

too thin for parachutes to slow a heavy vehicle properly.

The heaviest spacecraft landed on Mars up to the early 2020s, the Curiosity and Perseverance rovers, each weighed around one metric ton.

A crewed lander, carrying people, supplies and a way to leave again, would likely weigh many times more, probably tens of tons.

Engineers are exploring large inflatable heat shields and firing rocket engines while still traveling faster than sound to brake through the thin air.

Both approaches have been tested at smaller scales, but setting down a load that heavy on Mars would be a first.

Living Off the Land

Every kilogram launched from Earth is costly, and bringing enough propellant for the trip home would make a Mars mission enormous.

That is why planners want to use local resources, an approach called in-situ resource utilization. The main targets include:

• Oxygen from the air: The Martian atmosphere is about 95 percent carbon dioxide. An experiment called MOXIE aboard Perseverance showed that oxygen could be extracted from it, producing small amounts on the surface.
• Water from ice: The Phoenix lander scooped up water ice just below the surface in 2008, and orbiting radar has found buried ice in many regions. Water could be used for drinking, growing food and radiation shielding.
• Rocket propellant: Combining carbon dioxide with hydrogen from water can produce methane and oxygen, a propellant pair that could fuel the trip home.
• Power: Mars receives less than half the sunlight Earth does, and dust storms can darken the sky for weeks or even months. Compact nuclear reactors, one of which was tested on the ground in 2018, are among the options studied.

Keeping a Crew Healthy and Sane

A medical emergency on the space station can end with the crew returning to Earth within hours.

On a Mars mission, there would be no quick way home.

Crews would need broad medical training, compact diagnostic tools and supplies for problems ranging from dental emergencies to broken bones,

all with advice from Earth arriving many minutes late.

Isolation is its own challenge. Crews would spend years confined with the same few people, with no fresh faces,

no weekend away and a home planet shrinking to a blue dot. Researchers have studied this in long simulations.

In one experiment in Moscow, six men lived sealed inside a mock spacecraft for 520 days between 2010 and 2011,

and several showed disrupted sleep and reduced activity over time.

Isolated habitats in Hawaii's volcanic landscapes and research stations in Antarctica have offered further lessons about teamwork, boredom and conflict.

A Hostile Surface

Mars itself is unforgiving. The average temperature sits around minus 60 degrees Celsius (minus 80 degrees Fahrenheit),

the air cannot be breathed and fine dust clings to everything, wearing down seals, suits and machinery.

In 2008, the Phoenix lander also found perchlorates in the soil, chemical compounds that are toxic to people in significant amounts.

Scientists also worry about contamination in both directions.

If microbes from Earth hitch a ride, they could muddy the search for native Martian life.

Mission planners follow planetary protection guidelines designed to keep Mars as clean as possible and to handle anything brought back to Earth with great care.

Why People Keep Trying

Sending humans to Mars would demand solutions to problems that no crew has faced before: radiation far from Earth's shield,

a landing heavier than any attempted, years of self-reliance and a surface hostile to life.

Rovers, space station research and simulations on Earth have steadily chipped away at those unknowns.

Whenever people finally make the journey, they will be standing on decades of careful groundwork.