The idea of sending humans to Mars has been studied by scientists and engineers since the late 1940s as part of efforts to explore the planet. Some long-term plans have suggested sending people to live on Mars and changing the planet to make it more like Earth. So far, only robots, such as landers, rovers, and a helicopter, have been sent to Mars. The farthest humans have traveled beyond Earth is to the Moon, during the U.S. NASA Apollo program (1968–1972) and the Artemis II mission (2026).

Plans for missions that would send humans to space began in the early 1950s. These missions were usually expected to happen 10 to 30 years after they were first proposed. Many organizations and space agencies have created plans for crewed Mars missions. These plans include short scientific trips, where a small group of astronauts (two to eight people) would visit Mars for a few weeks or longer, and longer-term goals like building research stations or living on Mars. Some plans also include exploring Mars’ moons, Phobos and Deimos. By 2020, ideas for visiting Mars using touch-based technology had been suggested.

Uncrewed exploration of Mars has been a goal of many countries for many years. The first mission to reach Mars was in 1965, when the Mariner 4 spacecraft flew by the planet. Humans have imagined traveling to Mars in stories since the 1880s. In books, comics, and movies, Mars is often a place where people explore or live. The idea of Martians, or living beings on Mars, comes from fiction. Proposals for sending humans to Mars have been made by groups like NASA, the European Space Agency, China’s CNSA, companies like Boeing and SpaceX, and organizations such as the Mars Society and The Planetary Society.

Travel to Mars

The energy required to move a spacecraft between planetary orbits, called delta-v, is lowest during specific times determined by the synodic period. For trips between Earth and Mars, this period occurs every 26 months (2 years, 2 months). Missions are usually planned to begin during these intervals. Because Mars' orbit is not perfectly circular, the energy needed during these low-energy periods changes over roughly a 15-year cycle. The easiest periods require about half the energy of the most difficult ones. In the 20th century, low-energy launch windows occurred in 1969 and 1971, then again in 1986 and 1988. This pattern repeated over time. The most recent low-energy launch window was in 2023.

Several mission plans have been suggested, including opposition class, conjunction class, and the Crocco flyby. The Hohmann transfer orbit is the most energy-efficient way to reach Mars. It is a conjunction class mission that would take about 9 months to travel from Earth to Mars, approximately 500 days (16 months) at Mars to wait for the next transfer window to Earth, and another 9 months to return to Earth. This would make the total trip last about 34 months.

Shorter Mars missions could have round-trip flight times of 400 to 450 days (about 15 months) for an opposition-class mission, but these would require more energy. A faster mission with a 245-day (8-month) round trip might be possible using on-orbit staging. In 2014, a method called ballistic capture was proposed. This could lower fuel costs and allow for more flexible launch windows compared to the Hohmann transfer.

In the Crocco grand tour, a crewed spacecraft would pass by Mars and Venus in less than a year. Some flyby mission designs could also include a Mars landing using a special spacecraft called a flyby excursion lander. In 1966, R. Titus proposed a plan involving a short-stay lander that would separate from the main Earth-Mars transfer spacecraft before reaching Mars. This lander would arrive earlier, either orbit Mars or land, and then return to the main spacecraft within 10 to 30 days, depending on the design.

In the 1980s, scientists suggested that using Mars' atmosphere to slow down a spacecraft, called aerobraking, could reduce the mass needed for a human Mars mission by up to half. Because of this, spacecraft and landers designed for Mars missions now include features that allow for aerobraking.

Landing on Mars

Many spacecraft without people have landed on Mars, but some, like Beagle2 (2003) and the Schiaparelli EDM (2016), had problems during landing. Successful missions include:

• Mars 3 – 1971
• Viking 1 and Viking 2 – 1976
• Mars Pathfinder and its Sojourner rover – 1997
• Spirit and Opportunity rovers – 2004
• Phoenix lander – 2008
• Curiosity rover – 2012
• InSight lander – 2018
• Tianwen-1 lander and Zhurong rover – 2021
• Perseverance rover and Ingenuity helicopter – 2021

When spacecraft reach Mars, they must slow down to enter orbit. Two methods are used: rockets or aerocapture. Scientists studied aerocapture for human missions in the 20th century. In a review of 93 Mars studies, 24 used aerocapture for Mars or Earth return. A key concern for crewed missions is the maximum force astronauts can safely experience. Scientists agree that 5 g (five times Earth’s gravity) is the highest safe deceleration.

Safe landings require understanding Mars’s atmosphere, first studied by Mariner 4, and identifying good landing sites. Major surveys were done by Mariner 9, Viking 1, and orbiters that supported Viking landers. Later orbiters, like Mars Global Surveyor, 2001 Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter, mapped Mars in greater detail. These surveys found likely locations of water, a vital resource.

Sending humans to Mars will be very expensive. In 2010, one estimate was about $500 billion, but costs may be higher. In the late 1950s, space exploration was a competition between nations, driven by political goals. This was not sustainable, and now, international cooperation is common, as seen in projects like the International Space Station and the proposed Lunar Gateway.

Some argue that the high cost of human missions to Mars may not be worth the benefits and that money could be better spent on robotic exploration. Others believe that human presence in space could inspire public interest and global teamwork. Some also say long-term space travel investments are needed for humanity’s survival.

One way to lower costs could be space tourism. As more people use space travel, technology may improve, reducing costs. This is similar to how personal computers became cheaper and more powerful as their use grew.

Key challenges for human missions to Mars include:

• Health risks from cosmic rays and other radiation. In 2013, NASA found that a Mars mission might expose astronauts to 0.66 sieverts of radiation round-trip, which is close to the 1 sievert limit for astronauts. In 2017, NASA reported higher radiation levels on Mars due to a solar storm.
• Kidney health risks from long exposure to radiation and microgravity. A 2024 study found that prolonged exposure to these conditions may harm kidney function.
• Health effects of long-term weightlessness, such as bone loss and vision changes. A 2019 study found blood flow and clotting issues in astronauts during six months on the International Space Station.
• Psychological and social effects of long isolation and lack of real-time contact with Earth.
• Social challenges of living in tight spaces for over a year.
• Limited medical resources.
• Risks of equipment failure, such as propulsion or life-support systems.

Some of these issues were studied in the HUMEX project. Scientists like Ehlmann have examined political, economic, technological, and biological challenges. While fuel for round-trip missions may be difficult, methane and oxygen could be made using Martian water ice and carbon dioxide with advanced technology.

Spacecraft sent to Mars must be sterilized to avoid contamination. General spacecraft can have up to 300,000 spores on their surfaces, but stricter rules apply to spacecraft heading to areas with water. Contamination could harm experiments or the planet itself.

Sterilizing human missions to this level is impossible because humans carry many microorganisms. Containment is the only option, but it is hard to manage during accidents. Scientists have discussed this issue, but no final guidelines exist. Humans could also risk bringing Martian microbes back to Earth.

Mission proposal

Over the past 70 years, many different plans have been suggested or studied for sending humans to Mars. These plans include using chemical, nuclear, and electric propulsion systems, as well as various methods for landing on Mars, living there, and returning to Earth.

Several countries and groups have long-term goals to send humans to Mars.

• The United States has robotic missions exploring Mars now, with plans to bring samples back to Earth in the future. The Orion Multi-Purpose Crew Vehicle (MPCV) is designed to carry astronauts to and from space, while a Deep Space Habitat module would provide extra living space for the 16-month journey to Mars. A first crewed mission to Mars, which would involve sending astronauts to the planet, orbiting it, and returning to Earth, is planned for the 2030s. Work is being done to develop technology for U.S. government missions to Mars, but there is no fully funded plan to complete the mission with human landings on Mars by the mid-2030s, as stated as the goal. NASA engineers are researching ways to build possible human habitats on Mars by making bricks from pressed Martian soil.

• The European Space Agency (ESA) has a long-term goal of sending humans to Mars but has not yet created a spacecraft for human travel as of October 2024. It sent robotic probes, such as ExoMars, in 2016 and planned to send another probe in 2022. However, the project was paused because of Russia’s invasion of Ukraine. As of November 2022, the ESA was planning to send the probe in 2028 with help from NASA.

Technological innovations and hurdles

Significant technological challenges must be solved for humans to travel to Mars.

Entering Mars' thin and shallow atmosphere during re-entry will be difficult. Compared to Earth's much thicker atmosphere, spacecraft will fall quickly to the surface and must slow down. A heat shield is needed. NASA is studying technologies that use rockets to slow spacecraft during Mars' atmospheric entry. A major issue with these rocket-based methods is managing the movement of air around the spacecraft and controlling its direction during the fast descent.

A return mission from Mars would require landing a rocket on the surface to carry astronauts back. This rocket could be smaller than rockets used to launch from Earth to space. Launching from Mars might also be done with a single stage. However, landing such a rocket back on Mars will still be very challenging.

In 2014, NASA proposed a project called the Mars Ecopoiesis Test Bed.

A medical supply that may be needed is a large amount of intravenous fluid, mostly water but containing other substances to be added directly to the blood. If this fluid could be made from water already on Mars, it would reduce the need to carry it from Earth. A prototype for this was tested on the International Space Station in 2010.

When people are inactive for a long time, they lose strength, muscle, and bone mass. Spaceflight causes astronauts to lose bone density, increasing the risk of fractures. Recent studies predict that 33% of astronauts may be at risk for osteoporosis during a Mars mission. A device similar to the Advanced Resistive Exercise Device (ARED) would be needed on the spacecraft, but it would not completely prevent bone loss.

While humans can breathe pure oxygen, breathing mixtures usually include other gases like nitrogen. One option is to use nitrogen and argon from Mars' atmosphere, but these gases are hard to separate. A Mars habitat may use air made of 40% argon, 40% nitrogen, and 20% oxygen.

To remove carbon dioxide from breathing air, reusable amine-bead scrubbers could be used. While one scrubber filters the air, the other releases the carbon dioxide into the Mars atmosphere.

If humans live on Mars, growing food there may be necessary, but this has many challenges. Making Mars soil useful for plants is difficult because the soil lacks organic material and contains about 0.5% perchlorates, a toxic salt that harms the thyroid, kidneys, and cells. The environment is also very cold and has little water, except possibly at the poles.

In 2022, NASA helped fund a multi-year grant of US$1.9 million to Arizona State University, the University of Arizona, and the Florida Institute of Technology. The project aims to use bacteria like Dehalococcoides mccartyi to reduce perchlorates in simulated Mars soil. These bacteria break down perchlorates into harmless chloride and oxygen. They also leave organic material in the soil when they die or excrete it, which could help make the soil more suitable for growing plants.

Related missions

Some Mars missions may be seen as major steps on their own, while others are smaller parts of larger plans. Examples include missions that fly by planets, explore Mars's moons, or study how the Martian environment affects spacesuit materials, like the Perseverance rover does.

Many Mars mission ideas include earlier steps to explore the moons of Mars, such as a mission to bring samples back from Phobos, one of Mars's moons. This could help prepare for future missions to Mars's surface. Lockheed Martin, in their "Red Rocks Project," suggested exploring Mars from Deimos, the other moon, using robotic tools.

Using fuel made from water found on Phobos or Deimos has also been suggested as a way to support future missions.

An uncrewed mission to bring samples back from Mars (called an MSR) has sometimes been seen as a step before sending humans to Mars. In 2008, the European Space Agency (ESA) said such a mission was essential to help connect robotic and human missions to Mars. One example is the Sample Collection for Investigation of Mars mission. Mars sample return was the top priority for NASA, as outlined in the Planetary Decadal Survey 2013–2022. However, these missions are difficult and expensive. One ESA plan required five separate uncrewed spacecraft.

Sample return missions also raise concerns about the possibility, though very small, of bringing something harmful from Mars to Earth. Guidelines for handling such samples have been created, depending on where the samples come from (e.g., asteroids, the Moon, or Mars).

At the start of the 21st century, NASA proposed four possible ways to send humans to Mars. Three of these included a Mars sample return mission as a necessary step before landing humans on Mars.

The Perseverance rover, which landed on Mars in 2021, has a tool to collect rock samples. These samples could be returned to Earth in a future mission. Perseverance was part of the Mars 2020 mission and launched on an Atlas V rocket on July 30, 2020.

Since 2004, NASA scientists have suggested exploring Mars using remote control from human astronauts in orbit. A similar idea was the "Human Exploration using Real-time Robotic Operations" mission.

To reduce delays in communication, which can take between 4 and 24 minutes, a crewed station in orbit around Mars has been proposed. This station would help control robots and Mars aircraft without long delays.