Killing radiation in the cosmos, potential loss of vision and atrophic bones are just some of the challenges that scientists have to overcome before any future astronaut can walk on Mars, experts and NASA officials said on Tuesday.
The US space agency believes it can put people on the red planet in 25 years, but technological and medical obstacles are immense.
"The costs of solving these things mean that current budgets or slightly expanded budgets will take about 25 years to resolve them," said former NASA astronaut Tom Jones, who flew four space shuttle missions before retiring in 2001.
"We have to start now with some key technologies," he told reporters in Washington.
At an average distance of about 225 million kilometers, Mars raises scientific issues with an order of magnitude larger than anything encountered by Apollo's monthly missions.
With the help of today's rocket technology, it would take an astronaut of up to nine months to reach Mars – the floating physical charge that would have a long zero gravity
For example, scientists believe that prolonged weight can cause irreversible changes in blood vessels in the retina, leading to vision degradation.
And after a while in zero G, the skeleton begins to flow through the calcium and bone mass.
With the gravity of only a third of Earth, scientists do not yet know the effects of a supposed one-year mission on the surface of Mars.
One way to reduce wear on the human body is to drastically reduce travel time on Mars.
Jones has demanded nuclear propulsion systems that would have the added advantage of generating electricity during flights.
"If we start now, in 25 years, we could have these technologies available to help us and protect us from these long transit times," he said.
Under current conditions, a trip to Mars will take so long that any astronaut would receive the same amount of radiation as would normally have been considered safe throughout an entire career.
"We do not have the solution yet in terms of shielding, in terms of protecting you from the cosmic rays and solar rays that you experience during this transit time," Jones said.
Aerospace experts have identified several technologies that require rapid development, including spacecraft that can survive the rugged entry to Mars and land slowly, as well as the ability to lift people off the surface and return to Earth.
NASA currently has a new robotic lander, called InSight, approaching Mars due to land on November 26 after taking off California on May 5th.
The $ 993 million project aims to extend human knowledge about Martian indoor conditions, to inform efforts to send explorers there and to reveal how rocky planets like the Earth were formed billions of years ago.
Jim Garvin, chief scientist at NASA's Goddard Space Flight Center, said InSight would complement the "Critical Unknowns" and help build a key understanding of Mars.
In 2020, another mission will make NASA send a Martian navigator, which aims to determine the habitats of the Martian environment, search for ancient signs of life, and assess natural resources and dangers for future human explorers.
In addition, private firms such as SpaceX and a number of other nations are building technologies that could be used in Mars' future missions.
Some experts believe that exploring the Moon as a key to a future Mars mission, astronauts could learn about water extraction or use of technology and apply them to Mars's future missions.