It looks like NASA’s plan to land humans on Mars by around 2030 could be delayed for at least 20 years, and maybe indefinitely. It seems that a problem NASA has known about for at least 10 years—radiation— is more serious than the space agency thought, even though they were warned about it at the time by Frank Cucinotta, the radiation health officer at the Johnson Space Center.
Space experts agree that during the four years or more in space astronauts would experience on the trip to and from Mars, they would be exposed to a variety of radiation: the sun’s radiation, especially from flares, which could kill astronauts if they were large enough; and cosmic rays, a more common and insidious form of radiation that can increase a person’s chances of contracting cancer and tumors.
Galactic cosmic rays, especially those such as iron atoms shorn of electrons and travelling extremely fast enough, and with lots of energy (millions of MeV), can damage and mutate strands of DNA inside astronaut’s bodies if a strand is struck by them. Estimates are that after a single day in space, far from a planet or other object large enough to attract the rays and protect a space crew, every single cell in every crewman would have been struck by a high-energy proton (hundreds of Mev). And over the course of a year, each of their cells would have been hit by at least one damaging iron nuclei.
Translating those odds into risks of long-term damage to the crew, which is not an exact science, reveals that astronauts going to Mars and back would absorb one sievert of radiation. (This compares to the approximately 3 millisieverts humans get from background radiation on Earth every year.) If you exposed 100 people to one sievert of radiation, 61 would be expected to get cancer. That’s 20 more than the 41 who would get cancer under normal circumstances, given none of the extra exposure of travelling in space.
Cucinotta, now a radiation expert at the University of Nevada, says that a trip to Mars and back would shave 15 to 24 years off an astronaut’s life. He’s also done experiments on mice that show cosmic radiation can also lead to brain damage and cognitive losses (think onset dementia) without any signs of cancer. This is unacceptable to NASA, given their guidelines prohibiting missions that increase an astronaut’s chances of getting cancer by more than 3%.
There are potential solutions to this problem. The trips could be planned to coincide with the peak of the sun’s natural 11-year solar cycle. At that time, strong solar winds would blow against the incoming galactic rays and reduce astronauts’ exposure to them. Unfortunately, the peak of the sun’s solar cycle is also when solar flares are most common, and one of those could kill the entire crew in seconds.
Another mitigation scheme is to shield the crew from cosmic rays. Hydrogen is a good material for this, and it is quiet stable and safe when used in the form of water. Researchers estimate that cosmic rays would be reduced by half if forced to travel through two meters of water. A cubic meter of water weighs more than 2,200 lb., which is light compared to sand or metal, but covering a spaceship in two meters of water would add quiet a weight penalty. It would be a formidable engineering challenge to build and launch such a spaceship. Perhaps engineers can come up with a hydrogen-rich polymer that will serve as an effective shield against cosmic rays and not weigh so much or be too bulky. Perhaps.
Travelling to Mars faster—i.e., making the trip in 100 days or less, rather than a year—would also limit the crew’s exposure and overall danger of the trip. But developing the fuel and engines is another formidable challenge, and one that can’t be solved in a year (regardless of funding).
There are also other medical problems that are caused by space travel, and one of the major culprits, right behind radiation, is zero gravity. Bones and muscles weaken, and while the muscles can be exercised and get back close to normal once in a stronger gravity field (i.e., on Earth), the bones might not ever fully recover. Cerebral-spinal fluid does not circulate when a person is in zero g, and this can damage the optic nerve and lead to blindness. The static cerebral spinal fluid is also thought to contribute to dementia. Spinning a spaceship to create gravity has never been done and artificial gravity has not been invented.