Until humanity builds a laboratory in open space, researchers reproduce space conditions on Earth. Staff at the Medical College of Wisconsin (USA) conducted such an experiment on rats to understand what a long expedition to Mars would mean for future travelers. The results were published in the journal PLoS ONE.
A method developed by NASA was used as a model of weightlessness: the animals' hind legs were raised at a 30-degree angle using a special harness attached to the tail. This continued for 30 days. Middle-aged rats were chosen - roughly the same age as most professional astronauts. Radiation was recreated at an accelerator at Brookhaven National Laboratory: the facility simulated cosmic rays, including protons and heavy ions of silicon and iron capable of penetrating the ship's hull.
First the animals were kept in a "suspended" state for five days, then for several minutes they were irradiated at a dose that was meant to correspond to many months of exposure. After that the rats remained with their hind legs raised for another 25 days, and then lived normal lives under observation for about a year - comparable to about 25 years of human life.
The main surprise was changes in the heart. At a dose of 1.5 gray (a unit of absorbed radiation), the rats developed perivascular fibrosis - scar tissue that wraps around the coronary vessels supplying the heart muscle. By day 270 after irradiation, the scars had doubled in size and compressed the vessels so much that the risk of heart attack rose sharply. The simulation of weightlessness, strangely, had almost no effect on this process - everything was determined by radiation.
At a lower dose, close to what an astronaut would receive over a three-year mission to Mars, the scars did not appear immediately. However, a year after irradiation, fibrosis was found in 83% of the rats. This means the consequences may manifest decades later.
Immunity was also affected. Months after exposure, only a third of the natural killers remained in the rats' spleens - cells that destroy virus-infected cells and, especially importantly, cancer cells. Weightlessness by itself did not hinder recovery, but in combination with irradiation the cell composition never returned to normal: cosmic rays apparently suppressed the bone marrow's ability to respond to stress.
Finally, analysis of the genome of white blood cells revealed persistent mutations in a number of genes. Some of them are associated with clonal hematopoiesis of indeterminate potential (CHIP) - a condition in which one mutated blood cell begins to displace its neighbors. On Earth this happens with age, but it carries an increased risk of leukemia and quadruples the likelihood of heart attacks and strokes due to inflammation of artery walls. In the irradiated rats these changes arose much earlier than is usual for their species.
The authors acknowledge the limitations of the work. Radiation was given as a single dose rather than stretched over months or years, so it is unknown whether cellular repair mechanisms would have had time to cope with the damage. In addition, only males took part in the experiment - a female body might have reacted differently. So before the first flight to Mars, scientists still have many questions to answer.