A Medical College of Wisconsin study exposed rats to a mix of cosmic-ray ions and simulated weightlessness, finding fibrosis that appeared months later, even at the lower dose.

As NASA and its partners plan crewed missions back to the Moon and, eventually, on to Mars, one hazard looms larger than most: radiation. Beyond the protection of Earth’s magnetic field, astronauts are exposed to galactic cosmic rays, high-energy particles hurled across the galaxy by exploding stars. A round trip to Mars could take two to three years, and no practical shielding can block these particles completely.

Most concern about space radiation has centred on cancer. A new study from the Medical College of Wisconsin adds to growing evidence that the heart may also be at risk. Published in PLOS One, the research found that rats exposed to simulated galactic cosmic radiation developed scarring around the blood vessels of the heart that appeared months after exposure, and that even the lower of two doses tested caused it.

Recreating deep space on Earth

Galactic cosmic rays are not a single type of radiation. They are a mix of protons, helium nuclei and heavier ions such as iron, each depositing energy in tissue differently. To mimic that complexity, researchers used a NASA-developed galactic cosmic ray simulation, known as simGCRsim, which delivers a sequence of ion beams designed to approximate the deep-space environment.

The heaviest of these particles are especially damaging. As they pass through tissue, they leave dense tracks of ionization that can break DNA strands and disrupt cells in ways that are harder to repair than the damage caused by X-rays or gamma rays. Radiation doses are measured in gray, a unit of absorbed energy equal to one joule per kilogram of tissue.

The researchers, led by Marek Lenarczyk with John Baker as corresponding author, exposed 72 male rats of a strain called WAG/RijCmcr to this simulated radiation at doses of 0.75 or 1.5 gray.

Spaceflight involves more than radiation. Weightlessness causes fluids to shift toward the head and reduces the load on the heart and muscles. To simulate that, some rats underwent hindlimb unloading, a standard technique in which the animals’ hind legs are gently suspended so they do not bear weight, for 30 days. The team then followed the rats for up to 360 days, a long stretch of a rat’s life, to look for delayed effects.

Astronauts aboard the International Space Station still benefit from much of Earth’s magnetic protection, so most of what is known about the health effects of long missions comes from conditions that do not fully reproduce deep space. In 2022, NASA adopted a single career radiation limit of 600 millisieverts for all astronauts, a standard that a long Mars mission could approach.

Scarring that took months to appear

The main finding concerned perivascular fibrosis, the buildup of stiff, collagen-rich scar tissue around blood vessels in the heart. Fibrosis can stiffen heart tissue and is associated with impaired heart function in humans.

At the higher dose, perivascular fibrosis roughly doubled compared with unirradiated rats. The lower dose also caused fibrosis by the 360-day mark, with 83 percent of rats affected at 0.75 gray compared with 100 percent at 1.5 gray. That the effect emerged at the lower dose, and only after a long delay, is notable: damage that takes months to appear could easily be missed in shorter studies, and could surface in astronauts well after they return to Earth.

Simulated weightlessness alone did not cause fibrosis. But it did alter the immune landscape. Hindlimb unloading lowered the levels of 20 of 27 cytokines measured, signalling molecules that coordinate inflammation and immune responses. What that means for long-term heart health is not yet clear.

Mutations in blood cells

The team also looked for genetic changes in blood cells associated with clonal hematopoiesis of indeterminate potential, or CHIP. In CHIP, a blood stem cell acquires a mutation that gives it a growth advantage, and its descendants come to make up a growing share of a person’s blood cells. In humans, CHIP is common with aging and is linked to a higher risk of cardiovascular disease and blood cancers.

The researchers found seven CHIP-related gene variants that appeared only in irradiated rats. That raises the possibility that radiation could promote CHIP, which might in turn contribute to cardiovascular damage. But the genome sequencing involved only three rats per group, so this part of the study is best treated as preliminary.

What it means for astronauts

Animal studies of space radiation have well-known limits. Rats are not people, and the ground-based simulator compresses exposure into a much shorter time than a real mission, which may change how tissue responds. The study used only male rats. And research on astronauts themselves has produced mixed results: some analyses of Apollo astronauts suggested higher cardiovascular death rates among those who travelled beyond low Earth orbit, but the numbers are small and other studies have not confirmed the pattern.

Still, the findings add to a body of work suggesting that cardiovascular risk deserves a place in planning for long-duration missions. They also point to possible strategies, from monitoring astronauts for CHIP mutations to developing drugs that could protect the heart from radiation-induced fibrosis.

The research was funded by NASA and the Foundation for Heart Science. Unusually, the EurekAlert release carried only a headline and a link to the study, so this account is based on the published paper.

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Notes

1. Lenarczyk, M., et al. “Multiple simulated spaceflight stressors impact cardiac fibrosis, calcium dynamics, immune function, cytokines and gene variants in rat, Rattus norvegicus.” PLOS One, 2026. DOI: 10.1371/journal.pone.0357485.
2. PLOS. “Simulated space radiation may damage rat hearts long after exposure.” EurekAlert!, 30 September 2026. https://www.eurekalert.org/news-releases/1145089.

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