Hematopoietic stem cells in the blood can be compared to factory managers: they function most efficiently when they are at rest and able to recover. However, when stress arises—such as from infection or injury—these cells become activated and begin producing immune cells to defend the body. Over time, excessive stress wears them out, accelerates aging, and reduces their effectiveness.

Now, imagine sending these “managers” into space. In low Earth orbit, familiar gravity disappears, radiation levels increase, and biological laws shift. NASA’s Twins Study revealed that astronauts who spent nearly a year in space experienced changes in telomere length, chromosomal inversions and instability, as well as elevated levels of inflammatory cytokines.

It was previously known that microgravity affects the immune system, but how spaceflight impacts the stem cells that form this system remained unclear. Until recently, no one had studied how space conditions influence the aging and viability of human hematopoietic stem and progenitor cells (HSPCs).

To answer this question, the Sanford Stem Cell Institute at the University of California, San Diego, partnered with NASA and Space Tango to create the Integrated Space Stem Cell Orbital Research (ISSCOR) center. Across four SpaceX cargo missions to the International Space Station (ISS), scientists examined how low Earth orbit conditions affect HSPC viability before, during, and after spaceflight.

For the experiment, researchers developed bone marrow nanobioreactors that allowed real-time monitoring of HSPCs in CubeLabs, laboratories managed by artificial intelligence. This platform includes miniature 3D biosensor systems that enable human stem cells to grow in space. Using AI-powered cameras, scientists observed cell behavior in orbit.

The results showed that HSPCs returning from orbit exhibited clear signs of wear: they partially lost the ability to produce new healthy cells, became more vulnerable to DNA damage, and showed evidence of telomere shortening—the protective ends of chromosomes. All these changes point to accelerated aging. It’s as if young cells are sent into space, but they return aged and fatigued.

The director of the Sanford Stem Cell Institute and professor of medicine at UC San Diego called space “the ultimate stress test for the human body.” She emphasized that microgravity and cosmic radiation are key factors in stem cell aging.

Understanding these changes not only helps protect astronauts during long missions but also allows scientists to model aging processes and diseases like cancer here on Earth. This is especially important in the era of commercial space travel and research in low Earth orbit.

Building on the results of the NASA Twins Study and data from the Space Omics and Medical Atlas group, this new research provides a detailed picture of how space triggers molecular aging.

After just 32–45 days in space, HSPCs began to show signs of premature aging. The cells became hyperactive, quickly depleted their energy reserves, and lost the ability to rest—a crucial trait for long-term recovery. Their capacity to generate new healthy cells declined, while molecular damage accumulated: DNA breaks occurred, telomeres wore down, and stress signals intensified in the mitochondria—the cell’s powerhouses. Even typically “silent” regions of the genome became activated, disrupting cellular stability.

Together, these changes mimic aging processes on Earth and can weaken immune defenses or increase disease risk, suggesting that space not only challenges the body but can also accelerate its biological clock.

One of the main reasons for stem cell aging in space appears to be genotoxic stress caused by increased exposure to cosmic radiation. During these missions, NASA recorded radiation levels between 7.6 and 10.7 milligray—about the same dose as a routine medical scan, such as a CT or X-ray. While this may seem minor, even small doses of cosmic radiation can cause cellular problems, especially when combined with other space stressors.

Interestingly, when cells exposed to space returned to a young and healthy environment on Earth, some of the damage began to repair itself. This is a positive sign: under the right conditions or interventions, even aging cells can recover—much like astronauts returning from orbit and entering a “cellular spa.”

These findings highlight the need to protect stem cells from the harsh conditions of space. It is equally important to search for biological “signal lights”—early markers that can indicate stress-induced aging before it gets out of control.

The research team does not plan to stop here. Building on 17 missions to the ISS, they aim to launch even more studies, now involving astronauts. The goal is to track molecular changes in real time and explore pharmaceutical or genetic tools that could protect human health from the stresses of space.

The new study was published in the journal Cell Stem Cell.