Traveling through deep space raises challenges to biological systems that have not been fully appreciated or addressed. In addition to the lack of gravity, the space environment includes exposure to charged remnants of supernova explosions beyond our solar system that travels with enormous velocities and energies, called HZE (high atomic number Z and energy E) particles and have the potential to disrupt chemical bonds within the human body though ionization. As a process, the collision of charged particles with matter is not new to physicists and biologists on Earth, and we have extensive data on low-linear energy transfer (LET) ionizing radiation from both accidental and deliberate exposures, dating back to the discovery of radioactive isotopes by Madame Curie. One of the primary morbidities associated with radiation exposure is the challenge to the hematopoietic system. The purpose of the current review is to discuss some of the basic tenants of hierarchical tissue systems by elaborating the effects of radiation damage to the hematopoietic stem cell and how terrestrial radiation and space radiation differ.The last few decades of research in the field of space radiation, which consists of high-LET ions of 4He, 12C, 16O, 28Si, 48Ti, and 56Fe, and low-LET protons, have shown that there is a significantly more deleterious impact on the hematopoietic system by the high-LET ions compared to protons, X-rays, and γ-rays. Ground-based high-LET radiation experiments have shown not only in vitro and in vivo adverse effects on hematopoietic stem cells, but also that human leukemia can be induced in humanized mouse models.High-LET space radiation is more lethal to hematopoietic stem cells compared to low-LET radiation, but further research is required in order to understand the impact of high-LET radiation on hematopoietic malignancies. Most of the ground-based studies, because of technical difficulties and cost issues, have been carried out at high dose rates with only one ion species at a time. What remains to be clearly described, however, is the potential damage to the hematopoietic system from exposure to the more complex types of radiation at low dose rates that will occur during space travel and how space agencies can sufficiently protect our astronauts
