**Abstract**

Space radiation is an unavoidable health risk during space activities. Hematopoietic cells are sensitive to radiation including proton and oxygen radiation and so on. Understanding the mechanisms responsible for detrimental effects of space radiation is important to achieve countermeasures protecting hematopoietic stem cells (HSCs), which generates different hematopoietic populations. However, the biological effects of various sources of space radiation on HSCs are not understood well. Induction of cellular apoptosis, reactive oxygen species (ROS), and DNA damage upon space radiation is believed to be critical mediators for HSC damage. In this chapter, we will mainly discuss the biological effectiveness of proton and oxygen radiation on the numbers and function of HSCs. Space radiation-induced apoptosis, ROS, and DNA damage were examined as well, which will provide foundation to develop novel strategies protecting HSCs from space radiation.

**Keywords:** space irradiation, proton irradiation, oxygen irradiation, apoptosis, reactive oxygen species, hematopoietic stem cells, bone marrow
