The spaceflight environment presents many challenges to the human body. Bone loss in astronauts is a well known consequence of reduced loading in the weightless environment of low-earth orbit. Recent studies have also indicated that spaceflight relevant types of radiation (at relatively high doses) have deleterious effects on trabecular bone. Both of these represent potential skeletal challenges during long-duration spaceflight. The objective of these studies is to examine the response of bone to models of the spaceflight environment.

Astronauts will likely absorb doses of protons and heavy ions during lengthy missions outside the Earth’s magnetosphere. Following exposure to a range of doses of proton radiation, mouse bones displayed deterioration after receiving doses of one and two Gray (Gy), but not at one-half Gy. A one Gy absorbed dose is a potential scenario during a solar particle event. Mixed radiation source exposure (e.g. neutrons, protons, iron ions) characterize galactic cosmic rays. Trabecular bone loss in mice occurred after receiving a dose modeling what an astronaut may absorb during a mission to Mars. Additionally, the same dose led to cortical bone loss, which has not previously been demonstrated at spaceflight-relevant doses. When mice were irradiated with protons (one Gy) followed by unloading using a ground-based model of microgravity, bone loss was again observed. The unloading-induced bone loss in this study was much more severe than that which occurs during long-duration spaceflight. Because of the severity of this bone loss, it is not clear if radiation leads to additional bone loss when combined with unloading. However, there are indications of an additive negative effect of radiation and unloading.

These studies demonstrate that spaceflight-relevant doses and types of radiation have deleterious effects on the skeletal system. Further study is necessary to understand the mechanisms behind radiation-induced bone loss. Additionally, further examination of the combined effect of radiation and unloading is necessary in order to understand spaceflight-induced bone loss and effectively develop countermeasures.