The effects of spaceflight on the human body are complex and largely harmful over both short and long term.[1] Significant adverse effects of long-term weightlessness include muscle atrophy and deterioration of the skeleton (spaceflight osteopenia).[2] Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells (space anemia),[3] balance disorders, eyesight disorders and changes in the immune system.[4] Additional symptoms include fluid redistribution (causing the "moon-face" appearance typical in pictures of astronauts experiencing weightlessness),[5][6] loss of body mass, nasal congestion, sleep disturbance, and excess flatulence. A 2024 assessment noted that "well-known problems include bone loss, heightened cancer risk, vision impairment, weakened immune systems, and mental health issues... [y]et what’s going on at a molecular level hasn’t always been clear",[7] arousing concerns especially vis a vis private and commercial spaceflight now occurring without any scientific or medical research being conducted among those populations regarding effects.[8]
Overall, NASA refers to the various deleterious effects of spaceflight on the human body by the acronym RIDGE (i.e., "space radiation, isolation and confinement, distance from Earth, gravity fields, and hostile and closed environments").[3]
The engineering problems associated with leaving Earth and developing space propulsion systems have been examined for more than a century, and millions of hours of research have been spent on them. In recent years, there has been an increase in research on the issue of how humans can survive and work in space for extended and possibly indefinite periods of time. This question requires input from the physical and biological sciences and has now become the greatest challenge (other than funding) facing human space exploration. A fundamental step in overcoming this challenge is trying to understand the effects of long-term space travel on the human body.
In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars.[9][10]
On 12 April 2019, NASA reported medical results from the Astronaut Twin Study, where one astronaut twin spent a year in space on the International Space Station, while the other spent the year on Earth, which demonstrated several long-lasting changes, including those related to alterations in DNA and cognition, after the twins were compared.[11][12]
In November 2019, researchers reported that astronauts experienced serious blood flow and clot problems while on board the International Space Station, based on a six-month study of 11 healthy astronauts. The results may influence long-term spaceflight, including a mission to the planet Mars, according to the researchers.[13][14]