Short answers, pulled from the story.
Spaceflight causes muscle atrophy, bone loss of approximately 1.5% per month, fluid redistribution that produces facial puffiness, vision disorders, disrupted sleep, weakened immune function, and psychological stress. NASA groups these risks under the acronym RIDGE, covering radiation, isolation, distance from Earth, gravity fields, and hostile environments.
Astronauts lose approximately 1.5% of bone tissue per month in microgravity, concentrated in the lower vertebrae, hip, and femur. On Earth, cortical bone loss occurs at around 3% per decade. After a 3-4 month spaceflight, it takes roughly 2-3 years to regain the lost bone density.
On the 12th of April 2019, NASA reported results from the Astronaut Twin Study, in which one twin spent a year aboard the International Space Station while the other remained on Earth. The study documented several lasting changes between the twins, including differences in DNA and cognition.
The three crewmembers of the Soyuz 11 spacecraft, Vladislav Volkov, Georgi Dobrovolski, and Viktor Patsayev, are the only people known to have died from vacuum exposure in space. On the 30th of June 1971, a pressure-equalisation valve in their descent module opened unexpectedly at 168 km altitude, causing rapid depressurisation and the death of all three.
Space radiation damages lymphocytes, weakening the immune system and allowing dormant viruses to reactivate. It has been linked to a higher incidence of cataracts, potential acceleration of Alzheimer's disease onset, and chromosomal aberrations in cells central to the immune system. Beyond Earth's magnetosphere, a single powerful solar flare could cause acute radiation sickness or death.
A round trip to Mars with current technology is estimated to require at least 18 months in transit alone. Risks over that duration include significant bone and muscle loss, radiation exposure far beyond what ISS crews receive, potential gastrointestinal tissue damage, reactivation of latent viruses, and cognitive changes documented in long-duration spaceflight research.