Zero gravity.
Astronauts experience nausea, loss of proprioception, change in vision, possible retinal edema, abnormalities in cognition, circadian rhythms, sleep deprivation, nasal congestion and facial edema.
Threats in the space environment very according to the duration of the flight and range from physiological and adaptive alterations of the human body to the psychological challenges of isolation and distance from earth.
Astronauts experience anxiety, depression, and insomnia.
Astronauts lose 1-2% of bone mass per month while in space.
In space bone osteoclast activity is accelerated, calcium levels rise with increased risk of kidney stones.
When astronauts return to earth, bone calcification resumes, but takes 2-3 years to return to normal.
Lower extremity muscle atrophy occurs in astronauts during space travel, as does the heart muscle,
Emergency situations in spaceflight include loss of integrity of the pressurization system which can result in severe hypoxemia or decompression illnesses.
Space travel includes a suborbital space flight which by definition reaches an altitude of no more than 100 km above mean sea level, and they are typically of short duration, whereas orbital spaceflights can last from days to many months
Missions to the moon will typically be of extended duration, and missions to Mars are likely require require several years.
Flights that are suborbital or in low Earth orbit the Earth’s magnetic field and atmosphere provides shielding from space radiation.
Missions beyond low Earth orbit such as flights to the moon or Mars are challenges because of factors such as substantial radiation exposure from Galactic cosmic radiation and possible coronal mass ejections from our sun, and the risks increase with prolonged exposure.