Sunita Williams and Barry Wilmore were meant to spend eight days in space in what was a routine trip conducting scientific experiments. They splashed down off the coast of Tallahassee a full 286 days later; not a record duration by itself. Valeri Polyakov spent 437 days in the Mir station in 1994; Sergei Avdeyev spent 379 days in 1998-1999. The honour of a record went to Williams for the most time spent space walking; 62 hours and 6 minutes, surpassing Peggy Whiston’s previous record. Previous extended stays followed appropriate preparatory training for the astronauts; in this case, neither anticipated or catered for. Yet at no stage in those 286 days did the media forget them; the primary reason being the uncertainty involved. They were stranded there because of repeated technical failures, during the various attempts to rescue them, increasing the element of uncertainty even though food, water, oxygen etc. were regularly serviced by resupply missions.
Being stranded in space is no longer confined to fictional books and films. Gravitational forces are essential for the normal functioning of our locomotor system. Muscles and bones function best when subjected to stress in the form of work, such as normal activity and exercise. Without gravity, there is less demand and reduced mechanical loading leading to loss of bone density and muscle atrophy. On earth, this occurs when there is prolonged immobilisation of a patient for whatever reason. The turnover and renewal of bone cells is affected, with increased excretion of calcium and risk of kidney stones. The weakening often persists after the astronauts return to earth. They lose around 1-1.5% bone density for every month spent in space. Research in 2022 documented bone loss in 17 ISS astronauts in missions averaging about 5-1/2 months. A year after returning to Earth, there was an average 2.1% reduced bone mineral density of the tibia, and 1.3% reduced bone strength. 9 did not recover bone density.
Prolonged exposure to microgravity conditions also affects vestibular function - the inner ear's ability to sense movement and orientation. This is dependent on peripheral sensory feedback which is related to gravity; and without which the body is unable to interpret position, balance and coordination. This explains why astronauts are able to maintain weird positions for prolonged periods; which would otherwise have been impossible on earth.
The heart muscle weakens and coupled with the loss of gravity, the normal distribution of blood in the body, and blood pressure control gets disturbed. Without gravity, bodily fluids shift upward, leading to facial swelling and increased intracranial pressure. More blood reaches the head along with accumulation of fluid. Overall blood volume is reduced and possibilities of clot formation increase. The increased volume in the head gives the feeling of a permanent cold and headaches. Of the 24 astronauts who travelled aboard the ISS for up to 26 weeks, all but two reported headaches. A study in 2023 found that astronauts who travelled on the ISS or NASA space shuttle on missions lasting at least six months experienced expansion of the cerebral ventricles.
Many physiological systems gradually return to normal. But some issues persist, being directly proportionate to the time spent in space. For short-duration missions of a few days in low-Earth orbit, about 95% of the biological damage appears to be reversed upon return.
One example is Spaceflight-Associated Neuro-Ocular Syndrome (SANS), linked to visual impairment due to microgravity-induced fluid shifts and changes in intracranial pressure distorting the shape of the eyes. As a result, many astronauts’ experience difficulty in walking, dizziness and visual disturbances. Research suggests that dysfunction in sub-cellular structures called mitochondria plays a role in SANS. Some astronauts experience lasting impairment requiring corrective lenses. Countermeasures based on mitochondrial research to mitigate space-induced damage are being actively pursued. Research published in 2024 detailed changes in the brain, heart, muscles, kidneys, skin, immune regulation, stress levels and a breakdown in the activity of mitochondria among crew members who participated in SpaceX's three-day Inspiration4 mission in 2021; the first all-civilian team to orbit Earth.
Relatively little is known about how lung function is affected. While it is known that space radiation elevates cancer risk, accelerates aging and induces cellular damage, the precise biological mechanisms remain elusive. Research has shown that mitochondria play a central role in spaceflight-induced health effects, but the precise mechanism is still unknown. Scientists also lack a comprehensive understanding of how microgravity, radiation exposure and isolation impact cognitive function, mental health and neuroplasticity - the brain's ability to change and adapt - over long durations.
Visions of space colonisation are no longer confined to fiction; but the effects on human fertility, reproduction and embryonic development remain unknown. Research has been largely confined to limited studies mainly on mice. Astronauts are required to live in confined and isolated environments with limited social interaction and exposure to natural stimuli. This can lead to psychological stress, sleep disturbances, cognitive performance declines and mood disorders. Interpersonal conflicts could get aggravated further impacting mental well-being and mission performance.
Questions remain about prolonged exposure to radiation on longer missions. Without effective countermeasures, recovery could be problematic. Exposure to high-energy radiation can lead to DNA damage, increased cancer risk, neurodegenerative effects, cardiovascular issues and immune system dysregulation. Earth's magnetosphere - the region of space dominated by the planetary magnetic field - provides some protection for astronauts in low-Earth orbits. But astronauts traveling beyond that would experience much higher radiation doses. "It's going to be space radiation exposure that's going to be the big limiting factor for how well astronauts do or how long they're going to be able to actually be in space," said Susan Bailey, a radiation biologist from Colorado State University.
We are a long way from home in resolving the health hazards of space travel.
(The author is the Past IMA Goa State President, founder member and Past President of VHAG, and a past member of the Central Executive
Committee of VHAI.)