Humans on International Space Station: 25 years, and counting...

Eighteen thousand miles an hour

Fuelled by science and solar power

The oceans racing past

At half a thousand tonnes

Ninety minutes Moon to Sun

A bullet can’t go half this fast!

 

On February 8, 2013, these lyrics, written across continents and orbits, were sung as a duet between Earth and space. Canadian astronaut Chris Hadfield, floating in the “Cupola”―a small module to observe operations outside the International Space Station (ISS)―strummed his guitar in microgravity while Ed Robertson, frontman of the Barenaked Ladies, joined in from down below. Their song, a lyrical tribute to the ISS, became a metaphor for humanity’s most extraordinary outpost: a place where science, solitude and starlight converge.

 

This year marks the 25th anniversary of continuous human presence aboard the ISS, orbiting about 400km above Earth. And in this silver jubilee year, India is excited about sending its first astronaut to this shared home above the skies. Group Captain Shubhanshu Shukla’s ISS trip aboard the Dragon capsule on a SpaceX Falcon 9 rocket from the Kennedy Space Center in Florida comes at a time when gravity-bound eyes on Earth are increasingly seeking breakthroughs that may shape humanity’s future colonies on the Moon and Mars. This is the first time India, Poland and Hungary are part of an ISS mission. Remarkably, it signals a return to human spaceflight for all three nations, each undertaking its first government-sponsored mission in over 40 years. The mission, Axiom-4, is run by the private aerospace company Axiom Space.

 

Peggy Whitson of the United States, former NASA astronaut and director of human spaceflight at Axiom Space, is the commander of the commercial mission, while Group Captain Shukla is the pilot. The two mission specialists are Slawosz Uznanski-Wisniewski of Poland and Tibor Kapu of Hungary.

 

Our species first experienced microgravity through the legendary cosmonaut Yuri Gagarin of the Soviet Union, who became the first person to travel to space on April 12, 1961. En route to the launch pad at the Baikonur Cosmodrome in present-day Kazakhstan, Gagarin famously asked the bus driver to pull over, stepped out and urinated on the right rear tyre of the bus. He then went to space and returned without a scratch.

 

That spontaneous act became a quirky pre-flight ritual, religiously followed by Soviet and later Russian cosmonauts. Interestingly, in the 2010s, American astronauts, who make up the largest share of those who have travelled to the ISS, also adopted this tradition. After the United States retired its Space Shuttle programme in 2011, American astronauts and others relied on the Russian Soyuz spacecraft launched from Baikonur. This dependence, which lasted until SpaceX’s Crew Dragon Demo-2 mission in 2020, meant that even American male astronauts stepped out to urinate before launch. “The tradition is so well respected that women space travellers bring a bottle of urine or water to splash on the tyre rather than getting entirely out of their suits,” writes astronaut Scott Kelly in his memoir, Endurance.

 

A week before his flight, Gagarin planted a tree. Two days before, he got a haircut. On the night before the launch, he watched a film. These, too, became part of pre-flight rituals. The Americans have their own exclusive rituals. These include astronauts sitting down for a last-minute poker game, which must continue until the commander plays the worst hand. Since the dawn of the SpaceX era, new traditions have emerged. Astronauts now ride to the launch pad in SpaceX-branded Tesla Model X vehicles and place a mission sticker inside the ISS at the docking port where the Crew Dragon capsule arrives. Each crew member typically signs the sticker, marking their mission’s arrival and their participation in ISS operations.

 

Though many of these traditions may seem like superstitions, ironically observed by people of science, they are deeply rooted in the high risks and uncertainties of space travel. For astronauts and cosmonauts, pre-flight rituals serve crucial psychological and social functions. These rituals help them cope with the stress, anxiety and unpredictability of each mission. After all, the ISS is just a 420-tonne structure made predominantly of aluminium alloys. It orbits Earth approximately every 90 minutes and remains vulnerable to a host of dangers.

 

COLLABORATION AT THE CORE

The ISS began as a dream in 1984, when US president Ronald Reagan declared in his state of the union address that NASA would build a permanent space station, named Freedom, within a decade. But the road to orbit was anything but smooth, and for nearly ten years that dream hovered just beyond reach. Then came a turning point in 1993. President Bill Clinton announced a new direction: the United States would join forces with Russia, merging Freedom with Russia’s proposed Mir-2 station. Soon, space agencies from Europe, Japan and Canada joined the effort. What emerged was not just a station but the biggest international peacetime project in history.

 

Building the ISS took more than a hundred rocket launches and spacewalks. Piece by piece, in the harsh vacuum of space, humans assembled one of the greatest engineering feats of all time. This spirit of international collaboration continues with every mission to the ISS. Like past crews, the Axiom-4 team underwent months of intensive training. Inside simulators replicating Dragon’s interior, they practised launch protocols, orbital manoeuvres and ISS docking. Their training modules focused on crew coordination, systems management and real-time decision-making. Each astronaut trained on Dragon’s touchscreen interface, responded to simulated anomalies and maintained communication with SpaceX Mission Control.

 

Compared with the advanced spacecraft of SpaceX and Boeing Starliner, which are both spacious and sleek, the Soyuz of the Russian space agency Roscosmos is cramped, hot, bumpy and noisy, with manual switches, dials and analogue instruments. The landing is worse. “It’s kind of like going over Niagara Falls in a barrel but the barrel is on fire,” NASA astronaut Douglas H. Wheelock once said. Nevertheless, Soyuz has proven reliability and still remains one of the limited options to reach the ISS. In fact, despite the Russia-Ukraine war, NASA and Roscosmos signed a seat barter agreement in July 2022. This agreement allows Russian cosmonauts to fly on SpaceX’s Crew Dragon and American astronauts on Russia’s Soyuz. Extended through 2027, the deal ensures that at least one American and one Russian are always aboard the ISS, maintaining operations even if one spacecraft is grounded. The agreement highlights how the ISS remains a rare zone of cooperation even amid deep geopolitical rifts. However, cultural differences are evident even during launches. Americans embrace the drama of a detailed and publicised countdown. In contrast, Russian procedures are more reserved and less theatrical, with a subdued countdown.

 

HELLWAY TO HEAVENS

Cosmonauts and astronauts aboard Soyuz wear bulky Sokol suits. First designed in the 1970s, these suits are less comfortable than the modern sleek designs from commercial space firms like Axiom or SpaceX. The intense rumble and vibrations are felt right from the launch pad, followed by a powerful thrust that presses the crew firmly into their seats as acceleration builds. Some have described it as a “kick in the pants” moment.

 

The capsule is enclosed in a metal shell called fairing, which shields it from aerodynamic stress until it exits the atmosphere. As the rocket accelerates to three times Earth’s gravity, some astronauts report breathing difficulty and aching knees. However, astronauts, being high-performing individuals, do not always report every inconvenience or difficulty they experience. “Many suffer from space sickness during launch―much like motion sickness on Earth, but it often goes undocumented,” says Brinda Rana of the department of psychiatry at the University of California San Diego School of Medicine, who was one of the principal investigators of NASA’s landmark identical twin study involving astronauts Scott and Mark Kelly. The study followed Scott’s 340-day stay aboard the ISS while his twin, Mark, remained on Earth. “Even seasoned astronauts like Scott sometimes recorded symptoms in personal journals that never made it into official medical logs. We had to turn to his memoir to uncover some of those unreported experiences,” she says.

 

The second stage of launch, which lasts more than four minutes, is more intense. Explosive charges jettison the fairing in two pieces. The third stage takes over after 4.8 minutes, followed by a four-minute burn. There is a loud bang and a jolt, and then weightlessness sets in. After enduring intense g-forces, a sudden silence and stillness envelops them. “This is the moment we call MECO, pronounced ‘mee-ko’, which stands for main engine cutoff. It’s always a shock. The spacecraft is now in orbit around the Earth,” writes Scott Kelly. At this stage, the debris that has been hiding in the spacecraft becomes visible. NASA calls these fragments “foreign object debris”.

 

Space-based films often depict spacecraft docking as a swift process, completed within minutes. In reality, it can take anywhere from three to thirty hours, depending on the launch vehicle and the spacecraft’s point of origin. Soyuz launches from Baikonur closely match the ISS’s 51.6 degrees orbital inclination, reducing the need for major course corrections and enabling fast-track docking in three to six hours. In contrast, Crew Dragon launches from Florida require more orbital adjustments, often taking 16 to 30 hours for docking.

 

HOUSE OF WONDERS

Though the ISS is a single facility, it has two main sections: the Russian Orbital Segment (ROS) and the US Orbital Segment (USOS). The ROS includes modules for propulsion, docking and life support, such as Zvezda and Zarya. The USOS houses laboratories like Destiny (US), Columbus (Europe) and Kibo (Japan), and supports research, habitation and power systems. Russian spacecraft dock to the Russian segment while US and partner spacecraft dock to the US segment.

 

Once the hatches between the spacecraft and the station open, the first thing astronauts often notice is a strong metallic scent, sometimes described as the “smell of space”. This has been likened to gunpowder, welding fumes or even burnt steak. The new crew members are then welcomed by those already aboard the station.

 

New crew members are guided by existing ISS residents on how things operate aboard the station. Even veteran astronauts returning to the ISS undergo this refresher. The orientation usually starts with the “waste and hygiene compartment”, or the space toilet, before moving on to other essential systems and equipment. To urinate in microgravity, astronauts use a personal funnel attached to a hose, which suctions urine into a collection system where it is recycled into drinking water. For defecation, they sit on a small toilet seat with a sealed bag that uses airflow to direct waste into a storage container, preventing it from floating around the cabin. The collected waste is either stored for disposal in resupply spacecraft or processed for recycling. Water is precious on the ISS, so the crew recycles nearly everything, including breath, sweat and urine. Astronauts often joke about the process, remarking that “yesterday’s coffee is tomorrow’s coffee”.

 

Cosmonauts live and work in the Russian segment of the ISS while astronauts from the US and other countries primarily operate in the American segment. The station’s living and working space includes seven sleeping quarters, two bathrooms and a gym. In the US segment, each astronaut has a private crew quarters (CQ) for sleeping, relaxing and personal time. They sleep in secured sleeping bags attached to the walls using velcro or bungee cords to remain properly oriented in microgravity. The CQ also serves as a space for changing clothes, using laptops, private communication and basic hygiene. By contrast, the Russian segment uses a tent-like sleeping system, where cosmonauts sleep in secured sleeping bags.

 

Astronauts on the ISS spend most of their time conducting scientific research, performing maintenance and maintaining their health through mandatory exercise and other activities such as sleep. As the ISS orbits Earth once every 90 minutes, the crew experiences 16 sunrises and sunsets each day, rendering Earth’s usual rhythm of day and night meaningless in space. To maintain structure, the station’s clocks are set to Coordinated Universal Time (UTC), and a scheduling programme divides each astronaut’s day into precisely timed segments for equipment repairs, scientific experiments, meals, exercise and sleep. If an astronaut takes longer than planned to complete a task―such as an experiment, repair or spacewalk preparation―time must be reallocated from other activities such as meals, exercise or sleep. Although this tightly structured lifestyle can be stressful, it also has scientifically proven benefits. Rana notes that Scott, who spent nearly a year in space, maintained more stable physiological markers such as lipid levels and cardiovascular indicators.

 

In contrast to the early days of spaceflight, when piloting expertise was the top requirement, ISS astronauts today are chosen for their adaptability. They must be capable of carrying out complex scientific tasks, working well in teams, adjusting quickly to changing conditions, managing risk and maintaining time discipline.

 

“The ISS functions as an enormous laboratory,” says Rana. “Hundreds of experiments are under way at any given moment, covering human biology, plant growth, materials science and engineering. While the astronauts participate in studies related to their own health and adaptation to space, they also conduct research that will shape the future of human spaceflight and benefit life on Earth.” NASA organises ISS research into two broad categories: one aims to improve life on Earth by developing new medicines, cleaner energy and advanced materials; the other addresses challenges for future space missions, such as testing life support systems, studying food production in space, solving technical issues and understanding the human body’s response to space. The Axiom-4 mission includes around 60 scientific studies and activities from 31 countries, including seven from India’s ISRO focused on advancing microgravity research.

 

Some experiments cannot be carried out inside the space station and require spacewalks, formally known as extravehicular activities (EVAs). These are among the most challenging tasks an astronaut undertakes. Besides supporting scientific studies, spacewalks are essential for maintaining the ISS and installing new hardware.

 

Preparing for a spacewalk is a multi-hour operation that involves at least three crew members on board and support from dozens of personnel on the ground. Conditions during a spacewalk―extreme temperatures, radiation and risk of equipment malfunction―mean that even a minor error can be life-threatening. One of the most serious ISS incidents occurred in July 2013, when Italian astronaut Luca Parmitano’s helmet began to fill with 1.5 litres of water from a suit coolant leak during a spacewalk. The water covered his eyes, nose and ears, making it difficult to see, hear or breathe. Despite poor communication, he managed to alert fellow astronaut Chris Cassidy, who guided him back to the station. Parmitano later described the near-death experience using a playful analogy: “Like a goldfish inside a fishbowl―from the goldfish’s point of view.”

 

Aptly, the “Cupola” in the station’s Tranquillity module offers astronauts a stunning, fishbowl-like view of Earth. Most ISS modules are windowless, lit with fluorescent lights and lined with sterile white walls, which can create a cold and impersonal atmosphere. The Cupola, therefore, offers more than just a window; it serves as a psychological refuge, helping astronauts cope with isolation and maintain a sense of connection to Earth. This bond with the planet is so important that astronauts often listen to recordings of natural sounds such as birdsong, wind and rain. Many astronauts describe the ISS as a place with its own character, not just a machine controlled remotely from Earth. “Each module has its own story and its own quirks,” writes Scott.

 

The ISS usually houses a crew of seven, although this number can temporarily increase during crew rotations or special missions. The amount of carbon dioxide produced on board is directly related to crew size. Astronauts often have a difficult relationship with carbon dioxide, as elevated levels have been linked to headaches, eye irritation and impaired concentration. While carbon dioxide-related problems tend to be short-term, astronauts face long-term physiological and psychological challenges in microgravity from the very first day in space. “It is important to remember that life on Earth evolved under the constant pull of gravity. Our entire physiology is shaped by it,” says Arvind Ramanathan, principal investigator of the ‘Effect of Metabolic Supplements on Muscle Regeneration under Microgravity’ study, which will be conducted by the Axiom-4 team for ISRO. “So, when the body is suddenly in a gravity-free environment, it becomes disoriented. The result is a kind of physiological breakdown because the body does not know how to respond.”

 

All ISS crew face a range of physical challenges, especially muscle atrophy and loss of bone density. Without gravity, the body no longer needs to support its own weight, prompting it to reduce muscle and bone tissue. To counter this, astronauts must adhere to a strict exercise routine―typically two hours per day, six days a week―consisting of treadmill running, cycling and resistance training, combined with a nutrition plan developed by experts.

 

One lesser-known effect of microgravity is its impact on the body’s ability to regenerate muscle tissue. “We want to understand why muscle regeneration is compromised in space,” says Ramanathan. “Muscle communicates with other organs and plays a key role in overall health. So, when its balance is disrupted, it triggers a chain reaction that can affect multiple organs, even changing how the heart functions.”

 

Vision problems and increased intracranial pressure are also common among ISS crew, partly due to a phenomenon called “fluid shift”. “One of the first things that happens in microgravity is that bodily fluids migrate upwards,” says Rana. “On Earth, gravity pulls fluids downwards and our bodies are adapted to that. In space, without that pull, fluids collect in the upper body. This causes puffy faces, headaches and, in some cases, visual changes.” Interestingly, studies suggest female astronauts are less likely than their male counterparts to experience these vision issues. Some have even proposed that, unless science uncovers the cause of this difference, an all-female crew might be better suited for long-duration missions such as the Mars one.

 

Researchers observe that many of the physiological changes experienced by astronauts resemble those seen in ageing or bedridden patients on Earth. “The physical changes astronauts undergo are often forms of accelerated ageing,” says Rana. Studying these changes in space helps us understand similar processes on Earth. “For example, understanding blood distribution in microgravity could improve treatment for conditions like orthostatic hypotension or oedema. Likewise, bone loss research aboard the ISS supports the development of therapies for osteoporosis.”

 

Although many effects of space travel were already known, NASA’s twins study broke new ground by combining molecular and cellular analysis with physical observation. “We tracked proteins, RNA and DNA to examine gene expression and other biological markers,” says Rana. (Gene expression is the process cells use to convert the instructions in the DNA into a functional product, such as protein.) “One of the most striking results was a notable change in Scott’s gene expression compared to his twin, Mark. But what was even more remarkable was the resilience of the human body. Most of Scott’s changes returned to baseline within six months of his return, which shows how incredibly adaptable we are under extreme conditions.”

 

Beyond physical changes, astronauts must also contend with disrupted sleep cycles, higher radiation exposure and the psychological strain of isolation. Rana says unexpected mission extensions, such as the recent one involving Sunita Williams and Butch Wilmore, add further psychological pressure. “We are now seeing cognitive and neurological changes that may be linked to gene expression shifts during long missions. But studying these effects is difficult―most standard cognitive tests are too easy for astronauts, even when they are under stress. This is why NASA is now creating more advanced, specialised assessments.”

 

Ramanathan, meanwhile, describes astronauts as “the alpha of our species”. “These are some of the toughest and most resilient people alive. Yet, even they are challenged by space. If our very best are pushed to their limits, it tells us that we must understand the root causes. Only then can we prepare for a future where many more people venture into space.”

 

BUMBLE, HONEY AND QUEEN

 

Astrobees are robotic assistants in the ISS that help astronauts with their chores. These free-floating, cube-shaped robots use electric fans powered by rechargeable batteries to navigate in microgravity. Equipped with a variety of attachments, these robots help in completing tasks such as taking inventory, documenting experiments conducted by astronauts with their built-in cameras or moving cargo throughout the station. The trio―named Bumble, Honey and Queen―has been operational since 2019 and can be remotely controlled by astronauts, flight controllers or researchers on Earth. When not in use, they magnetically dock to a charging station.

 

HOUSTON, WE NEED LIGHTSABERS

 

Star Wars Day was celebrated on the ISS, too. For the uninitiated, May 4 is celebrated as Star Wars Day by fans of the iconic sci-fi franchise created by filmmaker George Lucas. The date comes from the pun “May the Fourth be with you,” a playful twist on the legendary phrase “May the Force be with you” from the Star Wars franchise. On Star Wars Day this year, astronaut Nichole Ayers distributed Star Wars-themed socks to crew members aboard the ISS and the crew then posed for a photo wearing them with Earth as a stunning backdrop. NASA astronaut Anne McClain shared the image on social media with the caption: “May the 4th be with you!”

 

AXIOM MISSION 4 CREW

 

Peggy Whitson

commander

 

America’s most experienced astronaut with 675 days in space, more than any other American and more than any woman in the world. Called “American Space Ninja” by fellow astronauts, she has held posts such as NASA’s chief of Astronaut Office, chair of Astronaut Selection Board and chief of Operations Branch. She is the first woman commander of the ISS, the only woman to command the ISS more than once and the first woman, non-military chief of the Astronaut Office. She holds the record for most spacewalks by a woman (10). She has degrees in biology and chemistry, and a doctoral degree in biochemistry. She grew up on a farm outside Iowa and decided to become an astronaut after watching the first moon landing (1969) on TV as a child.

 

Tibor Kapu

mission specialist

 

A mechanical engineer from Hungary. He also has a master’s degree, specialising in polymer technology. He has worked in pharmaceuticals and logistics, and on hybrid car battery development. From 2022, Kapu has focused on space radiation protection at an aerospace technology company. In 2023, he emerged as one of four Hungarians selected from a pool of 247 candidates for the astronaut programme.

 

Shubhanshu Shukla

pilot

 

Born on October 10, 1985, in Lucknow, Shukla was commissioned into the Indian Air Force in June 2006. He has 2,000 hours of flight experience across aircraft, including the Su-30MKI, MiG-21, MiG-29, Jaguar, Hawk, Dornier and An-32. He trained for a year at the Yuri Gagarin Cosmonaut Training Center in Star City, Moscow, ahead of the Gaganyaan mission, scheduled for launch in 2025.

 

Sławosz Uznanski-Wisniewski
mission specialist

 

Polish scientist and engineer. A member of the European Space Agency’s Astronaut Reserve Class of 2022, he emerged from a pool of more than 22,500 candidates. He has an MSc with honours, another MSc, an engineering diploma, and a PhD, focusing on radiation-tolerant designs for space applications. He was a reliability expert and project lead at the European Organisation for Nuclear Research (CERN). From 2018 to 2020, he was engineer-in-charge of the Large Hadron Collider.

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