All you need to know about the International Space Station's 25 years in orbit -BBC

The International Space Station (ISS) is humanity's most expensive object and has been in orbit for 25 years. Read its fascinating history, told in 25 numbers.

Orbiting some 400km (250 miles) above the Earth, the International Space Station (ISS) represents one of mankind's most ambitious engineering projects. Since the first Expedition 1 mission, more than 280 astronauts and cosmonauts have visited the ISS, and it has now been continuously occupied for 25 years. If you were born after 2 November 2000, for your entire life, there has always been someone living in space.

The ISS demonstrates what can be achieved through international cooperation and proves what humans working together can do when they put their minds to it. Not that it's always been easy.

From power supply to habitable volume, didgeridoos to toilets, here is our take on 25 years of the ISS in 25 numbers:

27: Age of the oldest ISS module (years)

The first stage of the ISS, Zarya, was launched on a Proton rocket from the Bakinonur Cosmodrome in Kazakhstan on a bleak November morning in 1998. For those of us watching from the snow-covered mound of a disused nuclear missile silo (or at least we were told it was disused), it was an inauspicious affair. At the time, most of the stories written about the ISS were far from positive. The project – a mash-up between an abandoned US space station programme of the 1980s called Freedom, and a successor to Russia's Mir space station – had been mired in delays and cost overruns. There was no shortage of political opposition to abandoning the programme completely.

3: Members Expedition 1

When Bill Shepherd, Sergei Krikalev and Yuri Gidzenko switched on the lights in November 2000, the ISS consisted of just three modules bolted together. Zarya, Zvezda and Unity continue to form the heart of the station, providing power, life support and connections to all the other sections that have been added since. Despite their cramped quarters, the astronauts spent almost five months on the embryonic station, conducting some 22 scientific investigations, seven spacewalks and welcoming two visiting Space Shuttle crews. Today, there are typically seven astronauts on the ISS at any one time – this website tracks who is in space.

42: Assembly flights

Depending on how you count them, there are between 16 and 20 pressurised modules on the ISS, eight major solar arrays and many more interlocking components. In short, some assembly was required. Fortunately, the US had the Space Shuttle which not only delivered various sections of the ISS, but its robotic arm could also be used to help slot them into place. Considering parts for the station were built in Russia, North America, Europe and Japan, they have all fitted together with minimal drama.

388: Habitable volume (cubic metres)

The volume available to astronauts is equivalent to a six-bedroom house, or 13,705 cubic feet if you're old school. But in microgravity, you can use all the available space – floor, walls and ceiling are all arbitrary. In practice that means that almost every interior surface is covered in equipment, experiments and storage. To avoid astronauts getting completely disorientated – and to match the training modules on the ground – the ISS is designed with floors and ceilings. But crews have been known to mix things up and have days where everything is upside down.

2: Exercise (hours per day)

If long duration space missions have taught us anything, it's that space is bad for you. Prolonged exposure to weightlessness leads to loss of muscle mass, bone density and deteriorating eyesight. So that they can return to Earth without serious health problems, astronauts need to work out for two hours a day. The gym on the ISS includes a treadmill, cycle and resistance machines. Astronauts are sufficiently motivated, disciplined and competitive to take exercise seriously. In 2016, British astronaut Tim Peake even set a new record for running a marathon in space, completing the 42km (26.1 miles) in three hours and 35 minutes.

98: Percentage of breath, sweat and urine recycled

Today's urine is tomorrow's coffee, or so the saying in space goes. The water recovery system on the ISS is a marvel of engineering that recovers almost all the water astronauts expel. The technology extracts the water vapour and astronaut sweat from the air, and the urine from the toilets, to transform it into clean drinking water. Alongside the complex plumbing, the air conditioning system maintains a constant breeze through the station. Despite weekly cleaning sessions, the ISS is far from bug-free. A recent study found there were 55 different types of microbes co-existing with humans.

4,400: Research papers published

Astronauts spend much of their working day conducting, monitoring and analysing experiments. Microgravity research ranges from medical studies on bone and muscle, to investigations of materials and pharmaceuticals. Some experiments – like this one with worms – are aimed at combating diseases on Earth, while others involve developing new technologies. Several private companies are building on ISS research to manufacture advanced materials in space.

70: Oldest astronaut (years)

When Nasa selected its first astronauts in 1959, candidates had to be young and physically perfect. When John Glenn, was asked at a press conference which test he found worst during the medical screening he replied: "If you figure out how many openings there are on a human body, and how far you can go into any one of them…you answer which would be the toughest for you." Thanks to his sacrifice, it turns out you do not have to be young or at the peak of fitness to be an astronaut. You do, however, need to be good at your job, which is why astronauts are retiring much later in their careers. The oldest astronaut to live on the ISS is currently Don Pettit, who returned to Earth on his 70th birthday. John Glenn, incidentally, flew his second mission aged 77.

371: Longest mission (days)

Ever find yourself staring out of the office window, longing for the end of the working day? Imagine being stuck at your workplace for more than a year, only occasionally able to go outside. Missions lasting six months or more are now the norm and the current record holder for the single longest ISS flight is Nasa's Frank Rubio, who returned to Earth in September 2023. Cosmonaut Valeri Polyakov still holds the all-time record for the most days spent in space in a single mission. He lived onboard Russia's much smaller Mir space station for 437 continuous days in orbit between 1994 and 1995.

25: Thickness of a pane of Cupola glass (mm)

There are few more popular places to hang out on the ISS than the Cupola. This European-built bay window on the Earth provides astronauts with one of the best views in the known Universe. But the distraction of staring out of the window can ruin the strict timelines that govern the astronaut day. Nicole Stott confessed to my Space Boffins podcast that she had to set an alarm to remind her to get back to work. Astronauts have become increasingly skilled at photography, sharing their perspective with the world. You can see what they see on this feed.

1: Didgeridoos played

Hobbies on the ISS range from watching movies to watercolour painting, but many astronauts also bring musical instruments with them. Canadian Chris Hadfield famously recorded his version of Space Oddity on the ISS. Other performers have included Cady Coleman who played flute on the station and Thomas Pesquet who brought his saxophone. Don Pettit, however, probably goes down as the ISS's greatest musical innovator. He fashioned the station's vacuum cleaner into a digeridoo to demonstrate a science experiment – part of the extensive education programme carried out by astronauts on the ISS. You can read all about space music firsts here.

If I didn't fix the toilet, it wasn't getting fixed – Chris Hadfield

211: Length of astronaut sleeping bag (cm)

Astronauts are each allocated a private living area on the ISS – around the size of a classic British phone box – which they can decorate with pictures and mementoes from home. They are also issued with specially designed sleeping bags, which fasten to the wall. But the noise of the ventilation fans, the glare of the Sun and the lack of gravity all conspire to make sleeping a challenge. At least on the ISS astronauts have a private space. For the forthcoming 10-day Artemis II mission, the four crew of the Orion spacecraft will be suspended in hammocks just a few centimetres apart.

4: Toilets

It was, Chris Hadfield once told me his "proudest day in space". During his 2012 mission, he was co-ordinating a Russian spacewalk, when the toilet in the US section of the station broke down. "If I didn't fix the toilet, it wasn't getting fixed.

"I was elbow deep in a non-gravity driven, non-water driven toilet, rebuilding it, then I'd have to clean myself up and whip down to the other end of the station and help set up for the spacewalk." By the end of the day, the spacewalk went without a hitch... and the toilet worked. "It was my highlight of six months of orbiting the world." You can watch a tour of an ISS toilet here.

5: Chocolate chip cookies baked

Space food has come a long way since the early days of gloopy pastes (you can read more about early space food here). While US astronauts tend to celebrate with pizza nights, European Space Agency (Esa) astronauts can claim more sophisticated tastes. Italian Samantha Cristoforetti's mission included the first espresso machine in orbit and French astronaut Sophie Adenot plans to dine on gourmet meals developed by an award-winning chef. In future, the aim is to grow food in space and cook raw ingredients. Salad has already been successfully cultivated on the ISS and, in 2019, astronauts even managed to bake five cookies… but were not allowed to eat them. They had to be returned to Earth to see if they were safe.

40: Manoeuvres to avoid space junk

The 2013 film Gravity highlighted what can go wrong when space junk gets out of control. The story imagines a "Kessler syndrome" event, with cascading impacts of orbital debris destroying spacecraft. With some 28,000 objects in orbit, the risks of crewed spacecraft being hit by debris is a growing concern. The ISS has avoided major damage but has still been scarred by impacts. During his 2015 mission, Tim Peake took this disconcerting picture of a chip in a Cupola window.

9: Longest spacewalk (hours)

Since the first assembly mission in December 1998, there have been more than 270 spacewalks at the ISS. Initially, most of them were to help piece the station together but, today, they are increasingly to repair or replace ageing parts. The longest spacewalk remains an eight-hour-and-56-minute excursion by Susan Helms and Jim Voss in 2001. During a spacewalk in 2013, Luca Parmitano almost drowned when his helmet began filling with water.

7: Joints on Canadarm2

Apart from providing singing astronauts, one of Canada's greatest contributions to the ISS is a complex multi-jointed robotic arm. Canadarm2, a development of the original robotic arm fitted to the Space Shuttle, is used for everything from assembly missions and spacecraft docking to carrying spacewalkers to different parts of the ISS – a truly memorable ride. Canada has also developed what it calls "the most sophisticated space robot ever built". Known as Dextre, it has two hands, five eyes and more attachments than a Swiss Army knife.

13: Space tourists

Visiting the ISS is no longer only the preserve of space agency astronauts and cosmonauts, 13 private individuals have also flown to the ISS. Companies offering the rides prefer to call them spaceflight participants, rather than tourists. And even billionaires need to undergo months of training. Nasa is reported to charge around $35,000 (£26,500) a night for accommodation, although that does include meals and an view of the Earth.

16: Orbits in 24 hours

At 28,000km/h (17,500mph), ISS astronauts experience 16 sunrises and 16 sunsets every day. The orbital track of the station takes it over some 80% of the Earth, which gives us a chance to see them pass overhead. The ISS is one of the brightest objects in the night sky. You can use a free Nasa app to alert you to viewing opportunities.

55m: Cheapest flight ($)

Reaching the ISS does not come cheap but it is less than it used to be. According to a 2018 report, each Space Shuttle flight cost in the region of $1.5bn (£1.13bn) – although the Shuttle could carry both astronauts and a payload. Today, the going rate for a seat in SpaceX's Dragon is around $55m (£41.6m) and a less comfy Russian Soyuz trip comes in at around $80m (£60.6m). Although competition has driven down costs, rockets are always going to be expensive with existing technology.

More like this:

• What happens when astronauts get stuck in space

• How the ISS will meet a fiery end

• What comes after the ISS? 

286: Duration of Suni and Butch's 'eight-day' mission (days)

Starliner has been described as the next big thing in human spaceflight. After plenty of setbacks, Boeing's new spacecraft was finally due for its maiden crewed test flight in June 2024. But Nasa would come to regret billing it as an eight-day mission. Multiple failures during docking led officials to decide to send Starliner back to Earth without a crew, leaving astronauts Sunita Williams and Butch Wilmore stuck on the station and spending Christmas in space. The two astronauts finally made it back to Earth in March 2025.

735,000: Power generated (KWh/year)

At 109m (356ft) the wingspan of the ISS solar arrays is longer than an Airbus A380. The arrays, which tilt to face the Sun, are connected to banks of batteries to ensure a continuous power supply. The ISS is also fitted with insulation and a cooling system to stop it overheating, with a bank of ammonia-filled radiators to dissipate heat into space. Ammonia leaks have been a cause of concern, resulting in emergency spacewalks to patch them up.

3,000,000: Lines of software code

In 1969, a computer with the equivalent of only around 74 kilobyte ROM and 4 kilobyte RAM memory (a tiny fraction of what you'd find in your smartphone) enabled three astronauts to navigate the roughly 380,000km (236,000 miles) from the Earth to the Moon and descend to a precise spot on the lunar surface. The ISS requires rather more computing power and encompasses both the operating system on the station itself and mission control on the ground. In fact, there are multiple computers on the ISS – from clunky Soviet-era electronics to tablets. There are even adapted Raspberry Pi computers. Renamed Astro-Pi they have been used to run student experiments.

486,000: Items onboard

We all lose things at home – but imagine if you put them down and they drifted away. That is a frequent occurrence on the ISS and the reason why surfaces and astronaut clothes are covered in patches of Velcro. To keep track of what's onboard, almost every item – from critical hardware to toilet roll – is given a bar code and/or serial number. The whole system is overseen by the Inventory Stowage Officer in mission control. So, if astronauts run out of toilet roll, they know who to call. (Kindly provided by Nasa's mission control centre in Houston, this is by far our favourite ISS fact.)

150bn: Total cost ($)

The most difficult number to calculate or verify, we may never know the full cost of the ISS. We do know that Nasa alone spends between $3bn and $4bn (£2.27bn to £3bn) each year to operate the ISS. Has it been worth it? You decide. - BBC