At 6.41pm this Thursday, a small bright light will appear low in the night sky before vanishing in the darkness. Few people will notice and even fewer will care, but for a handful of souls that speck on the horizon is a place called home.
From down here on Earth there is little more to see, but close up the spot takes on a more complex form: a shiny hulk of interconnecting tubes and metal trusses bracketed by giant wing-like panels. As roomy as a five-bedroom house, these are the most extreme living quarters ever built. What looks like a wandering star in the heavens is sunlight reflecting off the International Space Station.
With more than a decade of construction now coming to an end (next week's shuttle mission leaves only two more before the fleet is mothballed), astronauts can finally look forward to stretching out and using the space station to the full. If the experiences of those who helped build and man the station are anything to go by, they are in for an extraordinary time. "I still can't believe what I've seen sometimes," says Piers Sellers, the Sussex-born-boy-turned-Nasa-astronaut who took part in the most recent shuttle mission to the station in May. "Often it all comes back to me in dreams."
Next week, Nasa will commemorate 10 years of life on the space station (the first residents arrived on 2 November 2000), but fewer than 200 people have first-hand knowledge of life on board. Only a fraction have stayed more than six months on the largest orbiting spacecraft ever built. The longer the stint, the closer these veterans come to perfecting the art of life in freefall.
And falling it is. The footage of weightless, grinning astronauts pulling somersaults and chasing food through the air make it seem as though the space station is floating free from the pull of gravity. Nothing could be further from the truth. The orbiting outpost – all 450 tonnes of it – is forever falling to Earth and would crash-land were it not moving so fast as to maintain a gentle curve around the planet. In orbit, things are weightless simply because they are all falling at the same velocity.
To get to the space station takes two days, a journey dictated as much by its speed as its altitude. The station flies at an altitude of 220 miles or so (that's more than 30 times the cruising height of a jumbo jet), but is travelling at a breakneck 17,500mph. Before astronauts can clamber aboard, they first have to chase it down and pull alongside. Go by shuttle and you will need 900 tonnes of solid rocket fuel and half a million gallons of liquid oxygen and liquid hydrogen to burn in the main engine.
The shuttle approaches the space station from below and performs a graceful backwards somersault as it closes in. This gives the station crew a chance to take around 400 pictures of the shuttle's heat-shielded underbelly, which are beamed down to Nasa staff and examined for cracks and holes. The backflip was introduced after damage to protective tiles caused the space shuttle Columbia to burn up on re-entry in 2003.
The docking procedure is as slow and cautious as you might expect given the price tags of the spacecraft involved: $1.7bn (£1.1bn) for a shuttle and around $100bn (£64bn) for the space station. Once they are locked together – a move that ends with a gentle lurch – it takes half an hour or so to equalise the pressure and finally open hatches that separate the two crews. "You see these pale faces on the other side and they're always excited to see you. Sometimes it's been three months since they've seen anyone else," says Sellers.
In all, the living space on the station amounts to the equivalent of roughly one-and-a-half Boeing 747s. The main area was constructed piece by piece, by bolting giant can-shaped modules to each other to form one 74m-long tube. At one end are the Russian-built modules, with practical names such as Zvezda (Star) and Zarya (Sunrise). These connect (via an adaptor, naturally) to the more touchy-feely sounding US and European-built modules Unity, Destiny and Harmony. Storage facilities, laboratories and siderooms jut off this main tube, to give astronauts room to go about their business, do experiments and operate the space station's two robotic arms. Bolted across the station, making a cross at the point where the Russian and US sections meet, is a huge truss that carries 16 solar panels to provide electrical power.
The space station has a permanent crew of six, so the arrival of new faces is a cause for celebration. That said, even the most welcome visitors can cause havoc if they are inexperienced. There is a subtle art to moving around without crashing into anything – or, more annoyingly, others – knocking computers, equipment and other objects off the walls to which they are attached with Velcro pads. One serving shuttle pilot confessed to leaving a wake of laptops and other vital belongings behind him the first time he tried to fly from one room to another. "When you first turn up, you are like a bull in a china shop," he said. "I had no idea where to put any of it back."
In time, people hone the skill and can fly down the length of the station, straight as an arrow, without touching anything, except with their fingertips. People sit in mid air, tapping away at a computer, with only a toe hooked under a wall strap to anchor themselves. Then, with a flick of the hand, they'll float up to another computer and carry on typing there. Getting from one place to another is all the more difficult because up and down (and so left and right) have no absolute meaning. The ability to form a mental map of the space station – and then rotate it in 3D to suit your perspective – is a priceless skill for an astronaut.
In such close quarters personal hygiene is a must, but the weightless conditions make washing a delicate chore. Water droplets can cause choking if inhaled and can short-circuit equipment, so many astronauts use the music-festival favourite: moist wipes. All-male crews have been known to strip off and get wiping en masse, but mixed crews tend to take turns in a dedicated hygiene station. Hair-washing is trickier. Men tend to get military buzz cuts before a mission. Even Sunita Williams, who spent 195 consecutive days on the space station – a female record – had her long dark hair chopped to shoulder length but still had problems. "Washing took time. I'd squirt a little water under my hair, pat it down with my hand so it wasn't splashing everywhere, then put some shampoo in my hand and moosh it around. Then I'd wet a towel and try and soak it up. I usually did it on a weekend when we didn't have a whole lot of other things to do," she says. Going to the toilet takes a little practice too, but is less traumatic following a redesign that saw plastic bags replaced with a suction-system toilet, like the ones used on planes. The astronauts' urine, incidentally, is recycled into clean water.
Living in a weightless environment does curious things to the body; after all, our entire physiology evolved in the presence of gravity. On their first day or two in space, some astronauts feel queasy, a condition referred to in Nasa-speak as "stomach awareness". Body fluids that are settled on Earth move up to the head, leaving astronauts with scrawny-looking chicken legs and bloated faces, which has the happy side-effect of erasing wrinkles and making space station crews look years younger, if only temporarily.
On the downside, many astronauts feel congested in space and lose much of their sense of smell. Unless there is a problem with the station's plumbing (and there has been), or someone's lunch has floated off and got lost in a nook or cranny (as has happened), there isn't much to smell on board, because air scrubbers filter out any odours as the air is circulated. Taste is another casualty. "We get a drawer with our name on it and select all our meals before we go, but nothing tastes like it does on Earth. It all tastes like cardboard," says Sellers. "We get through gallons of Tabasco sauce." If you want to know how hard it is to swallow in space, try eating while lying on one side, he suggests.
With no gravity exerting itself on the body, both bones and muscles begin to waste. For every month in space, astronauts lose around 2% of their bone mass. On long stays aboard the space station, crews spend at least two hours a day exercising to keep themselves from becoming frail, splitting their time between a treadmill, a bicycle and a machine that simulates weightlifting.
It takes the space station one and a half hours to fly around the planet, making for 16 complete laps a day. For those on board, the visual effect is spectacular. Open the covers over the windows and the light can be so blinding that astronauts reach for their sunglasses. But after 45 minutes of daylight, a dark line appears on the planet, dividing Earth into night and day. For a couple of seconds, the space station is bathed in a coppery light and then complete darkness. Another 45 minutes later, and just as abruptly, the sun rises to fill the station with brilliant light again.
The onslaught of apparent days and nights would play havoc with astronauts' body clocks, so a shutters-down and bedtime schedule is imposed by mission controllers. Each of the crew has a closet-like cabin where they can hook a sleeping bag to the wall and settle down for the night. Some strap pillows to their heads to make it feel more like lying down. The lights don't go out completely, though. People dozing in orbit see streaks and bursts of bright colour caused by high-energy cosmic rays painlessly slamming into their retinas. Fans and air filters add to the distractions, so some astronauts wear ear plugs to block out the constant hum.
Unsurprisingly, falling asleep can take some getting used to. Just as you are nodding off, you can feel as though you've fallen off a 10-storey building. People who look half asleep will suddenly throw their heads back with a start and fling out their arms. It gets easier with time. One Russian crew member is renowned for doing without a sleeping bag and falling asleep wherever he ends the day. Anyone still awake after bedtime would see his snoozing form drift by, slowly bouncing off the walls, his course set by the air currents that gently pushed and pulled him.
Newcomers are always in awe of the views. It is the sight of our planet that takes the breath away. On board, the best vantage point is from the gun-turret-like cupola whose six windows look down on a panoramic view of Earth. But for the really exceptional vistas, you need to step outside.
During the last shuttle mission in May, a major computer failure left Nasa astronaut Garrett Reisman stuck with what must be one of the most striking views imaginable. The station's 65ft-long robotic arm is used to move equipment from one place to another, but sometimes an astronaut climbs on the end to help. On 17 May, Reisman was standing there, his feet clipped into a footplate, when a computer crashed and the arm froze. Reisman had 25 minutes to take in the scenery.
Space walkers see whole continents, mountain ranges, cities, aircraft contrails and the wakes of ships crossing the oceans. Though hurtling through space, the senses are rarely aware of the velocity. "Sometimes you feel that you are on this big flying building and it's going round the world, but most commonly you feel that someone is rolling this huge ball-shaped map beneath you. You have no feeling of motion," says Sellers.
In lieu of an alarm clock, sleeping crews are woken by music played over the PA system by Nasa staff on the ground. Each day the track is dedicated to a particular astronaut and chosen by their spouse or a colleague. It was Reisman's turn the morning after his eventful spacewalk and so, thanks to his wife, the orbiting vessel of sleeping bodies was roused at 1.50am central daylight time by the Village People singing Macho Man. "That was a total surprise to me," says Reisman. Later on in the mission, when the shuttle had undocked, its own crew woke one morning to the theme tune from Wallace and Gromit. The next day it was Muse's Supermassive Black Hole.
Most shuttle missions take astronauts to the space station for two weeks or so, during which every working day is intense. As soon as the wake-up music begins, printers start chattering out instructions for the day ahead. Almost every hour is scheduled, with crew members' tasks and the tools they will need choreographed by logistics experts on the ground making sure no one gets in anyone's way. At least that is the theory. The crews meet for breakfast, get briefed on the day's jobs, then scatter, breaking only for lunch and dinner.
Short visits to the space station are relentless but easier to cope with psychologically than longer ones. Frank de Winne, a Belgian astronaut and former test pilot, spent nine days on the space station in 2002 and returned for a six-month trip last year, when he became the first European commander of the space station. "If you are there for a week or two, you are basically on a high the whole time. It's not the same when you're there for six months. You need to manage your mood and motivation despite inevitable setbacks. Things that are difficult in the short term, such as not having a shower or any fresh fruit, become part of normal life. The things you really miss are close contact with your wife, your kids and your family and friends," he says. The crews are not completely cut off from those back home, and use email and the station's phone to get in touch when there is time.
The space station will be orbiting Earth for at least another five years, but probably much longer. Of the agencies that pay for it, only the 18-member European Space Agency has yet to finalise plans to keep it in orbit until 2020. Further moves are afoot to keep the station flying until 2028; with the space shuttle about to retire, it will be left to the Russian Soyuz capsule to ferry people back and forth.
For those who built the space station, and the thousands of support staff at agencies around the world, seeing its bright light shooting across the sky at night evokes feelings few others will understand. "You can go out on a quiet night and see the thing flying over and you think, my goodness, I was there, I helped put that together," says Sellers. "When we see it here in Houston, we think of them on board, all in their sleeping bags, tumbling around the Earth. Everybody here feels they own a little piece of it. It's a lasting achievement."