Twenty-five years ago, in April 1981, the first space shuttle was launched.
Two days later, after orbiting the Earth 36 times, it completed a successful re-entry. Since then, the shuttle has taken hundreds of men and women into space. As well as the highly trained astronauts who pilot the craft and oversee the missions, they have included scientists, doctors, veterinarians, politicians, engineers and even a Saudi prince.
What makes these people risk the dangers of space travel? Do they ever get scared? How do they survive in the hostile environment of space? What effect does weightlessness have on their bodies? How do they eat, sleep and go to the toilet? Do they get chance to relax and have fun? And what is the view like from up there?
Countdown to lift off
The launch is the most exciting and dangerous part of a shuttle mission.
For many astronauts it is the pinnacle of their career. The wait on the launch pad is an uncomfortable and nervous one. The shuttle is launched vertically, so everyone sits facing upward while waiting for the countdown.
The metal seats are unyielding, and lumpy parachutes and bulky pressure suits add to the discomfort. Back pain builds up to excruciatingly high levels.
"Every launch countdown is a fearful event," says astronaut Mike Mullane.
That is hardly surprising considering the shuttle sits on top of almost two million kilograms (four million pounds) of explosive chemicals. But once the launch is underway there is no time to worry.
The blast-off is awesome. The shuttle leaps from the launch pad in huge clouds of steam and a blaze of fire. It takes just four seconds for it to reach 160kmh (100mph) and less than a minute to go supersonic.
"The launch looks kind of slow to an observer," says shuttle veteran Scott Kelly. "But when you're inside there ain't nothing slow about it. I mean, you feel all seven million pounds of that thrust. It's like a giant hand grabs the orbiter and throws it into space."
The ride is noisy and rough. A shuttle launch exceeds a deafening 150 decibels and the vehicle's vibrations set the astronauts' heads rattling inside their helmets.
As the shuttle accelerates, the crew are pushed back in their seats. The force gradually builds to three times the strength of gravity, which means that everyone feels three times heavier than they do on Earth. It becomes very difficult to move.
Just over eight minutes into the flight, the shuttle reaches orbit. The main engines shut down and the acceleration force disappears. Suddenly everything is weightless. Pencils and notebooks begin to float around the orbiter's cabin.
Life in a weightless world
Once the shuttle is in stable orbit, the ride becomes smooth and quiet.
Although the craft travels at five miles per second, there is no sensation of movement.
In space, astronauts spend most of their time in the pressurised crew compartment. Seven people share an area the size of an average living room.
But in orbit the compartment seems larger than it does on Earth, because not only the floor, but also the walls and ceiling can be used.
"While one of us works strapped to a wall, another sits on the ceiling eating peanuts, and a third runs on a treadmill anchored to the floor,"
explains astronaut Sally Ride.
Inside the crew compartment, the astronauts are cocooned from the hostile environment of space. They have breathable air at normal atmospheric pressure and are shielded from harmful cosmic radiation. Any sudden loss of cabin pressure would prove fatal, because body fluids evaporate in the vacuum of space, meaning that the astronauts' blood would boil.
Weightlessness takes some getting used to. Anything that is not stowed or tied down will float away and may get lost. Good housekeeping is essential.
For the first couple of days, about 50 per cent of astronauts suffer space sickness. In zero gravity, internal organs shift position, bodily fluids are redistributed and the balance organs of the inner-ear stop working properly. This can cause nausea, headaches and vomiting.
Weightlessness also causes the vertebrae in the spine to float apart.
Astronauts grow a few centimetres taller in space and often suffer back pain for the duration of the mission. But sickness and back pain notwithstanding, weightlessness is tremendous fun. With a one-fingered push, an astronaut can send himself floating through the cabin, or turning slow-motion somersaults.
"I tell people that zero-g is like swimming in air, but it's not quite,"
says astronaut Guy Bluford. "You push off walls and ceilings and glide through open hatchways. It's almost like a seal gliding through the water."
Because it's so easy to move around, the heart and muscles don't have to work as hard as they normally do and can become weak. To compensate, periods of exercise are built into the mission schedule. On short flights these are not compulsory and some astronauts skip them, preferring instead to admire the view.
Eating feels the same in space as it does on Earth. Astronauts no longer eat food squeezed from a toothpaste tube as they did in the past. On the shuttle, the crew enjoy a menu of more than 70 foods, including spaghetti, meat, vegetables, fruit and chocolate pudding.
However, weightlessness makes food difficult to control. Most items are eaten straight from the packet to stop them floating away. Drinks have to be taken through a straw, because weightless liquids will not pour from an upturned cup or glass.
Astronauts love to play with their food. Sally Ride, the first American female astronaut, recalls her crew setting a cookie spinning in the middle of the room and then flying-in, open-mouthed, to devour it.
No water is taken aboard the shuttle. The craft's fuel cells combine liquid hydrogen and liquid oxygen to generate electricity, and drinkable water is a by-product of this process. Water doesn't flow in zero-g, so there are no sinks or showers. The only way to keep clean is to take a sponge bath.
"About the third day in orbit, astronauts would kill for a shower," says astronaut Mike Mullane.
Toothbrushing is more straightforward. An ordinary toothbrush and toothpaste are used, and the foam is spat into a tissue. The space shuttle's toilet is surprisingly Earth-like, but waste solids are carried away by air rather than water. Astronauts urinate into a vacuum hose which leads to a storage tank. When the tank is full, the urine is vented into space. It instantly freezes, forming ice-crystals - and is said to be a beautiful sight.
Sometimes it is necessary for a couple of crew members to venture outside the spacecraft to repair some space hardware, launch a probe, or test equipment. During extra-vehicular activities (EVAs), spacesuits must be worn. These self-contained units provide everything needed to survive outside the capsule: oxygen, carbon dioxide removal, temperature control, pressurisation and protection from radiation. EVAs are long, exhausting affairs and spacesuits are bulky and difficult to move in. Even so, spacewalks rank alongside launches as the highpoints of a mission. They offer the chance to escape from the confines of the orbiter, stand exposed to the vastness of space and gain unobstructed views of the Earth below.
Flying onboard a shuttle is not all work and no play. Even the busiest mission schedules leave a little time for recreation. And, of course, astronauts need to sleep. It takes a while to get used to sleeping, because in a weightless environment there's no such thing as lying down. It is possible to sleep anywhere, in any orientation. On most missions, astronauts sleep in lightweight bags they clip to the walls or ceiling to prevent them drifting around.
When they are not working or sleeping, astronauts have plenty to keep them amused. They can take CD players into orbit and sometimes even movies. But most free time is spent fooling around in zero-g or simply admiring the view. Mike Mullane says: "In space it's only necessary to look out of the window to be entertained. The sights are so beautiful, it's impossible to get your fill of them."
Sunrises and sunsets are among the most breathtaking sights. Gorgeous bands of changing colours appear just before or after the sun dips over Earth's horizon. Best of all, astronauts are treated to these stunning spectacles 32 times a day: one sunrise and one sunset during each 90-minute orbit.
Re-entry is the most dangerous part. The shuttle has to hit the atmosphere at just the right angle and must glide to a perfect runway landing. There are no second chances. Everyone wears g-suits - inflatable trousers that apply pressure to the lower body and prevent blood from pooling in the legs. This reduces the danger of astronauts fainting as they begin to feel the pull of gravity. When the shuttle hits the top of the atmosphere it begins to slow down. Air friction generates temperatures of up to 1,650C (3,000F) outside the vehicle, but the heat-resistant tiles keep the cockpit cool. Through the windows, astronauts can see the flickering orange glow of the super-heated air surrounding them.
Most of the landing is controlled by computer, but the commander takes control for the last six minutes of the glide home. The orbiter makes a very steep approach to the runway, causing the astronauts to slide forward in the seats.
By now everyone feels so heavy that it is difficult even to lift a hand.
Once the shuttle lands, the astronauts wait inside while the ground crew make safety checks. This gives them the chance to do gentle exercises to restore their "Earth-legs". They have to be very cautious when they eventually climb down the steps onto the runway, because their sense of balance has not yet adjusted to Earth's gravity.
HOW A SHUTTLE WORKS
Nasa's Space Transportation System, better known as the space shuttle, is the only re-usable spacecraft.
It is used to deploy satellites, fix broken space hardware, carry out scientific experiments in weightless conditions, conduct military missions, and maintain the International Space Station.
The space shuttle is extremely versatile: it launches like a rocket, manoeuvres in orbit like a spacecraft and lands like an aeroplane.
The shuttle has three main sections: the orbiter, the two solid-fuel rocket boosters and an external fuel tank. The three sections are linked together for only a short time. The boosters are released shortly after launch and the external fuel tank is jettisoned soon afterwards. Only the orbiter navigates through space. The orbiter contains a pressurised crew compartment and a huge cargo bay that can carry satellites, spacecraft and scientific laboratories.
The shuttle is launched vertically like a rocket. The solid-fuel rocket boosters, with their combined six million pounds of thrust, provide most of the power for the first two minutes of the flight. Once spent, they are released and parachute into the ocean. They are then recovered and refurbished, ready to be re-used.
The shuttle now relies on the external tank for power. This holds more than 2.27 million litres of liquid oxygen and liquid hydrogen which are mixed and burnt to fuel the orbiter's three main engines.
Eight-and-a-half minutes into the flight, as the shuttle approaches its orbiting altitude, the external fuel tank is jettisoned and the main engines shut down. The fuel tank breaks up as it falls toward the ocean. It is the only section of the shuttle that is not re-used.
After main engine shutdown, the shuttle is powered by smaller space engines. Two orbital manoeuvring system engines provide the thrust to take the craft into low Earth orbit; 44 small rocket engines clustered around the nose and tail are used to make minor orbital adjustments.
Less than 10 minutes after take-off, the shuttle reaches the 28,160kmh (17,500 mph) speed required to maintain its orbit 322km (200 miles) above the Earth's surface.
The shuttle typically stays in orbit for about a week. Once the mission is complete the engines are fired to slow the craft down and begin the descent. After 30 minutes the shuttle re-enters the atmosphere. It is moving very fast, and generates intense heat as it collides with molecules of air. The 32,000 heat-resistant tiles glued to its surface prevent it from burning up. As it decreases in height, the atmospheric density increases. The control surfaces on the wings and tail, which were useless in the vacuum of space, are now used to manoeuvre the craft like an aeroplane. The engines shut down, and the shuttle glides to a horizontal landing on a conventional runway.
From Enterprise to Endeavour, Nasa has built six space shuttles. One was a test-vehicle, not designed for space travel. Two have been destroyed in accidents. The remaining three are operational.
As a test vehicle, Enterprise was not equipped for space travel. Nasa used the test craft to carry out approach and landing tests in 1977, launching it from the back of an airborne Boeing 747.
After completing trial flights and ground-based vibration tests, Enterprise made a series of appearances at air shows. The craft is now housed in a space museum in Virginia.
Columbia was the first space shuttle to reach orbit. It was launched from the Kennedy Space Centre in Florida on April 12, 1981, and completed 36 orbits of the Earth. Columbia landed at Edwards Air Force Base, California, two days later.
In 1983, Columbia became the first space shuttle to carry the European-built Spacelab module in its cargo bay. This provided extra facilities for carrying out onboard scientific experiments.
Columbia disintegrated during re-entry on February 1, 2003. All seven members of the crew were killed. An investigation showed that the left wing had been damaged during take off when it was struck by a piece of insulation foam from the external fuel tank.
Nasa's second operational shuttle, Challenger, was launched in April 1983. Later that year it carried America's first woman astronaut, Sally Ride, into space.
Challenger's 25th launch, in January 1986, aroused intense interest due to the inclusion of a civilian, Christa McAuliffe, in the crew. She had been selected the previous year to be the first "teacher in space".
Tragically, the shuttle was destroyed 73 seconds after take-off after a seal on one of the solid rocket boosters failed, and all members of the crew were killed. Shuttle flights were then suspended for 32 months while safety improvements were made.
Discovery was launched in August 1984.
In 1990 it carried the pound;1 billion Hubble space telescope into orbit and in 1998 transported veteran astronaut John Glenn back into space after a 36-year absence.
Discovery was the first shuttle to be launched into space after the 1986 Challenger disaster.
On its return-to-flight launch last year, a piece of insulation foam broke off from the external fuel tank in a chilling echo of the Columbia flight.
Although the vehicle was undamaged, further shuttle flights were suspended indefinitely.
Nasa hopes to recommence shuttle missions with a Discovery launch this May.
Atlantis (operational) Atlantis was launched in 1994. The following year it performed the first of a series of nine space shuttle dockings with the Russian space station, Mir, heralding a new era of international co-operation in space flight.
In 1998, Atlantis was modified to make it more compatible with Mir's successor, the International Space Station.
Since then it has carried out a number of construction and maintenance missions for the International Space Station.
Endeavour (operational) Endeavour was built to replace Challenger and was launched in 1992.
In 1998 it was used to attach Unity, a US-built element of the International Space Station, to Russia's Zarya control module.
In 2000 Endeavour used radar to make an extremely accurate topographical map of the Earth's surface.