The sensation of weightlessness, or zero gravity, happens when the effects of gravity are not felt. Technically speaking, gravity does exist everywhere in the universe because it is defined as the force that attracts two bodies to each other. But astronauts in space usually do not feel its effects.
The International Space Station, for example, is in perpetual freefall above the Earth. Its forward motion, however, just about equals the speed of its "fall" toward the planet. This means that the astronauts inside are not pulled in any particular direction. So they float.
Not having to bear weight on your feet sounds relaxing, but in the long term there are many health problems associated with it. Bones and muscles weaken, and other changes also take place within the body. One of the functions of the ISS is to study how astronaut health is affected by long periods in weightlessness. This will also be a huge focus of the first one-year ISS mission in 2015.
You don't have to leave Earth to (briefly) escape the bonds of gravity. Anyone who crested the top of the hill in a fast roller coaster, or who sat in a small plane pushed down suddenly by the wind, briefly experienced weightlessness.
More sustained periods are possible in planes that fly a parabola. NASA's reduced gravity flight program, for example, flies planes in a series of about 30 to 40 parabolas for researchers to conduct experiments on board. Each climb produces a force about twice the force of gravity for 30 seconds. Then, when the plane reaches the top of the parabola and descends, passengers feel microgravity for about 25 seconds.
The film crew and actors on the movie "Apollo 13" (1995) spent hours aboard a plane that flew parabolic flights over and over again. This allowed the actors to really "float" during their time in the movie spacecraft rather than relying on cumbersome wires.
Astronauts, however, experience weightlessness for much longer periods. The longest sustained time spent in space took place in 1994-95, when Valeri Polyakov spent almost 438 days in space.
Even a few days in space can present temporary health problems, as Heidemarie Stefanyshyn-Piper discovered after spending two weeks in space during STS-115 in 2006. During a press conference after the landing, Piper collapsed as she was not quite readjusted to gravity.
Temporary health effects
Weightlessness causes several key systems of the body to relax, as it is no longer fighting the pull of gravity. Astronauts' sense of up and down gets confused, NASA said, because the vestibular system no longer can figure out where the ground and the ceiling are. Spacecraft designers take this into account; the ISS, for example, has all of its writing on the walls pointing in the same direction.
Crewmembers also experience a disruption in their proprioceptive system, which tells where arms, legs and other parts of the body are oriented relative to each other. "The first night in space when I was drifting off to sleep," one Apollo astronaut said in a NASA interview, "I suddenly realized that I had lost track of ... my arms and legs. For all my mind could tell, my limbs were not there."
This disorientation can cause astronauts to become queasy for a few days. One famous example took place during Apollo 9 in 1969. Rusty Schweickart had to change a planned spacewalk because he was feeling ill. The concern was that if he vomited while in his spacesuit, the fluid could spread through his helmet (making it hard to see) or interfere with the breathing apparatus and cause him to potentially choke to death.
Spacecraft also must be designed to take microgravity into account. During spacewalks, for example, astronauts require extra handholds and footholds on the exterior of their spacecraft so that they can anchor themselves and not float away. (Astronauts also attach to them in tethers in case they lose their grip.)
Long-term health effects
Astronauts in space for weeks to months can run into trouble. Calcium in bones secretes out through urine. As the bones weaken, astronauts are more susceptible to breaking them if they slip and fall, just like people with osteoporosis. Muscles also lose mass.
Astronauts typically exercise two hours a day in space to counteract these effects, but it still takes months of rehabilitation to adjust on Earth after a typical six-month space mission. More recently, doctors have discovered eye pressure changes in orbit. NASA has tracked vision changes in astronauts that were on the space station, but nothing so serious as to cause concern.
The first one-year mission to the ISS will take place in 2015. One of the astronauts, Scott Kelly, has a twin, Mark, who will remain on Earth. The brothers have volunteered to be guinea pigs to look at effects on Scott Kelly's body in weightlessness in comparison to Mark Kelly's body on Earth.
"They will provide insight into future genetic investigations that can build on this study, but with a larger study population of unrelated astronauts," NASA said in 2013.