What Is Zero Gravity?
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Zero gravity, more accurately called microgravity, is the apparent weightlessness astronauts experience in orbit. Despite the name, gravity is still very much present (only slightly weaker than on Earth's surface). The astronauts feel weightless because they and their spacecraft are continuously falling around Earth together.
Zero gravity is one of the most famous features of being in space, but the name is misleading. Astronauts in orbit aren't actually experiencing zero gravity; Earth's gravity at the ISS altitude is about 90 percent as strong as at the surface. What they're experiencing is free fall: the spacecraft and astronauts are continuously falling toward Earth at the same rate, creating the sensation of weightlessness.
Is there really zero gravity in orbit?
No, the name is misleading. According to NASA, gravity at the altitude of the International Space Station (about 250 miles up) is about 90 percent as strong as at Earth's surface. The astronauts aboard aren't escaping gravity; they're falling, just sideways fast enough to keep missing Earth as they fall. The constant falling, combined with the spacecraft falling at the same rate, creates the sensation of weightlessness. The technical term is microgravity, not zero gravity.
Why do astronauts float?
Because they're in free fall. When you fall, you don't feel your weight, even though gravity is still pulling on you. Astronauts and their spacecraft are continuously falling toward Earth, but because they're moving so fast (about 17,500 mph at ISS altitude), they keep missing the planet as they fall. The result is endless free fall, which feels exactly like weightlessness to people inside. The same effect can be experienced briefly in airplanes on parabolic flights, sometimes called vomit comets.
Can you experience zero gravity on Earth?
Only briefly, in free fall. Skydivers experience effective weightlessness for the seconds before their parachutes open. Riders on tall amusement park drops feel weightless during the descent. Special airplanes flying parabolic trajectories can create brief periods of weightlessness lasting 20-30 seconds. NASA uses such flights to train astronauts and conduct research. True extended weightlessness requires being in orbit, where the free fall continues indefinitely without anything to stop it.
What does microgravity do to the body?
Several things, mostly negative. Without gravity pulling them down, body fluids shift upward, causing puffy faces and stuffy noses for new astronauts. Muscles and bones weaken from lack of use against gravity. Astronauts can lose 1-1.5 percent of bone density per month without countermeasures. The cardiovascular system also adapts to the lower load. Long-duration stays on the ISS require careful exercise and monitoring to manage these effects. Most changes reverse after returning to Earth, though some bone loss may be permanent.
Zero gravity is more accurately called microgravity. Astronauts in orbit aren't escaping gravity; they're in continuous free fall with their spacecraft, which feels like weightlessness. The effects on the human body are significant, including muscle and bone loss that astronauts have to work hard to counteract. True weightlessness on Earth is only possible briefly, in free fall or parabolic flight trajectories.
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