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What Causes Weightlessness?
The reason astronauts are weightless while in orbit is identical to the reasons that people feel the sensations they do while riding roller coasters. While on Earth, external forces are pushing or pulling on a person's body, however when a spacecraft enters orbit, the people and objects aboard the craft enter a state of free fall. Essentially, the vehicle and all of its contents are falling towards the Earth causing the sensation of weightlessness similar to the state a person feels when enjoying the amusement park ride.
Many people believe that a lack of gravity is the root cause for weightlessness in space. However, a spacecraft needs gravity in order to orbit around the Earth. Gravity supplies a centripetal force which is responsible for the orbital motion. This means that the spacecraft is falling towards the Earth without colliding with it due to tangential velocity.
Despite this fact, spacecraft in orbit around Earth still experience a certain amount of weighted force. Known as centrifugal force, the furthest side away from the surface of the planet experiences greater pull as the vehicle orbits. Observations aboard the various spacecraft that have achieved orbit have shown that objects will still fall towards the part of the ship oriented towards motion.
Above right: Weightless hair. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/7/79/Weightless_hair.jpg)
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Creating the Effects of Weightlessness in Space on Earth
Since 1959, scientists working in the space program have known of ways to duplicate the effects of weightlessness in space within the atmosphere of Earth. Using airplanes dubbed the “vomit comet", researchers can analyze the conditions of weightlessness in space and train potential astronauts and tourists for space travel. The key to making this event occur is to use a six-mile long parabolic arc flight path. The plane climbs to a high altitude and then arcs towards the surface of the ground in an elliptical flight pattern relative to the center of the planet. This essentially puts the plane and its contents into free fall, simulating the effects of weightlessness for approximately 25 seconds.
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Can We Create Artificial Gravity?
As of 2010, there has only been limited success at creating some form of artificial gravity that would limit the effects of weightlessness in space. Using centrifugal force, a spacecraft that spins can create a pull on objects within the vehicle During the Gemini 11 mission, NASA attempted to create artificial gravity by rotating a space capsule around the Agena Target Vehicle, attached with a tether. Although the astronauts aboard the ship did not experience any gravitational force, objects were pulled towards the center of the connection.
Although limited to science fiction, other laws of physics could be used to create artificial gravity. Strong enough linear acceleration would theoretically create a gravitational force opposite to the direction of travel. Essentially, if a spacecraft was able to create enough force from speed to create a single-g environment, people and objects would experience gravity.
NASA's Jet Propulsion Laboratory was able to levitate a mouse in 2009 by canceling out a portion of the gravity of the Earth using magnetism. With the same technology, it could be possible to create artificial gravity in an object by reversing the concept. However, the system is extremely elaborate and could not be placed into spacecraft using modern technology.
In all, weightlessness in space is a major factor in the exploration of the worlds beyond our planet. While scientists wrestle with the physics of weightlessness, methods to limit the effects will continue to be explored.
Above left: Agena Target Vehicle. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/8/80/Profile_of_Agena_Docking_Target_-_GPN-2000-001345.jpg)
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"US Scientists Levitate Mice to Study Low Gravity" Reuters: http://www.reuters.com/article/idUSTRE58A02G20090911
Jefferson Labs: http://education.jlab.org/qa/gravity_01.html
"Weightlessness" ESA for Kids: http://www.esa.int/esaKIDSen/SEMG2JWJD1E_LifeinSpace_0.html
Weightlessness in Space
Space travel comes with a variety of risks and challenges caused by the effects of weightlessness. The lack of gravity impacts both people and objects at all times during the flight of a spacecraft. One of the most important aspects of weightlessness are the health impacts on humans.