Weightlessness

A sensation of weight is also produced, even when the gravitational field is zero, when contact forces act upon and overcome a body's inertia by mechanical, non-gravitational forces- such as in a centrifuge, a rotating space station, or within an accelerating vehicle.

Also, if they were some vertical distance apart the lower one would experience a higher gravitational force than the upper one since gravity diminishes according to the inverse square law.

To create a weightless environment, the airplane flies in a 10 km (6 mi) parabolic arc, first climbing, then entering a powered dive.

[5] NASA's current Reduced Gravity Aircraft, "Weightless Wonder VI", a McDonnell Douglas C-9, is based at Ellington Field (KEFD), near Lyndon B. Johnson Space Center.

[citation needed] The European Space Agency (ESA) flies parabolic flights on a specially modified Airbus A310-300 aircraft[6] to perform research in microgravity.

These campaigns are currently operated from Bordeaux - Mérignac Airport by Novespace,[7] a subsidiary of CNES; the aircraft is flown by test pilots from DGA Essais en Vol.

[12] These flights are sold by Avico, are mainly operated from Bordeaux-Merignac, France, and intend to promote European space research, allowing public passengers to feel weightlessness.

NASA's Marshall Space Flight Center hosts another drop tube facility that is 105 m tall and provides a 4.6 s free fall under near-vacuum conditions.

[14] Other drop facilities worldwide include: Another ground-based approach to simulate weightlessness for biological sample is a "3D-clinostat," also called a random positioning machine.

Unlike a regular clinostat, the random positioning machine rotates in two axes simultaneously and progressively establishes a microgravity-like condition via the principle of gravity-vector-averaging.

[19] Sub-orbital flights seize the approximated weightlessness, or μg, in the low Earth orbit and represent a promising research model for short-term exposure.

At the speed of light it would take roughly three and a half hours to reach this micro-gravity environment (a region of space where the acceleration due to gravity is one-millionth of that experienced on the Earth's surface).

Following the advent of space stations that can be inhabited for long periods, exposure to weightlessness has been demonstrated to have some deleterious effects on human health.

Astronauts subject to long periods of weightlessness wear pants with elastic bands attached between waistband and cuffs to compress the leg bones and reduce osteopenia.

[28] Other significant effects include fluid redistribution (causing the "moon-face" appearance typical of pictures of astronauts in weightlessness),[28][29] changes in the cardiovascular system as blood pressures and flow velocities change in response to a lack of gravity, a decreased production of red blood cells, balance disorders, and a weakening of the immune system.

[30] Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, excess flatulence, and puffiness of the face.

[38] On December 31, 2012, a NASA-supported study reported that human spaceflight may harm the brains of astronauts and accelerate the onset of Alzheimer's disease.

[44] SMS, along with facial stuffiness from headward shifts of fluids, headaches, and back pain, is part of a broader complex of symptoms that comprise space adaptation syndrome (SAS).

In some instances, it can be so debilitating that astronauts must sit out from their scheduled occupational duties in space – including missing out on a spacewalk they have spent months training to perform.

[47] Despite their experiences in some of the most rigorous and demanding physical maneuvers on earth, even the most seasoned astronauts may be affected by SMS, resulting in symptoms of severe nausea, projectile vomiting, fatigue, malaise (feeling sick), and headache.

[47] These symptoms may occur so abruptly and without any warning that space travelers may vomit suddenly without time to contain the emesis, resulting in strong odors and liquid within the cabin which may affect other astronauts.

[49] Graybiel and Knepton proposed the term "sopite syndrome" to describe symptoms of lethargy and drowsiness associated with motion sickness in 1976.

[50] Since then, their definition has been revised to include "...a symptom complex that develops as a result of exposure to real or apparent motion and is characterized by excessive drowsiness, lassitude, lethargy, mild depression, and reduced ability to focus on an assigned task.

"[51] Together, these symptoms may pose a substantial threat (albeit temporary) to the astronaut who must remain attentive to life and death issues at all times.

[53] In the space (or microgravity) environment the effects of unloading varies significantly among individuals, with sex differences compounding the variability.

[57] A more effective regimen includes resistive exercises or the use of a penguin suit[57] (contains sewn-in elastic bands to maintain a stretch load on antigravity muscles), centrifugation, and vibration.

In a micro-g environment, with the loss of a hydrostatic gradient, some fluid quickly redistributes toward the chest and upper body; sensed as 'overload' of circulating blood volume.

It is one of the only feasible options to combine with human experiments, making parabolic flights the only way to investigate the true effects of the micro-g environment on a body without traveling into space.

[68] Parabolic flight studies have provided a broad range of results regarding changes in the cardiovascular system in a micro-g environment.

Parabolic flight studies have increased the understanding of orthostatic intolerance and decreased peripheral blood flow suffered by astronauts returning to Earth.