Weightlessness in the context of "G-force"

In classical mechanics, free fall is any motion of a body where gravity is the only force acting upon it.A freely falling object may not necessarily be falling down in the vertical direction. If the common definition of the word "fall" is used, an object moving upwards is not considered to be falling, but using scientific definitions, if it is subject to only the force of gravity, it is said to be in free fall. The Moon is thus in free fall around the Earth, though its orbital speed keeps it in very far orbit from the Earth's surface.

In a roughly uniform gravitational field gravity acts on each part of a body approximately equally. When there are no other forces, such as the normal force exerted between a body (e.g. an astronaut in orbit) and its surrounding objects, it will result in the sensation of weightlessness, a condition that also occurs when the gravitational field is weak (such as when far away from any source of gravity).

In physics, apparent weight is a property of objects that corresponds to how heavy an object appears to be. The apparent weight of an object will differ from the ordinary weight of an object whenever the force of gravity acting on the object is not balanced by an equal but opposite contact force. By definition, the weight of an object is equal to the magnitude of the force of gravity acting on it. This means that even a "weightless" astronaut in low Earth orbit, with an apparent weight of zero, has almost the same weight as he would have while standing on the ground; this is due to the force of gravity in low Earth orbit and on the ground being almost the same.

An object that rests on the ground is subject to a contact force exerted by the ground. The contact force acts only on the boundary of the object that is in contact with the ground. This ground reaction force is transferred into the body; the force of gravity on every part of the body is balanced by stress forces acting on that part. A "weightless" astronaut feels weightless due to the absence of these stress forces.By defining the apparent weight of an object in terms of contact forces, one can capture this effect of the stress forces. A common definition is "the force the body exerts on whatever it rests on."

On a July 9, 1946, suborbital V-2 rocket flight, fruit flies became the first living organisms to go to space, and on February 20, 1947, fruit flies safely returned from a suborbital space flight, which paved the way for human exploration. Years before sending mammals into space, such as the 1949 flight of the rhesus monkey Albert II, the Soviet space dogs, or humans, scientists studied Drosophila melanogaster (the common fruit fly) and its reactions to both radiation and space flight to understand the possible effects of space and a zero-gravity environment on humans. Starting in the 1910s, researchers conducted experiments on fruit flies because humans and fruit flies share many genes.

At the height of the Cold War and the Space Race, flies were sent on missions to space with great frequency, allowing scientists to study the nature of living and breeding in space. Scientists and researchers from the Soviet Union and the United States both used fruit flies for their research and missions. These flies were used to further the understanding of the effects of weightlessness on the cardiovascular system, the immune system, and the genes of astronauts.

In classical mechanics, free fall is any motion of a body where gravity is the only force acting upon it.A freely falling object may not necessarily be falling down in the vertical direction. If the common definition of the word "fall" is used, an object moving upwards is not considered to be falling, but using scientific definitions, if it is subject to only the force of gravity, it is said to be in free fall. The Moon is thus in free fall around the Earth, though its orbital speed keeps it in very far orbit from the Earth's surface.

In a roughly uniform gravitational field, gravity acts on each part of a body approximately equally. When there are no other forces, such as the normal force exerted between a body (e.g. an astronaut in orbit) and its surrounding objects, it will result in the sensation of weightlessness, a condition that also occurs when the gravitational field is weak (such as when the body is far away from any source of gravity).

Astronautical hygiene evaluates, and mitigates, hazards and health risks to those working in low-gravity environments. The discipline of astronautical hygiene includes such topics as the use and maintenance of life support systems, the risks of the extravehicular activity, the risks of exposure to chemicals or radiation, the characterization of hazards, human factor issues, and the development of risk management strategies. Astronautical hygiene works side by side with space medicine to ensure that astronauts are healthy and safe when working in space.

Bone marrow adipose tissue (BMAT), also referred to as marrow adipose tissue (MAT), is a type of adipose tissue (fat deposit) found within the bone marrow. BMAT increases in conditions associated with low bone density, such as osteoporosis, anorexia nervosa and caloric restriction, and skeletal weightlessness such as that occurring during spaceflight. It has also been linked to certain anti-diabetic therapies.

Conversely, BMAT decreases in conditions such as anaemia, leukaemia, and hypertensive heart failure; in response to hormones including oestrogen, leptin, and growth hormone; with exercise-induced weight loss or bariatric surgery; following chronic cold exposure; and after treatment with pharmacological agents such as bisphosphonates, teriparatide, and metformin.