Weightlessness


Weightlessness is the complete or near-complete absence of the sensation of weight, i.e., zero apparent weight. It is also termed zero g-force, or zero-g or, misleadingly, zero gravity.
Weight is a measurement of the force on an object at rest in a relatively strong gravitational field. These weight-sensations originate from contact with supporting floors, seats, beds, scales, and the like. 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.
When the gravitational field is non-uniform, a body in free fall experiences tidal forces and is not stress-free. Near a black hole, such tidal effects can be very strong, leading to spaghettification. In the case of the Earth, the effects are minor, especially on objects of relatively small dimensions and the overall sensation of weightlessness in these cases is preserved. This condition is known as microgravity, and it prevails in orbiting spacecraft. Microgravity environment is more or less synonymous in its effects, with the recognition that gravitational environments are not uniform and g-forces are never exactly zero.

Weightlessness in Newtonian mechanics

In Newtonian physics the sensation of weightlessness experienced by astronauts is not the result of there being zero gravitational acceleration, but of there being no g-force that an astronaut can feel because of the free-fall condition, and also there being zero difference between the acceleration of the spacecraft and the acceleration of the astronaut. Space journalist James Oberg explains the phenomenon this way:
From the perspective of an observer not moving with the object the force of gravity on an object in free fall is exactly the same as usual. A classic example is an elevator car where the cable has been cut and it plummets toward Earth, accelerating at a rate equal to the 9.81 meters per second per second. In this scenario, the perception of the gravitational force by someone inside the elevator is mostly, but not entirely, diminished; however the force is not exactly zero. Since gravity is a force directed towards the center of the Earth, two balls a horizontal distance apart would be pulled in slightly different directions and would come closer together as the elevator dropped. 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. These two second-order effects are examples of micro gravity.

Weightless and reduced weight environments

Reduced weight in aircraft

Airplanes have been used since 1959 to provide a nearly weightless environment in which to train astronauts, conduct research, and film motion pictures. Such aircraft are commonly referred by the nickname "Vomit Comet".
To create a weightless environment, the airplane flies in a parabolic arc, first climbing, then entering a powered dive. During the arc, the propulsion and steering of the aircraft are controlled to cancel the drag on the plane out, leaving the plane to behave as if it were free-falling in a vacuum.

NASA's Reduced Gravity Aircraft

Versions of such airplanes have been operated by NASA's Reduced Gravity Research Program since 1973, where the unofficial nickname originated. NASA later adopted the official nickname 'Weightless Wonder' for publication. NASA's current Reduced Gravity Aircraft, "Weightless Wonder VI", a McDonnell Douglas C-9, is based at Ellington Field, near Lyndon B. Johnson Space Center.
NASA's Microgravity University - Reduced Gravity Flight Opportunities Plan, also known as the Reduced Gravity Student Flight Opportunities Program, allows teams of undergraduates to submit a microgravity experiment proposal. If selected, the teams design and implement their experiment, and students are invited to fly on NASA's Vomit Comet.

European Space Agency A310 Zero-G

The European Space Agency flies parabolic flights on a specially modified Airbus A310-300 aircraft to perform research in microgravity. Along with the French CNES and the German DLR, they conduct campaigns of three flights over consecutive days, with each flight's about 30 parabolae totalling about 10 minutes of weightlessness. These campaigns are currently operated from Bordeaux - Mérignac Airport by , a subsidiary of CNES; the aircraft is flown by test pilots from DGA Essais en Vol.
, the ESA has flown 52 scientific campaigns and also 9 student parabolic flight campaigns. Their first Zero-G flights were in 1984 using a NASA KC-135 aircraft in Houston, Texas. Other aircraft used include the Russian Ilyushin Il-76 MDK before founding Novespace, then a French Caravelle and an Airbus A300 Zero-G.

Commercial flights for public passengers

Novespace created Air Zero G in 2012 to share the experience of weightlessness with 40 public passengers per flight, using the same A310 ZERO-G as for scientific experiences. 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. Jean-François Clervoy, Chairman of Novespace and ESA astronaut, flies with these one-day astronauts on board A310 Zero-G. After the flight, he explains the quest of space and talks about the 3 space travels he did along his career. The aircraft has also been used for cinema purposes, with Tom Cruise and Annabelle Wallis for the Mummy in 2017.
The Zero Gravity Corporation operates a modified Boeing 727 which flies parabolic arcs to create 25–30 seconds of weightlessness.

Ground-based drop facilities

Ground-based facilities that produce weightless conditions for research purposes are typically referred to as drop tubes or drop towers.
NASA's Zero Gravity Research Facility, located at the Glenn Research Center in Cleveland, Ohio, is a 145 m vertical shaft, largely below the ground, with an integral vacuum drop chamber, in which an experiment vehicle can have a free fall for a duration of 5.18 seconds, falling a distance of 132 m. The experiment vehicle is stopped in approximately 4.5 m of pellets of expanded polystyrene, experiencing a peak deceleration rate of 65 g.
Also at NASA Glenn is the 2.2 Second Drop Tower, which has a drop distance of 24.1 m. Experiments are dropped in a drag shield in order to reduce the effects of air drag. The entire package is stopped in a 3.3 m tall air bag, at a peak deceleration rate of approximately 20 g. While the Zero Gravity Facility conducts one or two drops per day, the 2.2 Second Drop Tower can conduct up to twelve drops per day.
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.
Other drop facilities worldwide include:
Another ground-based approach to simulate weightlessness for biological samples 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.

Neutral buoyancy

Orbits

On the International Space Station. The symbol for microgravity, μg, was used on the insignias of Space Shuttle flights STS-87 and STS-107, because these flights were devoted to microgravity research in low Earth orbit.

Sub-Orbital flights

Over the years, biomedical research on the implications of space flight has become more prominent in evaluating possible pathophysiological changes in humans. Sub-orbital flights seize the approximated weightlessness, or μg, in the low Earth orbit and represent a promising research model for short-term exposure. Examples of such approaches are the MASER, MAXUS, or TEXUS program run by the Swedish Space Corporation and the European Space Agency.

Orbital Motion

is a form of free fall. Objects in orbit are not perfectly weightless due to several effects:
  • Effects depending on relative position in the spacecraft:
  • *Because the force of gravity decreases with distance, objects with non-zero size will be subjected to a tidal force, or a differential pull, between the ends of the object nearest and furthest from the Earth. In a spacecraft in low Earth orbit, the centrifugal force is also greater on the side of the spacecraft furthest from the Earth. At a 400 km LEO altitude, the overall differential in g-force is approximately 0.384 μg/m.
  • *Gravity between the spacecraft and an object within it may make the object slowly "fall" toward a more massive part of it. The acceleration is 0.007 μg for 1000 kg at 1 m distance.
  • Uniform effects :
  • *Though extremely thin, there is some air at orbital altitudes of 185 to 1,000 km. This atmosphere causes minuscule deceleration due to friction. This could be compensated by a small continuous thrust, but in practice the deceleration is only compensated from time to time, so the tiny g-force of this effect is not eliminated.
  • *The effects of the solar wind and radiation pressure are similar, but directed away from the Sun. Unlike the effect of the atmosphere, it does not reduce with altitude.
  • Other Effects:
  • *Routine crew activity: Due to the conservation of momentum, any crew member aboard a spacecraft pushing off a wall causes the spacecraft to move in the opposite direction.
  • *Structural Vibration: Stress enacted on the hull of the spacecraft results in the spacecraft bending, causing apparent acceleration.