From the reference of
wiki, “Artificial gravity (sometimes referred to
as pseudogravity) is the creation of an inertial force that mimics the effects
of a gravitational force, usually by rotation. Artificial gravity,
or rotational gravity, is thus the appearance of a centrifugal force in a rotating frame of reference (the transmission of centripetal
acceleration via normal force in the non-rotating
frame of reference), as opposed to the force experienced in linear
acceleration,
which by the equivalence
principle is
indistinguishable from gravity. In a more general sense, "artificial
gravity" may also refer to the effect of linear acceleration, e.g. by
means of a rocket engine. “
Artificial
gravity is defined as the simulation of gravitational forces aboard a space
vehicle that is in orbit (free fall) or in transit to another planet. The term
artificial gravity is reserved for a spinning spacecraft or a centrifuge within
the spacecraft such that a gravity-like force results. An important point is
that artificial gravity is not gravity at all. Rather, it is an inertial force
that is indistinguishable from the normal gravity experience on Earth in terms
of its action on any mass. A centrifugal force proportional to the mass that is
being accelerated centripetally in a rotating device is experienced rather than
a gravitational pull. Although the effect of artificial gravity on a human body
differs from that of true gravity, which will be discussed in some detail in
subsequent sections, the effects are equivalent for any given mass. Therefore,
one can think of artificial gravity as the imposition of accelerations on a
body to compensate for the forces that are absent in the microgravity of
spaceflight.
Mechanism
From the point of view of people rotating with the habitat,
artificial gravity by rotation behaves in some ways similarly to normal gravity
but with the following differences:
·
Centrifugal force varies with distance: Unlike real gravity,
which pulls towards a center of the planet, the apparent centrifugal force felt
by observers in the habitat pushes radially outward from the center, and
assuming a fixed rotation rate (constant angular velocity), the centrifugal
force is directly proportional to the distance from the center of the habitat.
With a small radius of rotation, the amount of gravity felt at one's head would
be significantly different from the amount felt at one's feet. This could make
movement and changing body position awkward. In accordance with the physics involved, slower rotations or larger
rotational radii would reduce or eliminate this problem. Similarly the linear
velocity of the habitat should be significantly higher than the relative
velocities with which an astronaut will change position within it. Otherwise
moving in the direction of the rotation will increase the felt gravity (while
moving in the opposite direction will decrease it) to the point that it should
cause problems.
·
The Coriolis effect gives
an apparent force that acts on objects that move relative to a rotating
reference frame. This apparent force acts at right angles to the motion and the
rotation axis and tends to curve the motion in the opposite sense to the
habitat's spin. If an astronaut inside
a rotating artificial gravity environment moves towards or away from the axis
of rotation, they will feel a force pushing them towards or away from the
direction of spin. These forces act on the inner ear and can cause dizziness, nausea and
disorientation. Lengthening the period of rotation (slower spin rate) reduces
the Coriolis force and its effects. It is generally believed that at 2 rpm or less, no adverse effects from the Coriolis forces will
occur, although humans have been shown to adapt to rates as high as 23 rpm.[4] It
is not yet known whether very long exposures to high levels of Coriolis forces
can increase the likelihood of becoming accustomed. The nausea-inducing effects
of Coriolis forces can also be mitigated by restraining movement of the head.
This form of artificial gravity has additional engineering
issues:
·
Kinetic energy and angular
momentum: Spinning up (or down) parts or all of the habitat requires
energy, while angular momentum must be conserved. This would require a
propulsion system and expendable propellant, or could be achieved without expending
mass, by an electric motor and a counterweight, such
as a reaction wheel or possibly another
living area spinning in the opposite direction.
·
Extra strength is needed in the structure to keep it from flying
apart because of the rotation. However, the amount of structure needed over and
above that to hold a breathable atmosphere (10 tons force per square meter at 1
atmosphere) is relatively modest for most structures.
·
If parts of the structure are intentionally not spinning, friction and
similar torques will cause the rates of spin to converge (as well as
causing the otherwise stationary parts to spin), requiring motors and power to
be used to compensate for the losses due to friction.
·
A traversable interface between parts of the station spinning
relative to each other requires large vacuum-tight axial seals.
How to
Generate Artificial Gravity
Artificial
gravity can be produced in a number of ways. In the following sections, we
discuss several interesting mechanisms that could, in theory, be used to
develop artificial gravity. However, the practical limitations imposed on
spacecraft mass, power, and cost means that achieving some of these designs
must wait until technology catches up with our imagination. These sections were
compiled using information from http://en.wikipedia.org/wiki/Artificial_gravity.
Manned
spaceflight
The engineering challenges of creating a rotating
spacecraft are comparatively modest to any other proposed approach.[original research?] Theoretical spacecraft designs using
artificial gravity have a great number of variants with intrinsic problems and
advantages. The formula for the centripetal force implies
that the radius of rotation grows with the square of the rotating spacecraft
period, so a doubling of the period requires a fourfold increase in the radius
of rotation. For example, to produce standard gravity, ɡ0 = 9.80665 m/s2 with
a rotating spacecraft period of 15 s, the radius of rotation would have to
be 56 m (184 ft), while a period of 30 s would require it to be
224 m (735 ft). To reduce mass, the support along the diameter could
consist of nothing but a cable connecting two sections of the spaceship. Among
the possible solutions include a habitat module and a counterweight consisting
of every other part of the spacecraft, alternatively two habitable modules of
similar weight could be attached.
Whatever design is chosen, it would be necessary for
the spacecraft to possess some means to quickly transfer ballast from one section to another, otherwise, even
small shifts in mass could cause a substantial shift in the spacecraft's axis,
which would result in a dangerous "wobble." One possible solution
would be to engineer the spacecraft's plumbing system to serve this purpose,
using drinking water and/or
wastewater as the ballast.
It is
not yet known whether exposure to high gravity for short periods is as
beneficial to health as continuous exposure to normal gravity. It is also not
known how effective low levels of gravity would be at countering the adverse
effects on the health of weightlessness. Artificial gravity at 0.1g and a rotating spacecraft
period of 30 s would require a radius of only 22 m (72 ft).
Likewise, at a radius of 10 m, a period of just over 6 s would be
required to produce standard gravity (at the hips; gravity would be 11% higher
at the feet), while 4.5 s would produce 2g. If brief exposure to high gravity can negate the harmful
effects of weightlessness, then a small centrifuge could be used as an exercise
area.
Gemini
mission
The Gemini 11 mission attempted to produce
artificial gravity by rotating the capsule around the Agena Target Vehicle to
which it was attached by a 36-meter tether. They were able to generate a small
amount of artificial gravity, about 0.00015 g, by firing their side thrusters
to slowly rotate the combined craft like a slow-motion pair of bolas.[5] The
resultant force was too small to be felt by either astronaut, but objects were
observed moving towards the "floor" of the capsule.[6] The Gemini 8 mission
achieved artificial gravity for a few minutes. This, however, was due to an
electrical fault causing continuous firing of one thruster. The acceleration
forces upon the crew were high (around 4 g), and the mission had to be urgently
terminated.
In
conclusion, the principle is to
generate a force, often
termed centrifugal force or
effect to "pin" the astronauts to the outside edge of the spinning
section of the station. By varying the radius and rotating speed you can directly affect the simulated "gravity" force.
Hence if we can overcome the effect of earth’s
gravity by any means we can create artificial gravity.
Beside centrifugal force we can use strong electromagnetic
force too. It seems to work but not to heavy bodies. For heavier mass upto
human body we have to maintain greater magnetic force which might be
challenging to create in normal house laboratory.
