A Primer on Artificial Gravity (AG)
What is Gravity?
A fundamental law of nature is that any two objects with mass are attracted to each other. A planet such as earth has a huge mass. We, as living organisms on the surface of the earth, are constantly pulled toward the center of the earth, downward. Collectively, every organ, cell and molecule of our bodies are also pulled toward the center of the earth. This “pull” is a form of acceleration known as gravity. Gravity is measured in “g’s”. By convention, Earth’s pull is defined as 1g.
Why is There No Gravity in Orbital Spaceflight?
Actually, there is gravity in orbital spaceflight. However, the orbiting object can be thought of as a centrifuge, with accelerations directed away from the center of the earth, essentially canceling out earth’s gravity, so the net effect is “microgravity” or virtual weightlessness. On the surface of the moon, also in orbital flight about the earth, one only feels moon’s gravity, about 1/6th of Earth’s. Similarly, we don’t feel the Sun’s gravity as the Earth is orbiting the Sun.
What is Artificial Gravity (AG)?
The concept of putting a crewmember on a centrifuge has come to be known as “Artificial Gravity”. There’s actually nothing artificial about it. As gravity is a type of acceleration, an alternative type of acceleration occurs when there is a change in speed or direction. Einstein showed there is actually no physical (or presumably physiological) distinction between these forms of acceleration.
When we drive in a car and step hard on the accelerator (note that term), we are pushed back into the seat. This is because there is a change in our speed. Car advertisers boast 0 to 60 mph in 6.8 seconds. Note this is simply a change in speed in a defined interval of time-the very definition of acceleration. Stepping hard on the brakes pulls us forward, a deceleration (negative acceleration).
When we drive in a car and make a right turn, even maintaining a constant speed, we are pulled to the left. This change in direction is yet another form of acceleration.
Virtual reality simulators move within earth’s gravity and fool us into thinking we are accelerating or changing directions when neither have actually occurred. Our bodies do not distinguish between Earth’s gravity acceleration and that of changing speed or direction.
The Space Cycle, like other centrifuges, simply provides a continuous change in direction, so as to provide continuous acceleration, as we have here on Earth. More correctly, the Space Cycle provides an alternative acceleration source. However, the term “Artificial Gravity” seems to be here to stay.
Why Artificial Gravity?
Every organism that has ever lived on Earth has known continuous 1g. It seems intuitive that, both at the cellular and organism level, we would adapt to and even rely on this pervasive and intrinsic component of our environment. Just as we require specific parameters of temperature, oxygen level and atmospheric pressure to survive, it has become apparent that specific parameters of acceleration are necessary for normal cell and life functions.
To our knowledge, a cellular structure that senses acceleration/gravity has not yet been identified. However, a seed (a single cell) in the ground knows to send the root down and the stalk up. Space based experiments have demonstrated basic cellular functions such as mitosis are hindered by lack of gravity. Clearly, there are acceleration/gravity receptor and signal mechanisms within single cells.
How Much Artificial Gravity?
The “prescription” of how much acceleration/gravity over what period of time and in what direction is required for maintaining normal health is currently unknown and logistically very difficult to determine. A rotating spacecraft that provides constant 1g acceleration would be ideal, but that is not yet on the drawing board. An onboard device used intermittently by crewmembers seems a logical intermediate solution.
What Doesn’t Work and Why
Exercise as a countermeasure was introduced in the days of the Gemini program. It has taken on various ingenious forms of elastic, pneumatic, mechanical, hydraulic and electrical devices. None have been demonstrated effective and at best, only partially protect crewmembers. However, exercise is still considered an integral part of any effective countermeasure, as we know it works on Earth-in 1g.
Devices have been created that can apply a force (not an acceleration) during exercise as countermeasures. Crewmembers wear a harness attached to an exercise bike or treadmill and are held “down” during exercise. Lower Body Negative Pressure, where the lower half of the body is in a chamber with decreased air pressure, can “suck” the crewmember “down” to a treadmill and provide simultaneous force with exercise.
Although clever, these devices predictably fail to achieve the desired effect, simply because as Newton formulated, Force = mass x acceleration. Force ? acceleration. Force and acceleration are fundamentally distinct physics concepts. Although related, force and acceleration are not interchangeable. Accelerations, produced either by Earth gravity or a centrifuge, are interchangeable. Elastic devices can effectively create force, but not sustained acceleration.
For Star Wars, its fine to utter “May the Force be With You”, but for real human spaceflight, the true axiom is “May the Acceleration be With You”.
Surface versus Body Forces
It may be that acceleration is the most efficient way to load the body. A 150 pound person can easily run, jump, climb, stand and walk. However, if carrying a 30 (20% body weight) pound back pack, these tasks are far more difficult. Doing such activities at 1.2g would be much better tolerated. Straps and harnesses apply forces through the surface, whereas accelerations act on every cell and molecule of the body and infinitely evenly distributed.
Consider an ordinary sealed glass jar containing water and a penny, lying on its side. Keeping the jar horizontal, one can squeeze the top and bottom with tremendous force, yet the penny will not move. Then tie a string to the top and spin the jar, keeping it horizontal. Immediately the penny travels to the bottom. This is because surface forces act on the surface and supporting parts of the structure, but body forces, induced by acceleration, act on every part of the structure.
Is There a Way to Simulate Microgravity on Earth?
Yes, but only for very brief intervals. There are no “anti-gravity” machines.
During free fall from a height, momentary weightlessness results. When an elevator begins its decent, we feel momentarily lighter. However, once the elevator is at constant speed, normal weight is felt. Skydivers experience weightlessness, but only until the parachute opens.
Special aircraft flying a parabolic flight profile can simulate microgravity, but only for about 30 seconds at a time.
The adverse physiologic consequences of spaceflight become relevant after about 2 weeks. For Shuttle missions, this is a trivial issue. For Space Station or interplanetary flight, it becomes critical. Earth-based models of microgravity are far to short-lived to study the effects.
Earth Based Countermeasure Studies
Earth-based analogs of microgravity have included bed-rest, fluid immersion, limb-suspension. These models can reduce muscle use and induce muscle and bone loss. However, test subjects remain in a 1g environment 100% of the time. Therefore, these studies are profoundly flawed. We use these models as they are the best available.
When the test subjects are allowed periodic exercise when at bed rest, much or all of the adverse consequences are eliminated. This is logical, as we know that exercise in a 1g environment is effective. However, it is foolhardy to extrapolate that the same exercise regimen will be effective in a 0g environment.