A common misconception about astronauts orbiting the earth is that they are in a gravity-free environment. Unfortunately, according to Newtonian physics, gravity is everywhere present in the universe. Generally, the phenomenon of weightlessness is associated with the term, "zero-G" which would apply to situations in which there is an absence of forces on a body.

Weight as we know it is the force on an object at rest in a relatively strong gravitational field. Earth's gravitational force at sea level exerts a force on every object that has mass. For example, a one-kilogram mass weighs about 2.2 pounds at sea level. If this one-kilogram mass were dropped from a height while sitting on a scale, its weight cannot be measured because the gravitational force is accelerating the mass downward at a rate of one-G.

The same mass, placed in a satellite travelling in a circular orbit about the Earth is, in fact, in free-fall as it travels about the planet. In other words, the force of gravity is balanced by the centrifugal force generated by the orbital speed as it circles Earth. This situation is not an example zero-G, but simply a case balanced forces.

In those cases where the gravitational field is not uniform, bodies in free fall experienceperturbation effects, sometimes called "microgravity" forces. In a large space station such as the International Space Station (ISS) in a circular orbit, gravity and centrifugal forces are exactly balanced to create a "zero-G" point, but only at the center of mass of the station.

An astronaut stationed at a point above the center of mass, will experience a slight force away from Earth, while an astronaut stationed below the center of mass will experience a slight force toward Earth. Such forces are referred to as microgravity forces. These are induced by the fact that the centrifugal force is slightly greater above the center mass while gravity forces are slightly higher below the center of mass.

The Earth's gravitation field is not uniform. Our planet's distribution of mass is not exactlyspherically symmetric, causing orbital shifts over time. If it were a perfect sphere of uniform mass density, or whose density varies solely with distance from the center, it would be sphericallysymmetry.

In such a case the gravitational field would act as a point mass with uniform magnitude in any direction and gravitational forces dependent only on the radial distance from the center. However, Earth is rotating and is not spherically symmetric. It is slightly flatter at the poles while bulging at the Equator, i. e., it is an oblate spheroid. The result is slight force disturbances on the orbits of satellite circling the planet. Such forces have been used to advantage in creating "sun synchronous" orbits" (SSOs).

SSOs are sometimes called "heliosynchronous orbits" and they are nearly polar orbits about the planet. In such orbits satellites pass over any given point of the Earth's surface at the same local mean solar time. This is caused by the "precessional" motion through one complete revolution each year, maintaining the same angular relationship with the Sun. As a result most Earth-observing satellites are placed in SSOs.

As a result SSOs Are quite popular for commercial, civil and national security spacecraft. In fact, a major challenge of a space traffic management program will be maintaining safe separation of satellites on SSOs.

Related Links
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The Physics of Time and Space

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