Gravity-gradient stabilization
Gravity-gradient stabilization (a.k.a "tidal stabilisation") is a method of stabilizing artificial satellites or space tether
Space tether
Space tethers are cables, usually long and very strong, which can be used for propulsion, stabilization, or maintaining the formation of space systems by determining the trajectory of spacecraft and payloads...

s in a fixed orientation using only the orbited body's mass distribution and the Earth's gravitational field. The main advantage over using active stabilization with propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

s, gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...

s or reaction wheels is the low use of power and resources. It was first used for low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

 and tested unsuccessfully for geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...

 in the Applications Technology Satellites ATS-2, ATS-4 and ATS-5 from 1966 until 1969.

The principle is to use the Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's gravitational field
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...

 and tidal forces to keep the spacecraft aligned in the desired orientation. The gravity of the Earth decreases according the inverse-square law
Inverse-square law
In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity....

, and by extending the long axis perpendicular to the orbit, the "lower" part of the orbiting structure will be more attracted to the Earth. The effect is that the satellite will tend to align its axis of minimum moment of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...


An example of gravity-gradient stabilization was demonstrated during NASA's TSS-1R mission. Just prior to tether separation, the tension on the tether was about 65 N (14.6 lbs).

External links

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