Gravity-gradient stabilization (a.k.a. "tidal stabilization") is a method of stabilizing

artificial satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisoto ...

s or

space tether Space tethers are long cables which can be used for propulsion, momentum exchange, stabilization and attitude control, or maintaining the relative positions of the components of a large dispersed satellite/spacecraft sensor system. Depending on t ...

s in a fixed orientation using only the orbited body's mass distribution and gravitational field. The main advantage over using active stabilization with

propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the ...

s, gyroscopes or

reaction wheel A reaction wheel (RW) is used primarily by spacecraft for three-axis attitude control, and does not require rockets or external applicators of torque. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be ...

s is the low use of power and resources. It can also reduce or prevent the risk of propellant contamination of sensitive components. The idea is to use the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

's gravitational field and tidal forces to keep the spacecraft aligned in the desired orientation. The

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...

of the Earth decreases according to the

inverse-square law In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be unde ...

, 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 The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular accele ...

vertically. The first experimental attempt to use the technique on a human spaceflight was performed on September 13, 1966, on the US

Gemini 11 Gemini 11 (officially Gemini XI) With Gemini IV, NASA changed to Roman numerals for Gemini mission designations. was the ninth crewed spaceflight mission of NASA's Project Gemini, which flew from September 12 to 15, 1966. It was the 17th crewed ...

mission, by attaching the Gemini spacecraft to its Agena target vehicle by a tether. The attempt was a failure, as insufficient gradient was produced to keep the tether taut. The technique was first successfully used in a near-

geosynchronous orbit A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbit ...

on the Department of Defense Gravity Experiment (DODGE) satellite in July 1967.Gunter's Space Page: DODGE
/ref> It was first used for

low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...

and tested unsuccessfully for

in the

Applications Technology Satellite The Applications Technology Satellites (ATS) were a series of experimental satellites launched by NASA, under the supervision of, among others, Wernher von Braun. The program was launched in 1966 to test the feasibility of placing a satellite int ...

ATS-2 ATS-2 (Applications Technology Satellite) was a communications satellite launched by NASA on April 6, 1967, on an Atlas-Agena D rocket from Cape Canaveral. Objectives The ATS-2 had the following objectives: test new concepts in spacecraft ...

ATS-4 ATS-4 (Applications Technology Satellite) also known as ATS-D was a communications satellite launched by NASA on August 10, 1968 from Cape Canaveral through an Atlas-Centaur rocket. Objectives The objective of ATS-4 was to investigate the poss ...

and ATS-5 from 1966 until 1969. The lunar orbiter Explorer 49 launched in 1973 was gravity gradient oriented (Z axis parallel to local vertical). Long Duration Exposure Facility (LDEF) used this method for 3-axis stabilization; yaw about the vertical axis was stabilized. An example of gravity-gradient stabilization was attempted during NASA's TSS-1 mission in July 1992. The project failed because of tether deployment problems. In 1996 another mission, TSS-1R, was attempted but failed when the tether broke. Just prior to tether separation, the tension on the tether was about 65 N (14.6 lbs).NASA
TSS-1R Mission Failure Investigation Board
Final Report, May 31, 1996 (accessed 7 April 2011)

External links

NASA on ATS-2A new kind of Earth Sensor using a proof mass on a MEMS created by EPFL students

References

Spacecraft attitude control Spacecraft propulsion Spacecraft components Tidal forces {{spacecraft-stub

See also

External links

References