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Gravity-gradient Stabilization
Gravity-gradient stabilization (a.k.a. "tidal stabilization") is a method of stabilizing artificial satellites or space tethers in a fixed orientation using only the orbited body's mass distribution and gravitational field. The main advantage over using active stabilization with propellants, gyroscopes or reaction wheels 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's gravitational field and tidal forces to keep the spacecraft aligned in the desired orientation. The gravity of the Earth decreases according to the inverse-square law, 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 vertically. The first experimental attempt to use the technique on a human spaceflight was performed on Sep ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 radioisotope thermoelectric generators (RTGs). Most satellites also have a method of communication to ground stations, called transponders. Many satellites use a standardized bus to save cost and work, the most popular of which is small CubeSats. Similar satellites can work together as a group, forming constellations. Because of the high launch cost to space, satellites are designed to be as lightweight and robust as possible. Most communication satellites are radio relay stations in orbit and carry dozens of transponders, each with a bandwidth of tens of megahertz. Satellites are placed from the surface to orbit by launch vehicles, high enough to avoid orbital decay by the atmosphere. Satellites can then change or maintain the orbit by propuls ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 more than about one-third of the radius of Earth. The term ''LEO region'' is also used for the area of space below an altitude of (about one-third of Earth's radius). Objects in orbits that pass through this zone, even if they have an apogee further out or are sub-orbital, are carefully tracked since they present a collision risk to the many LEO satellites. All crewed space stations to date have been within LEO. From 1968 to 1972, the Apollo program's lunar missions sent humans beyond LEO. Since the end of the Apollo program, no human spaceflights have been beyond LEO. Defining characteristics A wide variety of sources define LEO in terms of altitude. The altitude of an object in an elliptic orbit can vary significantly along the orbit. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spacecraft Propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry. Several methods of pragmatic spacecraft propulsion have been developed each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall-effect thrusters (two different types of electric propulsion) to great success. Hypothetical in-space propulsion technologies describe the p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spacecraft Attitude Control
Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle/satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc. Controlling vehicle attitude requires sensors to measure vehicle orientation, actuators to apply the torques needed to orient the vehicle to a desired attitude, and algorithms to command the actuators based on (1) sensor measurements of the current attitude and (2) specification of a desired attitude. The integrated field that studies the combination of sensors, actuators and algorithms is called guidance, navigation and control (GNC). Overview A spacecraft's attitude must typically be stabilized and controlled for a variety of reasons. It is often needed so that the spacecraft high-gain antenna may be accurately pointed to Earth for communications, so that onboard experiments may accomplish precise pointing for accurate collectio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tidal Locking
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon. Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking. The effect arises between two bodies when their gravitational interaction sl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 the mission objectives and altitude, spaceflight using this form of spacecraft propulsion is theorized to be significantly less expensive than spaceflight using rocket engines. Main techniques Tether satellites might be used for various purposes, including research into tether propulsion, tidal stabilization and orbital plasma dynamics. Five main techniques for employing space tethers are in development: ;Electrodynamic tethers Electrodynamic tethers are primarily used for propulsion. These are conducting tethers that carry a current that can generate either thrust or drag from a planetary magnetic field, in much the same way as an electric motor does. ;Momentum exchange tethers These can be either rotating tethers, or non-rotating ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gravity Gradiometry
Gravity gradiometry is the study and measurement of variations ( anomalies) in the Earth's gravity field. The gravity gradient tensor is the spatial rate of change of gravitational acceleration; as acceleration is a vector quantity, with magnitude and three-dimensional direction, the full gravity gradient is a 3x3 tensor. Gravity gradiometry is used by oil and mineral prospectors to measure the density of the subsurface, effectively by measuring the rate of change of gravitational acceleration due to underlying rock properties. From this information it is possible to build a picture of subsurface anomalies which can then be used to more accurately target oil, gas and mineral deposits. It is also used to image water column density, when locating submerged objects, or determining water depth (bathymetry). Physical scientists use gravimeters to determine the exact size and shape of the earth and they contribute to the gravity compensations applied to inertial navigation systems. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Long Duration Exposure Facility
NASA's Long Duration Exposure Facility, or LDEF (pronounced "eldef"), was a school bus-sized cylindrical facility designed to provide long-term experimental data on the outer space environment and its effects on space systems, materials, operations and selected spores' survival. It was placed in low Earth orbit by in April 1984. The original plan called for the LDEF to be retrieved in March 1985, but after a series of delays it was eventually returned to Earth by in January 1990. It successfully carried science and technology experiments for about 5.7 years that have revealed a broad and detailed collection of space environmental data. LDEF's 69 months in space provided scientific data on the long-term effects of space exposure on materials, components and systems that has benefited NASA spacecraft designers to this day. History Researchers identified the potential of the planned Space Shuttle to deliver a payload to space, leave it there for a long-term exposure to the harsh o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Explorer 49
Explorer 49 (also called Radio Astronomy Explorer-2, RAE-B) was a NASA satellite launched on 10 June 1973, for long wave radio astronomy research. It had four X-shaped antenna elements, which made it one of the largest spacecraft ever built. Mission The Radio Astronomy Explorer B (RAE-B) mission was the second of a pair of RAE satellites. It was placed into lunar orbit to provide radio astronomical measurements of the planets, the Sun, and the galaxy over the frequency range of 25-kHz to 13.1-MHz. The experiment complement consisted of two Ryle-Vonberg radiometers (nine channels each), three swept-frequency burst receivers (32 channels each), and an impedance probe for calibration. The experiment antenna package, made of BeCu, consisted of very long travelling wave antennas forming an X configuration: a upper V-antenna pointed away from the Moon; a lower V-antenna pointed toward the Moon; and a dipole antenna parallel to the lunar surface. There was also a boron libratio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 possibilities of a gravity gradient stabilization system (the method of stabilizing artificial satellites). Features The satellite has a cylindrical shape with a diameter and height (about considering the motor cover) with the surface covered by solar panels, and stabilized by gravity gradient. Instruments A total of four experiments were conducted during the mission: * Microwave Transponder * Gravity Gradient Stabilization * Image Orthicon (Day/Night) Camera * Ion Thruster Mission The Atlas and Centaur stages performed satisfactorily and placed the Centaur/ATS-4 in an elliptical parking orbit, the Centaur stage, however, failed to re-ignite after a 61-minute coast. The failure was determined to be caused freezing of the hydrogen peroxide ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 design, propulsion and stabilization; capture high quality images of cloud cover; collect data measurements in an aerospace environment; and test improved communication systems. Features The satellite had a cylindrical shape with a diameter and a height of . After including the motor cover, the satellite was about tall. The surface of the satellite was covered by solar panels, and it utilized gravity-gradient stabilization for control. Experiments The following 12 experiments were conducted: * Radio astronomy * Magnetospheric electric fields * Electron magnetic spectrometer deflection * Particle telescope * Omnidirectional proton and electron detectors * Very low frequency (VLF) receiver * Earth's albedo ( DoD) * Microwave comm ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
