They are usually used for high latitude communications particularly over Russia and China. The orbits are elliptical, so that they appear to hover above the earth in the same point for most of the day. They move in a figure of eight pattern which is centered on a fixed longitude.
The first Geo-synchronous satellite was Syncom 2. Most communications satellites are geostationary rather than geosynchronous. They are very similar in that their orbit also takes 24 hours to complete, however, geostationary satellites do not move. Video and TV communications use these as otherwise we would have to constantly be readjusting our satellite dishes.
They generally weigh several thousand kilograms. Launch vehicles such as the Arianne Rocket or the Space Shuttle are used to put them in orbit. Once they are in place most satellites will take their power from the sun using huge solar panels, those who are being sent deeper into space will often carry a nuclear power supply. They commonly move in a geostationary orbit. A remote Sensing instrument collects information about an object.
Global Positioning System GPS consists of up to 32 medium Earth orbit satellites in six different orbital planes, with the exact number of satellites varying as older satellites are retired and replaced. Operational since and globally available since , GPS is currently the world's most utilized satellite navigation system. Most satellites also work in this zone. Geostationary or geosynchronous orbit is the best spot for communications satellites to use, however. This is a zone above Earth's equator at an altitude of 35, km 22, mi.
At this altitude, the rate of "fall" around the Earth is about the same as Earth's rotation, which allows the satellite to stay above the same spot on Earth almost constantly.
The satellite thus keeps a perpetual connection with a fixed antenna on the ground, allowing for reliable communications. When geostationary satellites reach the end of their life, protocol dictates they're moved out of the way for a new satellite to take their place. That's because there is only so much room, or so many "slots" in that orbit, to allow the satellites to operate without interference.
While some satellites are best used around the equator, others are better suited to more polar orbits — those that circle the Earth from pole to pole so that their coverage zones include the north and south poles.
Examples of polar-orbiting satellites include weather satellites and reconnaissance satellites. There are an estimated half-million artificial objects in Earth orbit today , ranging in size from paint flecks up to full-fledged satellites — each traveling at speeds of thousands of miles an hour.
Only a fraction of these satellites are useable, meaning that there is a lot of "space junk" floating around out there. With everything that is lobbed into orbit, the chance of a collision increases.
Space agencies have to consider orbital trajectories carefully when launching something into space. Agencies such as the United States Space Surveillance Network keep an eye on orbital debris from the ground, and alert NASA and other entities if an errant piece is in danger of hitting something vital.
This means that from time to time, the ISS needs to perform evasive maneuvers to get out of the way. Collisions still occur, however. One of the biggest culprits of space debris was the leftovers of a anti-satellite test performed by the Chinese, which generated debris that destroyed a Russian satellite in Also that year, the Iridium 33 and Cosmos satellites smashed into each other, generating a cloud of debris.
NASA, the European Space Agency and many other entities are considering measures to reduce the amount of orbital debris. Some suggest bringing down dead satellites in some way , perhaps using a net or air bursts to disturb the debris from its orbit and bring it closer to Earth.
Others are thinking about refueling dead satellites for reuse, a technology that has been demonstrated robotically on the ISS. Most planets in our solar system have natural satellites, which we also call moons.
For the inner planets: Mercury and Venus each have no moons. Earth has one relatively large moon, while Mars has two asteroid-sized small moons called Phobos and Deimos. Phobos is slowly spiralling into Mars and will likely break apart or fall into the surface in a few thousand years.
Beyond the asteroid belt, are four gas giant planets that each have a pantheon of moons. As of late , Jupiter has 69 known moons, Saturn has 53, Uranus has 27 and Neptune has 13 or New moons are occasionally discovered — mainly by missions either past or present, as we can analyze old pictures or by performing fresh observations by telescope. Saturn is a special example because it is surrounded by thousands of small objects that form a ring visible even in small telescopes from Earth.
Scientists watching the rings close-up over 13 years, during the Cassini mission , saw conditions in which new moons might be born. Scientists were particularly interested in propellers, which are wakes in the rings created by fragments in the rings. Just after Cassini's mission ended in , NASA said it's possible the propellers share elements of planet formation that takes place around young stars' gassy discs.
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