Geostationary satellites take 24 hours to orbit the Earth, so the satellite appears to remain in the same part of the sky when viewed from the ground. Satellites in geostationary orbits are used for communications and satellite television. For an object to remain in a steady, circular orbit it must be travelling at the right speed. The diagram shows a satellite orbiting the Earth. The third Lagrange point is opposite the Earth on the other side of the Sun so that the Sun is always between it and Earth.
A satellite in this position would not be able to communicate with Earth. Closer to the Earth, satellites in a medium Earth orbit move more quickly.
Two medium Earth orbits are notable: the semi-synchronous orbit and the Molniya orbit. The semi-synchronous orbit is a near-circular orbit low eccentricity 26, kilometers from the center of the Earth about 20, kilometers above the surface. A satellite at this height takes 12 hours to complete an orbit.
As the satellite moves, the Earth rotates underneath it. In hours, the satellite crosses over the same two spots on the equator every day. This orbit is consistent and highly predictable. The second common medium Earth orbit is the Molniya orbit. Invented by the Russians, the Molniya orbit works well for observing high latitudes. The Molniya orbit offers a useful alternative. The Molniya orbit is highly eccentric: the satellite moves in an extreme ellipse with the Earth close to one edge.
As it moves away, its speed slows, so it spends more time at the top of its orbit farthest from the Earth. A satellite in a Molniya orbit takes 12 hours to complete its orbit, but it spends about two-thirds of that time over one hemisphere. Like a semi-synchronous orbit, a satellite in the Molniya orbit passes over the same path every 24 hours. This type of orbit is useful for communications in the far north or south.
Most scientific satellites and many weather satellites are in a nearly circular, low Earth orbit. Therefore, it has a relatively low inclination 35 degrees , staying near the equator. In this highly inclined orbit, the satellite moves around the Earth from pole to pole, taking about 99 minutes to complete an orbit. During one half of the orbit, the satellite views the daytime side of the Earth.
At the pole, satellite crosses over to the nighttime side of Earth. As the satellites orbit, the Earth turns underneath. By the time the satellite crosses back into daylight, it is over the region adjacent to the area seen in its last orbit.
In a hour period, polar orbiting satellites will view most of the Earth twice: once in daylight and once in darkness. Just as the geosynchronous satellites have a sweet spot over the equator that lets them stay over one spot on Earth, the polar-orbiting satellites have a sweet spot that allows them to stay in one time.
This orbit is a Sun-synchronous orbit, which means that whenever and wherever the satellite crosses the equator, the local solar time on the ground is always the same. When the satellite comes around the Earth in its next overpass about 99 minutes later, it crosses over the equator in Ecuador or Colombia at about local time.
The Sun-synchronous orbit is necessary for science because it keeps the angle of sunlight on the surface of the Earth as consistent as possible, though the angle will change from season to season. This consistency means that scientists can compare images from the same season over several years without worrying too much about extreme changes in shadows and lighting, which can create illusions of change. Without a Sun-synchronous orbit, it would be very difficult to track change over time.
It would be impossible to collect the kind of consistent information required to study climate change. The path that a satellite has to travel to stay in a Sun-synchronous orbit is very narrow. If a satellite is at a height of kilometers, it must have an orbital inclination of 96 degrees to maintain a Sun-synchronous orbit.
Any deviation in height or inclination will take the satellite out of a Sun-synchronous orbit. The amount of energy required to launch a satellite into orbit depends on the location of the launch site and how high and how inclined the orbit is. Satellites in high Earth orbit require the most energy to reach their destination. Satellites in a highly inclined orbit, such as a polar orbit, take more energy than a satellite that circles the Earth over the equator.
The International Space Station orbits at an inclination of Once a satellite is in orbit, it usually takes some work to keep it there. Throughout their lifetime, GOES satellites have to be moved three or four times to keep them in place.
Satellites in a low Earth orbit are also pulled out of their orbit by drag from the atmosphere. Though satellites in low Earth orbit travel through the uppermost thinnest layers of the atmosphere, air resistance is still strong enough to tug at them, pulling them closer to the Earth.
Over time, the satellite will eventually burn up as it spirals lower and faster into the atmosphere or it will fall to Earth. Atmospheric drag is stronger when the Sun is active.
Just as the air in a balloon expands and rises when heated, the atmosphere rises and expands when the Sun adds extra energy to it.
The thinnest layer of atmosphere rises, and the thicker atmosphere beneath it lifts to take its place. Now, the satellite is moving through this thicker layer of the atmosphere instead of the thin layer it was in when the Sun was less active. Since the satellite moves through denser air at solar maximum, it faces more resistance.
While polar orbits have an inclination of about 90 degrees to the equator, geostationary orbits match the rotation of the Earth. A sun-synchronous orbit passes by any given point with the same local solar time, which is useful for consistent lighting and sun angle. Out of the three types of orbits low, medium and high Earth orbits , polar orbits often fall into low Earth orbits. Learn more about geostationary and geosynchronous orbits.
The correct definition is that the orbital plane of a satellite in sun-synchronous orbit always has the same orientation with respect to the sun. Please let me know if it is changed. This statement is wrong. What happens in the case of a geostationary satellite is that the force of gravity at that height is exactly matched by the force of the satellite momentum, which is provided to the satellite by the launcher engines and the engine on board the satellite called the Apogee Engine.
There is no acceleration as such but a constant angular speed of degrees per 24 hours. Your email address will not be published.
0コメント