Just as the places in the theater allow you to take a different look at the performance, the different orbits of the satellites give perspective, each of which has its own purpose. Some seem to be hanging over a point on the surface, they provide a constant view of one side of the Earth, while others circle around our planet, sweeping over many places in a day.
Types of Orbits
At what altitude do satellites fly? There are 3 types of near-earth orbits: high, medium and low. As a rule, at the highest, farthest from the surface, there are many weather and some communication satellites. Satellites rotating in the middle Earth orbit include navigation and special ones designed to monitor a specific region. Most scientific spacecraft, including NASA’s Earth Observation System fleet, are in low orbit.
The speed of their movement depends on how high the satellites fly. As we approach the Earth, gravity becomes stronger and faster. For example, NASA's Aqua satellite takes about 99 minutes to fly around our planet at an altitude of about 705 km, and a meteorological device, remote at 35,786 km from the surface, will take 23 hours, 56 minutes and 4 seconds. At a distance of 384,403 km from the center of the Earth, the Moon completes one revolution in 28 days.
Aerodynamic paradox
A change in satellite altitude also changes its speed in orbit. There is a paradox. If a satellite operator wants to increase its speed, it cannot just start the engines for acceleration. This will increase the orbit (and altitude), which will lead to a decrease in speed. Instead, you should start the engines in the opposite direction to the direction of motion of the satellite, that is, perform an action that on Earth would slow down a moving vehicle. Such an action will move it lower, which will increase the speed.
Orbit Characteristics
In addition to altitude, the satellite path is characterized by eccentricity and inclination. The first relates to the shape of the orbit. A satellite with a low eccentricity moves along a path close to a circular one. The eccentric orbit is elliptical. The distance from the spacecraft to the Earth depends on its position.
Inclination is the angle of the orbit with respect to the equator. A satellite that rotates directly above the equator has a zero slope. If the spacecraft passes over the north and south poles (geographic rather than magnetic), its tilt is 90 °.
All together - height, eccentricity and inclination - determine the movement of the satellite and how the Earth will look from his point of view.
High earth
When the satellite reaches exactly 42164 km from the center of the Earth (about 36 thousand km from the surface), it enters the zone where its orbit corresponds to the rotation of our planet. Since the device moves at the same speed as the Earth, i.e., its orbital period is 24 hours, it seems that it remains in place above a single longitude, although it can drift from north to south. This special high orbit is called geosynchronous.
The satellite moves in a circular orbit directly above the equator (the eccentricity and inclination are equal to zero) and relative to the Earth it stands still. It is always located above the same point on its surface.
The geostationary orbit is extremely valuable for weather monitoring, since the satellites on it provide a constant view of the same surface area. Every few minutes, meteorological devices such as GOES provide information on clouds, water vapor and winds, and this constant flow of information serves as the basis for monitoring and forecasting the weather.
In addition, geostationary devices can be useful for communication (telephony, television, radio). GOES satellites provide a search and rescue beacon used to help locate ships and aircraft in distress.
Finally, many of the Earth’s high-orbit satellites monitor solar activity and monitor magnetic field and radiation levels.
GSO Height Calculation
A centripetal force F c = (M 1 v 2 ) / R and gravity F t = (GM 1 M 2 ) / R 2 act on the satellite. Since these forces are the same, it is possible to equalize the right-hand sides and reduce them by the mass M 1 . The result is the equality v 2 = (GM 2 ) / R. Hence the speed of movement v = ((GM 2 ) / R) 1/2
Since the geostationary orbit is a circle 2πr long, the orbital velocity is v = 2πR / T.
Hence, R 3 = T 2 GM / (4π 2 ).
Since T = 8.64x10 4 s, G = 6.673x10 -11 Nm 2 / kg 2 , M = 5.98x10 24 kg, then R = 4.23x10 7 m. If you subtract the Earth's radius equal to 6.38x10 6 m from R, you can find out at what altitude the satellites hanging above one point on the surface fly - 3.59x10 7 m
Lagrange Points
Other remarkable orbits are the Lagrange points, where the Earth's gravity is offset by the gravity of the Sun. Everything that is there is equally attracted to these celestial bodies and rotates with our planet around the luminary.
Of the five Lagrange points in the Sun-Earth system, only the last two, called L4 and L5, are stable. In the rest, the satellite is like a ball balancing on top of a steep hill: any minor disturbance will push it out. To remain in a balanced state, spacecraft here need constant adjustment. At the last two points of Lagrange, the satellites are likened to a ball in a ball: even after strong outrage, they will return back.
L1 is located between the Earth and the Sun, allowing the satellites located in it to have a constant view of our luminary. The SOHO Solar Observatory, a satellite of NASA and the European Space Agency, monitor the Sun from the first point of Lagrange, 1.5 million km from our planet.
L2 is located at the same distance from the Earth, but is located behind it. The satellites in this place need only one heat shield to protect themselves from the light and heat of the sun. This is a good place for space telescopes used to study the nature of the universe by observing the background of microwave radiation.
The third point of Lagrange is located opposite the Earth on the other side of the Sun, so the luminary is always between it and our planet. A satellite in this position will not be able to communicate with the Earth.
The fourth and fifth Lagrange points are extremely stable in the orbital trajectory of our planet 60 ° in front of and behind the Earth.
Middle Earth Orbit
Closer to Earth, satellites move faster. There are two medium near-Earth orbits: semi-synchronous and “Lightning”.
At what altitude do satellites in semi-synchronous orbit fly? It is almost round (low eccentricity) and is located at a distance of 26560 km from the center of the Earth (about 20200 km above the surface). A satellite at this altitude makes a complete revolution in 12 hours. As it moves, the Earth rotates beneath it. For 24 hours it crosses 2 identical points at the equator. This orbit is consistent and highly predictable. Used by GPS global positioning system.
The orbit “Lightning” (inclination 63.4 °) is used for observation at high latitudes. Geostationary satellites are tied to the equator, so they are not suitable for distant northern or southern regions. This orbit is very eccentric: the spacecraft moves along an elongated ellipse with the Earth located close to one edge. Since the satellite is accelerated by gravity, it moves very quickly when it is close to our planet. When moving away, its speed slows down, so it spends more time at the top of the orbit in the farthest edge from the Earth, the distance to which can reach 40 thousand km. The orbital period is 12 hours, but about two-thirds of this time the satellite spends over one hemisphere. Like a semi-synchronous orbit, a satellite passes along the same path every 24 hours. It is used for communication in the far north or south.
Low earth
Most scientific satellites, many meteorological and space stations are in almost circular low Earth orbit. Their slope depends on what they are monitoring. TRMM was launched to monitor rainfall in the tropics, therefore it has a relatively low inclination (35 °), remaining near the equator.
Many of the satellites of NASA's surveillance system have an almost polar, highly inclined orbit. The spacecraft moves around the Earth from pole to pole with a period of 99 minutes. Half of the time it passes over the daytime side of our planet, and at the pole goes over to the night.
As the satellite moves, the Earth rotates beneath it. By the time the device moves to the illuminated area, it is above the area adjacent to the zone of passage of its last orbit. Over a 24-hour period, polar satellites cover most of the Earth twice: once during the day and once at night.
Solar Synchronous Orbit
Just as geosynchronous satellites must be above the equator, which allows them to remain above one point, polar-orbiting satellites have the ability to remain at the same time. Their orbit is solar-synchronous - when the spacecraft crosses the equator, the local solar time is always the same. For example, the satellite Terra crosses it over Brazil always at 10:30 in the morning. The next intersection in 99 minutes over Ecuador or Colombia also occurs at 10:30 local time.
A solar-synchronous orbit is necessary for science, since it allows you to save the angle of incidence of sunlight on the Earth's surface, although it will vary depending on the season. Such constancy means that scientists can compare images of our planet at the same time of the year for several years, without worrying about too much leaps in lighting that can create the illusion of change. Without a sun-synchronous orbit, it would be difficult to track them over time and collect the information needed to study climate change.
The satellite path here is very limited. If it is located at an altitude of 100 km, the orbit should have a slope of 96 °. Any deviation will be unacceptable. Since the resistance of the atmosphere and the force of attraction of the Sun and the Moon change the orbit of the device, it must be regularly adjusted.
Orbiting: launch
Launching a satellite requires energy, the amount of which depends on the location of the launch site, the height and slope of the future trajectory of its movement. To reach the remote orbit, more energy is required. Satellites with a significant slope (for example, polar ones) are more energy consuming than those that circle above the equator. Orbiting with a low inclination helps the rotation of the Earth. The International Space Station is moving at an angle of 51.6397 °. This is necessary to make it easier for space shuttles and Russian rockets to get to it. The height of the ISS is 337–430 km. Polar satellites, on the other hand, do not receive help from the Earth's impulse, so they need more energy to rise the same distance.
Adjustment
After launching the satellite, efforts must be made to keep it in a certain orbit. Since the Earth is not an ideal sphere, its gravity in some places is stronger. This unevenness, along with the attraction of the Sun, Moon and Jupiter (the most massive planet in the solar system), changes the inclination of the orbit. Throughout its lifetime, the position of the GOES satellites has been adjusted three or four times. NASA Low Orbiters must adjust their tilt annually.
In addition, the atmosphere affects the Earth satellites. The uppermost layers, although sufficiently sparse, show strong enough resistance to draw them closer to Earth. Gravity accelerates satellites. Over time, they burn out, spiraling lower and faster into the atmosphere, or fall to Earth.
Atmospheric resistance 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 gives it extra energy. Sparse layers of the atmosphere rise, and their place is more dense. Therefore, satellites in orbit around the Earth should change their position about four times a year to compensate for atmospheric resistance. When solar activity is maximum, the position of the apparatus has to be adjusted every 2-3 weeks.
Space debris
The third reason forcing orbit to change is space debris. One of the Iridium communications satellites collided with a non-functioning Russian spacecraft. They crashed, forming a cloud of garbage, consisting of more than 2500 parts. Each element has been added to the database, which today has over 18,000 objects of technogenic origin.
NASA carefully monitors everything that might get in the way of satellites, because space debris had to change its orbits several times.
Flight control center engineers monitor the position of space debris and satellites, which can interfere with movement and carefully plan evasion maneuvers as necessary. The same team plans and performs maneuvers to adjust the tilt and height of the satellite.