A satellite is any orbiting object that has been placed in orbit by human effort. Sometimes they are also referred to as artificial satellites, to distinguish them from natural orbiting objects such as the moon. Satellites serve us day and night. When you call someone at the other side of the globe, your voice is carried deep into outer space, processed by a communications satellite, and relayed to another satellite or a base station, and then transmitted to the person at the other end of the line. The TV shows you watch are often relayed via satellites. The images of the earth that you see on Google Earth and related software are captured by satellites. Satellites also supply real time weather information.
There are two types of satellites – polar satellites and geostationary satellites. A polar satellite keeps orbiting the earth while the geostationary satellite stays in a constant position relative to the earth.
Geostationary/Geosynchronous
The geostationary earth orbit (GEO) is located at a height of 35786 kilometers above the earth. A satellite in geostationary orbit will always be at the same position relative to the earth (it has the same orbital period and in the same direction as the earth’s rotation). Often, weather and communication satellites are placed in this orbit so that antennas which communicate with these satellites can point towards them all the time.
There are also inclined geostationary orbits which lie at an angle other than zero degrees to the equatorial plane. Such orbits are known as inclined geostationary orbits, or inclined geosynchronous orbits.
This image will give you an idea about the relative positions of orbits: [LINK]
Polar
In order to maintain its height, a satellite must have an angular speed greater than the earth if it is below the geostationary orbit. If it is not moving at this speed, the centripetal force (provided by the earth’s attraction) will be greater than the centrifugal force (provided by the satellite’s circular motion) and it will start losing altitude. A polar satellite must be carefully launched in its orbit if it is to stay at that specific height.
Polar satellites often exist in groups, called constellations. In each constellation, there are often three planes, each perpendicular to each other. Having more than one satellite in each plane ensures global coverage all the time. Sometimes, there may be more than three planes, as in the case of Iridium.
Examples of some major polar satellites currently functioning are GPS, GLONASS, Iridium, Kosmos, the Hubble Space Telescope, and the mighty International Space Station. All serve different purposes. GPS Satellites provide street accurate global navigation service. GLONASS (Russian: Globalnaya Navigatsiya Sistemi Sputnikova) is the Russian alternative of GPS, but is mostly restricted for military use.
Iridium satellites are owned by Iridium LLC (United States) and provide a failsafe telecommunication service. Iridium LLC provides special phones which communicate using these satellites. At the moment, there are 66 Iridium satellites in the constellation (11 in each plane, 6 planes), excluding in orbit spares.
Kosmos is a name given to satellites operated by the former Soviet Union, and then Russia. Unlike Iridium, GPS, GLONASS, etc, the Kosmos satellites do not form part of a single mission or project. Instead, it includes almost ALL Soviet and Russian military satellites, as well as some scientific satellites. Presently, there are around 2500 Kosmos (Cosmos) satellites in orbit.
The International Space Station is a joint venture of the US and Russia. Previous space stations which have served before ISS include Salyut stations, Skylab and Mir. It is like a hotel in space. Space shuttles (like Buran, Discovery, Challenger, etc) can dock onto the ISS.
How can I see a satellite?
Satellite orbits are accurately planned and by doing some simple calculations, you can be exactly sure as to when a satellite will pass over your city. The path of satellites is very long and they travel at dazzling speeds of like 10-20 kilometers per second. So even if there are large errors in the measurement of their orbits, those errors scale down to nothingness when the observer is down there on the earth’s surface.
The International Space Station is amongst the lowest flying satellites. It orbits the earth at a height of 370 kilometers. It can be seen from the naked eye. When the ISS passes your sky, you see it like a star moving at a constant pace. This is because the outer shielding of the ISS and its modules reflect light from the sun.
Also visible from the naked eye are Iridium satellites. There are 66 Iridium satellites in orbit and they fly at a height of 500 kilometers above the earth surface. Iridium also has in orbit spare satellites which are usually stored at a height of 616 kilometers. In case a functioning satellite fails, the spare satellite will be brought down to the correct altitude.
Iridium satellites do not reflect light all the time. Rather, they are shaped such that they have three door sized antennas. When one of these will point directly towards the earth, it will produce a “flare”. This flare will be short lived, since the antenna will move away with the rotation of the satellite. Iridium flares can also be carefully calculated and seen.
Last but not least, the Hubble Space Telescope. We owe HST and its observations around 10,000 research papers. The HST is also visible from the earth since it is in low orbit (569 kilometers).
There are different websites and even Android apps which can tell you of possible sightings of ISS, HST and Iridium flares.
Satellite debris
After more than 5 decades of human activity in space, it is quite inevitable that we have accumulated lots of rubbish in space – orbiting rubbish. This debris has been a result of space accidents. In February 2009, a defunct Russian satellite, Kosmos 2251 collided with a privately owned Iridium 33 satellite with a relative velocity of around 11 kilometers per second, (a bullet flies at around 1.5 kilometers per second, and a satellite weighs around as much as a car, so imagine the mess). Tons of debris was leaked into space, some following the orbit of Kosmos 2251 and some following the orbit of Iridium 33. Luckily, the precious satellites like ISS and Hubble are in much lower orbit as compared to Iridium and Kosmos, but the debris can easily harm space shuttles and other satellites. In 1983, the windshield of the space shuttle Challenger was to be replaced when a small drop of paint hit it at 4 kilometers per second.
The ISS has carried out debris avoidance maneuvers some seven or eight times. These maneuvers take as much as 24 hours since the US first detects possible collision with debris and then Russia executes an avoidance maneuver.
Effort has been undertaken to remove pieces of debris from space, but it can never become hundred percent successful. Cleaning the cosmos is tough, and expensive, after littering it for fifty plus years.
