Satellites
Geostationary Satellites
A geostationary satellite, also known as a communication satellite, orbits the Earth in 24 hours. Since the planet revolves around its own axis during the same time,
a geostationary satellite appears to be parked, when seen from the Earth - and hence, the name.
In addition, both the Earth and the satellite moves anticlockwise. These satellites are placed at an altitude of 35,390 km above the equator.
Since a single satellite is capable of covering 1/3rd of the surface of the Earth, three satellites can cover the entire surface in satellite communication.
Since they are stationary relative to the Earth, the terrestrial antenna dishes do not have to be adjusted all the time in order to receive signals from a particular
communication satellite. The fact that every domestic antenna in the UK that receives the signals corresponding with SKY broadcasting satellite are erected in the
same direction illustrates the significance of the satellite being 'stationary'.
One of the limitations of geostationary satellites is the latency: the significant time taken by signals for a round trip - from the Earth to satellites and vice versa.
Even at a speed of 3x108m/s, the time taken by signals for the round trip is about 240 milliseconds.
Polar Satellites - Low Earth Orbit Satellites(LEO)
Polar satellites or Low Orbit Satellites - LEO - are satellites that orbit the earth a few times during a day. They are usually placed
a few hundred km above the earth. The period of orbit ranges from 90 minutes to a few hours, depending on the altitude, of course. They are usually spy satellites and weather satellites. Due to their relatively low orbit,
the latency is significantly reduced.
Recently, there was news that Google, the search giant, is going to use 180 Low Earth Orbit satellites - #GoogleSatellites - to guarantee the internet access for the whole planet. It shows the untapped potential of
the polar satellites.
The art and science of chasing a comet
Rosetta,the spacecraft of the European Space Agency finally managed to get closer to the comet that it has been pursuing for 10 years - comet 67P. As of August 7, 2014, the craft is scanning the comet for a possible landing site. The following video shows the art of chasing a comet: it is much harder than pursuing a satellite, like moon, or a planet like mars. The following video from the European Space Agency explicitly explains how to chase a comet!
Rosetta's plan in the coming months
The following video shows how Rosetta is orbiting the comet until it finds a suitable landing spot for a probe.