Satellites are launched to space from earth for more reasons than one. While some are designed for doing astronomy, others are for communication purpose. Communication satellites make up the GPS network. Then there are satellites to study the weather of the earth. Also, the military satellites are there for defense point of view to take covert images of sensitive areas.
The basic function of satellites is simple. Whatever type they are, they serve to transmit data from space to ground stations by using radio signals that are collected each time the satellite comes to its station. Typically, a satellite takes about 90 minutes to circle the orbit of the Earth once. This tells us that satellite can transfer its data to the space station multiple number of times in one day.
Sometimes, you can spot satellites up in the sky without binoculars or telescopes to help you. To begin with, you can see the Heavens Above astronomy site or the Starry Night app to see what satellites are up there currently. Then, if you are lucky, and the night is dark and cloudless, with little moonlight, you would be able to easily spot several satellites that are launched to Earth` orbit.
However, only handful of the hundreds of satellites that are up in the sky are large enough, reflect enough, and on orbits low enough to be seen by human naked eye. A good night can give sightings up to five to ten. A typical blinking satellite can be found visible for several minutes at a stretch.
The International Space Station is one of easily visible ones since it looks brighter than the stars around it. The other popular one that can be viewed with naked eye is “Iridium flares”. This phenomenon occurs when one of the many satellites launched by a company called Iridium is placed at the right part in the orbit. This means that the shiniest part of the satellite`s surface should face this side so that it reflects sunlight to where you are located. These particular Iridium flares twinkle in flashes from being invisible to naked eye, to being brighter than the Venus planet, and back to being invisible again, in seconds.
However, if you are not familiar with their presence, you might mistake them for being UFOs.