Satellites and Orbits – Background and Context
It was no lesser mind than the 17th Century super-genius Sir Isaac Newton who first suggested the possibility of a man-made object launched into orbit around the Earth. I.E. a satellite.
It took 300 years to see that vision become a reality. And to date, there have been around 6,600 satellite launches, with approximately 3,600 satellites still in orbit. Of these only 1,071, or so, are fully functioning.
Words like Low Earth Orbit and Geo Stationary Orbit are common expressions in the satellite and telecommunications communities. But what do they really mean?
This is where the period of rotation is not 24hrs but some multiple (or fraction) of 24hrs.
Such a satellite would pass over the same spot on the Earth at a given time (or times) each day. For example, a satellite in Equatorial Orbit with a period of rotation of 12hrs, would pass over the same spot twice each day.
Geostationary is a special case of “Geosynchronous orbit”.
In the 1940’s Arthur C Clarke, published an article in Wireless World. Clarke explained that a satellite orbiting the Earth at an altitude of 36,000 km above the equator would circle the Earth in exactly 24Hrs. Such a satellite would maintain a “geostationary position”. Thus, to observers on Earth, it would appear to remain motionless in the sky.
Of our 1071 satellites still functioning approximately 500 are in “Geostationary” or “Geosynchronous” orbit. I.E. around 36,000 km above our heads.
Medium Earth Orbits (MEO)
These are satellites orbiting at an altitude of between 10,000km and 20,000km. This distance places them between the Van Allen belts where radiation exposure is minimized. MEO’s are not necessarily limited circular orbits. We have approximately 50 satellites in MEO.
Low Earth Orbits (LEO)
Satellites such as the International Space Station (ISS), are in LEO. Typically, such orbits are 1000km or less and circular. This facilitates easy “re-visit” capability.
Low Earth Orbiting satellites have a relatively short period of rotation, usually of the order of 100 minutes or so. This means that they speed across the sky and disappear below the horizon quiet quickly. As such until recently they were thought inappropriate for Telecommunications. However, during the 1990’s a “constellation” of these satellites was deployed making them useful for telecommunications. (Eg: Iridium Satellite Constellation). There are approximately 500 satellites screaming across our sky in Low Earth Orbit at present.
And more recently Sir Richard Branson’s Virgin Galactic are investing heavily into Satellite Internet technologies, such as Halo-Fi.
Newton would be very proud……..
And what of the future of satellite technology? Have you heard of the amazing new Cubsat Technology?