Artificial satellites are sophisticated and intelligent machines. They carry complex instruments and operate fairly autonomously. They give us exhaustive information about our home planet and show us things which we could never imagine witnessing through in-situ observations on Earth. A few of them are out exploring our neighbours, some are capable of housing humans and a couple (the Voyagers) have even left our Solar System to enter into interstellar space. They are science laboratories flying above our heads, unnoticed, as we go through the humdrum of life. However, just like any other machine, they wear out and have a limited life.
In an ideal universe without disturbances, a satellite that is once injected into a particular orbit would endlessly revolve in that orbit until eternity. Real life is tough though, for satellites too, so they are subjected to external disturbances arising due to solar radiation, gravity gradient, aerodynamic drag, earth’s oblateness etc. These disturb the orbit of the satellite and periodic orbit corrections are necessary to maintain it’s intended orbit. This requires propellant and that is what limits the life of a satellite. In unforeseen situations, a faulty equipment may also result in an early and abrupt ending.
When a satellite’s useful life is over, it is decommissioned from service. In order to understand where these defunct satellites are disposed, it is first essential to understand the orbits these satellites reside in. To keep it simple, I would only be discussing the satellites orbiting in Low Earth Orbit (LEO) and Geostationary/Geosynchronous Equatorial orbit (GEO).
The satellites in LEO, orbit at heights a few hundred kilometres above the Earth’s surface and serve a variety of applications like weather monitoring, earth imaging, resource mapping etc. The easiest strategy to dispose a satellite in LEO is to lower it’s orbit into a gradually decaying one so that after a few years or decades it disintegrates on coming in contact with Earth’s atmosphere. Most small satellites fully break up on this fiery encounter owing to their non-aerodynamic box-like design. A few bigger chunks of massive space stations may survive and actually make it to the Earth’s surface. For these, proper trajectory analysis is done to ensure that they land up in the so-called designated spacecraft cemetery in the Pacific Ocean, away from all humanity. Some pieces of space junk may fall elsewhere and may even become a prized posession at a museum, but that’s fairly rare.
Satellites in GEO, orbit at heights of about 36000 km above the Earth’s surface and are used for communication, weather monitoring, DTH applications, etc. Owing to their distance from Earth, for a GEO satellite, lowering the satellite’s orbit would be an extremely costly affair in terms of time and propellant. So there is something called “graveyard orbit” (Yes, it’s for real and I promise I’m not making this up). It is a super-synchronous disposal orbit a few hundred kilometres above the operational orbit with an orbital lifetime of millions of years, where satellites are pushed to so that they aren’t a threat to other operational satellites.
Long story short, deorbiting a satellite involves changing it’s orbit. Both the approaches described above require propellant that must be budgeted for during the concept and design phase of a satellite. Then there are a few other important things that a defunct satellite must endure before it is disposed. Passivation involves getting rid of any unused propellant and disconnecting the batteries. And then the transponders (transmitter and receiver) are switched off, thereby ending all communication with the ground station and mission control. With this last step, life as we know it comes to a complete end for the satellite. Well, if you really think about it, the satellite body is still very much alive and continues it’s afterlife in outer space. it just doesn’t have anything to do or anyone it can ‘talk’ to or anyone that ‘listens’ to it.
Humankind became space faring around 65 years ago. Since then numerous objects have been launched into space for a variety of applications. In earlier times, satellite manufacturers weren’t really thinking about decommissioning and disposal while designing the satellites. However, in present times, with the ever increasing threat of orbital debris (read my previous post for more information), a satellite’s design process also requires to account for deorbiting it and the entire space community must resolve to disposing unusable satellites safely and responsibly by adhering to debris mitigation guidelines. This shall hopefully also ensure that our future generations don’t hate us for that mess we leave behind in outer space.