Sunday, April 25, 2010

The thruster that fell into sea

A file picture of GSat-4 at Sriharikota

Amid the din of big rockets and cryogenics, a small but very significant step into space, which plunged into the sea along with the GSat-4 satellite, went completely unnoticed.
Even its mention in the brochures and other publicity literature was so unobtrusive and couched in jargon that perhaps an enthusiast would probably have glossed over its significance.We are speaking of the ‘plasma thrusters'.Fuel is the key .First a little background. A satellite is put in a particular orbit the choice of which largely depends upon what the satellite is meant to do. In that orbit, it keeps circling Earth at a particular ‘orbital velocity', which is a function of Earth's gravity acting upon it. But, since in the cosmos a body is acted upon by a number of gravitational forces, a body like the satellite, often swerves from its orbit and could either crash into Earth's atmosphere or slowly spin away into space.
In order to prevent a satellite from thus getting lost, small engines are fitted in and these can be fired by signals from Earth. When a satellite begins to go astray, its trackers on Earth fire one of these engines to nudge it back in line. But once these engines run out of fuel, the trackers can't do this and the satellite goes out of control. Therefore, the fuel these engines contained determines the life of the satellite.
Typically, in a satellite, half the space is occupied by these engines, leaving only the rest for equipment such as transponders or cameras.
The GSat-4 was a little different. Instead of conventional chemical engines, it had four ‘plasma thrusters.' Because of this, the life of the satellite would have been seven years, instead of 4-5. At optimum use, these plasma thrusters could enhance the life of a satellite to even 15 years.
A plasma thruster is an engine that uses the discharge of plasma to propel an object. Plasma is a gas in which some electrons have been ripped off their atoms by the application of external energy. These electrons and the (remainder) ions co-exist, and this state is often referred to as the fourth state of matter, after solid, liquid and gas. Plasma exists everywhere. The sun, for instance, is a huge chunk of plasma.
Plasma, electrically charged gas, is influenced by magnetic field. In a plasma thruster (to put it in very simple terms), you create a magnetic field, with the help of which you can direct a jet of plasma out through a nozzle. A thrust, in the opposite direction, results.
Plasma thrusters are nothing new. They have been used off and on, even as early as the 1960s. There seems to be some resurgence in interest in them now. However, the GSat-4 was the first instance of them being used in India.
4 thrusters
The GSat-4 satellite had four of these thrusters – two made by Russians and two ‘made in India'.
Satellites launched by ISRO in future may be expected to use plasma thrusters for maintaining ‘attitude', or orientation. These thrusters last much longer than the chemical rockets used today, as they are powered by electricity that the solar panels generate from sunlight. Consequently, not only will the satellites live long. Also, more on-board space will be freed for instruments.
The GSat-4 would have been a good learning experience. Alas, it's gone!


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