NEWTON’S GREAT IDEA
The idea of a man-made satellite was proposed as early as the 17th century by Isaac Newton. The great English scientist imagined what would happen if a cannon ball was fired at the top of a very high moun- tain. If there were no gravity or air resistance, the cannon ball would follow a straight path in the direction it had been shot and travel away from earth. But in the presence of gravity and air resistance, the cannon ball will choose a curved path before falling back to earth (as shown on figure, at A or B); and the greater that firing speed, the further away it will fall from the launching spot.
But if the firing speed keeps on increasing, a point will eventually be reached when the cannon ball will miss earth all together and continue to circle around it! – just like the moon keeps circling round the earth.
HOW OUR SATELLITE HAS BEEN LAUNCHED
Just like Newton’s hypothetical cannon, a golf ball swung at the speed of 8 kilometres per second (by a very energetic golfer!) from a very high building would also continue to circle round earth indefinitely – in the absence of air resistance.
Evidently, our first satellite MIR-SAT1 (a small cube of 10 cm3 weighing around 1 kg) has not been launched from a high mountain or from a tall building! After being assembled at AAC-Clyde in Scotland, it has been transported to the Kennedy Space Centre in Florida – from where it was sent by rocket to the International Space Station (ISS) situated at an altitude of 400 km. And from there, it has been flung into space by means of a robotic arm (10 metre long).
ISS is a huge satellite that orbits round the Earth every 90 minutes. Because of its massive size, it can glow as brightly as Venus or Jupiter and can be seen in a clear sky without a telescope – if you know when and where to look for it!
HOW MANY VISITS PER DAY?
Our satellite will be flying at around 8 km/sec. At this speed it will take 92 minutes to go round the Earth and will thus do about 15 orbits every 24 hours. However, this does not mean that it will pass over Mauritius 15 times daily! This is because while the satellite orbit remains fixed in space, the Earth rotates inside this orbit. As a result, since the Earth moves from West to East, the satellite will have moved by a certain amount to the West one orbit later.
So, if at noon MIR-SAT1 is passing right over Mauritius (that is, at 57° E), at 13.30 it will be in the whereabouts of longitude 33° E – and hence will no longer be visible because it will lie below the horizon! But if at noon it was at 70° E (a few degrees above the horizon), at 13.30 it would be at 46° E and would still be visible. It is estimated that our satellite will be visible over Mauritius about 4 to 5 times daily.
HOW FAR CAN OUR SATELLITE ‘SEE’?
How far a satellite can ‘see’ will depend on the altitude of its orbit. By means of geometry, it can be shown that a circular region of radius 2200 km (with Mauritius at the centre) can be observed by MIRSAT1 when it is flying right overhead at an altitude of 400 km. It can also be deduced that our satellite will start to be visible as far away as 2200 km from our island. Thus if it is flying from North to South, it will appear over the horizon the moment it will be flying over Northern India and will disappear below the horizon when it is in the vicinity of Antarctica.
The longest track of our satellite across the sky will occur when it will be passing right overhead and the duration of that passage is approximately 10 minutes. In practice, however, the time during which we shall be able to capture signals being sent might well be shorter. This is because when a satellite is less than 20 degrees above the horizon, an antenna is not very receptive to the signals being transmitted.
THE LIFESPAN OF MIR-SAT1
The higher the orbit, the longer a satellite will stay in that orbit. This is because although space begins ‘officially’ at 100 km altitude, a satellite will still run into traces of the Earth’s atmosphere below 500 km, and the tiny amount of air present will cause a drag. Hence, the satellite will lose speed and fall back into the lower (and thicker!) layers of the atmosphere before eventually burning up. It is expected that MIRSAT1, flying at 400 km altitude, will stay in orbit between 1-2 years.
It might be argued of course why MIR-SAT1 could not have been sent into a higher orbit where the vacuum of space is nearly complete and where drag is almost non-existent. In fact, the best choice would have been a geosynchronous satellite. At 35,800 km above the Earth, this satellite travelling at the same speed as the Earth’s rotation would appear stationary; and right above our heads, this perpetual ‘eye in the sky’ would have sent - in a never ending stream - live pictures of our island and the surrounding region. But as it can be well imagined, a higher orbit (specially one at 35,800 km altitude) would have meant a much bigger budget!
TOO SHORT A TIME?
Although one or two years might look very short, much can be acquired during that time about Space/Satellite technologies. A dedicated team can learn how to control a satellite more efficiently, find better ways to improve communication between near-by islands, collect and analyse data in a more effective manner and perhaps more importantly impart the acquired knowledge to the younger generation. As a matter of fact, teams from several colleges have been trained how to set up and operate their own ground stations (with equipment donated by local Radio Amateurs) and are already mastering numerous modes of satellite communication. The aim is to encourage students to get more involved in Space/Satellite technology, and this is but the beginning. After all, Space/Satellite technology holds much promise because it can offer huge opportunities for creating new jobs and boosting investment.
We are at the start of a big adventure. After one or two years spent in monitoring MIR-SAT1, a dedicated team, besides acquiring new knowledge, would have developed better skills and competencies to perform at a greater capacity – thereby paving the way for more ambitious projects. This is what capacity building is all about. For, even with the most inspiring vision, one cannot go very far if there is no capacity to realize it. And our first satellite is a novel way to make it happen.
After every orbit, satellite will shift roughly by x° W where x = 360/15 = 24. Thus at 13.30 satellite will lie £on longitude (57° - x°) E = (57° - 24°) E = 33°E.