12-04-2008 It’s getting crowded up there! (Article132)

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Simon Kenny
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12-04-2008 It’s getting crowded up there! (Article132)

Post by Simon Kenny » Fri Jan 02, 2009 4:25 pm

Earth orbit: it’s getting crowded up there! (Article132)

Those of us heading for the big 6-0 or more, may be among the lucky ones who saw a piece of history in the tiny star that moved like no other, making steady progress from the North West horizon on the clear twilights of Autumn 1957 and within two minutes had vanished over the opposite horizon. Those were the heady days of Sputnik I, which opened for humankind the immense vistas and possibilities of space.

Now, on the 50th anniversary of Sputnik, a whole plethora of satellites pass overhead without meriting a comment. Every dawn or dusk, a casual skyward glance will reveal numerous satellites in low-earth orbit reflecting the sun’s light. Occasionally, there are flashes of light that merit a second look, though by then they may have vanished! In recent years people have been surprised, even alarmed by brilliant flashes of light in the twilight sky. Many people think the International Space Station is the cause, which may be true, but the flashes are much too frequent for the ISS to be the only cause. More likely, it is one of the over 70 Iridium satellites in polar orbit 780 km above the earth. These are the backbone of a mobile telephone network that can be used in any part of the world at any time. Each is distinguished by its highly reflective solar panels, which convert sunlight to electricity to power its circuits. There is at least one of these satellites above us at any time, which explains their frequent visibility each twilight.

For every visible satellite, there are numerous unseen ones. Satellites use a variety of orbital heights and directions to suit their purposes. The geostationary orbit is where a satellite completes an orbit in the same time the earth spins once on its axis. The effect is the satellite seems to hover permanently over a point on the Equator and from its orbital distance of 35,790 km, can survey nearly half the globe. This type of orbit is favoured for weather satellites, as it gives the big picture of weather patterns over a vast area of the world.

Satellites using polar orbits, like the Iridium satellites, need to provide detailed cover of every part of the earth, including the Poles. These orbit the Earth in a North-South plane. While they aren’t always over the same position, they usually operate in sun-synchronous orbits, making them pass over the same areas at the same times each day.

The increasing numbers of satellites in orbit has prompted the hitherto unthinkable scenario of overcrowding the space above Earth. Besides functioning satellites, there are over 9,000 pieces of ‘space junk’ raising fears of collision hazards. Luckily, most of the debris orbits between 900 and 1000 km and the orbits of manned spacecraft are below 600 km. While the risks are still very small, scientists are actively investigating economic ways to set up a Hoover-in-the-Sky!

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