Circling around the Earth far out in space are giant cameras which can see details on the ground only 12in (300mm) across. The cameras are fixed to satellites as big as a single-Decker bus 50ft (15m) long, and take up half the area of the satellites as ‘spies in the sky’ to check the extent of other countries’ arsenals.
But satellite photography has other purposes. Every day, TV weather forecasts show pictures of the Earth photographed by cameras on board satellites. Geologists and economists study photographs taken from space that reveal rocks and crops on Earth. And astronomers look at distant stars and galaxies, unhindered by the Earth’s atmosphere. but how do these images reach Earth?
The most common way to send photographs from space is to use radio waves and beam the pictures down in the same way that TV pictures are sent. The amount of detail that you can see depends on the spacing between the lines that make up the picture: the more lines, the more detail you can make out.
The world’s most advanced commercial satellite for surveying the ground, the French SPOT satellite, transmits 6000 lines per picture – nearly ten times as many as the 625 lines used on most of the world’s TV sets. This means that in a picture which covers an area of 40sq miles (100 sq km) and taken from a height of 570 miles (920 km
), details as small as 30ft (10m) across are visible. In a photograph of the whole of Paris, for example, you could pick out the Arc de Triomphe.
Military intelligence experts generally want to be able to make out even finer detail. When monitoring a war, they need detailed photographs that will enable them to count the number of troops on a battlefield, or reveal different types of aircraft or ship.
The most modern American ‘spy satellites’. The KH-11 series, relay their pictures by television techniques. But, in general television images cannot show as much detail as a fine-grained 16mm or 35mm film. When film is used, it has to be returned to Earth physically. If the photographs are being taken in manned spacecrafts, the cosmonauts can bring the film back with them, but this is obviously impossible with unmanned spacecraft. So the Americans and the Russians – and more recently the Chinese – have developed satellites that return a film to Earth automatically.
The American ‘Big Bird’ satellites have perfected this technique. The exposed film is out into one of six re entry capsules, which is then jettisoned and drops back into the Earth’s atmosphere. as it parachutes down, the capsule is captured, or lassoed, in a wire loop which trails behind a C-130 Gercules transport plane.
Picture Credit : Google
