How they build the International Space Station

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

17 June 2007

Paul Ngotho has some interesting questions: “what holds the International Space Station in place? Secondly, how was it constructed? Where did the people constructing it stay? Thirdly, what causes weightlessness in space?”

The International Space Station (ISS) is in an orbit 330 kilometres “up”. Ngotho is wondering why it does not fall back to the ground. The trick is that the ISS is not stationary: It is constantly moving around the Earth at a speed of about 28,000km/h. That motion counteracts the gravitational pull therefore the station doesn’t drop from the sky. But still, one may also wonder what keeps it in motion.

To answer that, we ask another question: What would stop it from moving?  We are so accustomed to seeing things slowing down that we don’t stop to ask what has slowed them. Objects are slowed by friction with the surface on which they are moving, or the air (in the case of flying objects).

But this is not to say that friction with the air is what makes things fall: No. Flying objects fall because they don’t have enough speed to counter the gravitational pull. It is obvious that the harder you throw a stone, the farther away it falls. It then follows that, since the Earth is round, it is possible to hurl the stone so fast that it never falls to the ground.

However, if you did that while standing on the ground, the stone would be slowed by air resistance. Eventually, its speed would go below the critical value and the stone would begin to fall down. But at 330 km up in the sky, it is almost a vacuum. So when the critical 28,000km/h is preached, the engines can be switched off and the object will not slow down.

Going to Ngotho’s second question, the ISS is built right here on Earth, then taken up into space piece by piece and reassembled. This construction is still going on and is expected to be completed in 2010. The people building it stay in the comfort of their homes.

However, those who reassemble it just float in space. They do not fall because they are also flying at the critical 28,000km/h. The principle is quite easy: the station is build on Earth and then disassembled (you don’t assume that things will fit in such a project!)

The first piece is put into the Space Shuttle and fired up into space. At 330km up, the Shuttle levels off, accelerates to 28,000km/h and the engines are switched off. The cargo bay is opened and that first piece of the ISS is eased out. The door is closed and the Shuttled returns home.

The second piece is put on board and again taken to 330km up. The Shuttle accelerates to28,000km again and catches up with the first piece. The second piece is eased off. Then one of the astronauts goes out with a spanner and bolts the two pieces together. This is what they call a space walk. This is repeated many times over and eventually the Station will be completed.