Magazine 雜志：Space
If you point a rifle straight up into the sky and shoot a bullet, what happens? The bullet starts going up really fast. If it's a good rifle it might be doing about 3,000 kilometres an hour. But it will immediately start slowing down, first, because of the Earth's gravity. You probably studied this in school: falling objects accelerate at 9.8 metres per second and things going up decelerate at the same rate. It will also slow down because of the air, which creates friction. So the bullet will slow down and eventually stop. If you try doing a rapid calculation you can quickly see that the bullet will not get into space, which begins at a height of about 100 kilometres. (It might go up 2 or 3 kilometres.) Instead it will come back down again and hit you on the head quite hard.
A really fast cannonball
Mathematically at least, you need something much bigger and more powerful than a rifle to get off the planet. In Jules Verne's story of the journey "from the Earth to the Moon" the travellers cover the distance in a big empty cannonball. According to his figures, the cannon ball had to reach a speed of 12,000 yards per second. The modern figure, 11,100 m/s or about 40,000 kilometres an hour, is called "escape velocity". I don't know if it is possible to make such a cannon, but I'm sure no one would get inside it.
Rockets
So, the only way to fly to a height of 100 kilometres is with an engine that will keep your craft moving upwards after it has left the ground. That way you can start reasonably slowly and just keep going until you get into space. As you go up the air gets thinner and it's harder for you to breathe. On the top of Everest, nearly 9 kilometres up, there is only about one third of the air at sea level. In a jet airliner, flying at a height of 11 kilometres, no-one can breathe without compressed air, or an oxygen mask in emergencies. The jet engine also needs air to burn its fuel, but it can still get enough because it is moving along horizontally at high speed. But if your aim is to carry on up - a jet will not take you far enough. The height record for a jet aeroplane is nearly 26 km. Rockets carry their own oxygen with them so they can fly vertically up through the very thin parts of the upper atmosphere.
Ready to fly into space
Let's suppose that you've done all the training and put on your space suit. You take the lift up to the top of your rocket. If this was a building, your capsule would be on about the thirtieth floor. Get into the capsule and strap yourself into the seat. Now you have to spend a few hours waiting while the countdown continues. The highly explosive fuel and liquid oxygen are pumped into the rocket. This is an extremely complicated machine, during the countdown a serious of minor problems will be found, checked and discussed. You read the instructions for what to do in an emergency. Inside your helmet you can hear your breathing get faster.
Into space
The engines start. For a few seconds everything shakes. Inside the capsule the noise is terrible but the rocket is still held onto the launch pad. Then the bolts holding you down are released and mission control announces "We have lift-off" The rocket starts to rise very slowly. From this moment, your rate of acceleration increases. It is a stronger version of the feeling you can have in a very fast car with a maniac driver when the traffic lights change from red to green. You are pushed back into your chair. The clouds come straight at your window. After 2½ minutes you are travelling at nearly 10,000 km an hour. You are 61 kilometres above the ground and the sky is changing colour from blue to black and stars are becoming visible. Very soon you will be in space.
Into orbit
The rocket uses an immense amount of fuel very quickly. At lift-off, the Apollo rockets weighed 2800 tons but they used about 2,000 tons of fuel in those first 2½ minutes. As the craft gets lighter, it accelerates more quickly. The feeling of acceleration gets stronger and stronger reaching 3 times gravity. You cannot move. The whole journey into orbit takes about 8 minutes. Then the engines stop and, instead of feeling incredibly heavy, you are suddenly weightless. There is perfect silence and everything around you in the capsule starts to float. Everyone who has been in space says it's beautiful.
