Getting away from Earth has other advantages, too. Modern industry uses manykinds of metal alloys (mixtures of metal that are better for certain purposes thanpure metals). Yet some metal alloys either can¡¯t be made or are very expensive to make on Earth because of gravity. For instance, certain metals don¡¯t mix well onEarth. But in zero gravity, molten (hot, liquid) metals mix more evenly. This is because there is no gravity to pull the heavier metals down, while the lighter ones float on top.

From space, too, we can look down on the Earth and study the atmosphere, its weather, and the effects of air pollution.

And because there is no strong gravity to break free from, our future homes away from Earth will be convenient starting points for travel to distant planets.

But, while going into space might solve some problems, outer space can also be a dangerous place. For example, in outer space, we have to protect ourselves from the dangers of ultraviolet light and cosmic rays. Ultraviolet light from the sun can give us bad sunburns right here on Earth. Yet, Earth¡¯s atmosphere screens out most of that harmful radiation. Cosmic rays are tiny high ª²energy particles from outer space. Again, the Earth shields us from most of them.

At Home in Space?

But in space, without special protection, we would be exposed to much stronger radiation from ultraviolet light and cosmic rays. Also, in the zero gravity of outer space, our bones will lose calcium and become weaker. This will be more of a problem the longer people stay out in space. Doctors are looking for a way to keep our bones from losing calcium in outer space. And a small spaceship just might ¡°drive you batty¡± after a while. But even on a short trip in outer space, you might not feelas well as you¡¯d like to. Space travel could make you seasick!

Yet, these risks won¡¯t keep people from going into space. Eventually, an Earth like environment will be built in space. And they will be populated by people with many different interests: medicine, construction, farming, teaching, mining, and so on.

The next hundred years will be filled with other worldly adventures, exciting scientific discoveries, and danger, as humans leave Earth¡ªperhaps forever.

Aging in Space

Suppose a space traveler is moving at a velocity of 186,200 miles per second.

For every hour that passes for him, 30 hours pass on Earth. If he travels for a year in this fashion (having accelerated instantaneously) and then turns around and comes back at this speed (having turned around instantaneously), he will find that while he has seemed to himself to have traveled two years, the men on Earth would claim he had been absent for 30 years.

Suppose the space traveler had left at the age of 30, leaving behind a twin brother also aged 30. When he returned he would be 32, but his stay at home twin

brother would be 60. That is why the ¡°clock paradox¡±, is sometimes called the ¡°twin paradox¡±.

Of course it takes quite a long while to accelerate to a high speed, and a long while to make a turn and head back again, so conditions aren¡¯t quite as clear cut as just described.