How can an Earth-like environment be simulated on a spacecraft?

Most of the things you need to do to make a spaceship environment ``Earthlike'' are actually pretty easy, at least for a short while. You can take your air, water and food with you, and with proper recycling you can reuse a lot of that. Energy can come from sunlight, or batteries, or nuclear fuel. But as you point out, gravity is a problem. There are really two choices: fake it, or do without.

Doing without isn't so bad on short trips. Astronauts have spent many months orbiting the Earth in microgravity (the official term for freefall nowadays) The problem is, after months and (theoretically) years, that starts to have bad effects. The most notorious is with the bones. Lack of sustained gravity causes calcium loss in the bones, which can be pretty bad if you ever plan on being in gravity again. There are a host of other problems as well, all of which have made people think of ways to fake gravity.

The best way to fake it is to spin the ship somehow. Centripetal force is what you feel when, for example, you are sitting in a car that makes a sharp turn and you are thrown to the outside of the curve. The amount of force depends on the speed of the object and the radius of the curve. So if you took a spacecraft that is basically a big cylinder and spun it around its long axis, people would feel a force outwards towards the cylinder wall. Have you ever seen ``2001: A Space Odyssey''? They do this trick in a giant space station as well as on the ship to Jupiter.

There are problems with this. Docking becomes hard on a rotating ship. It's a moving target! So you can put your docking moors on the ends, along the axis of the rotation, where the movement is smallest. Another problem is simply size. A small ship has to spin faster to feel the same force as a big ship spinning slower. The problem is that if the radius of the spin is too slow, you can actually feel it in your middle ear when you turn your head! It's like a miniature version of the Coriolis effect which makes hurricanes on the Earth (for more about that, try reading my web page about the Coriolis effect). So you really need a big ship to do this, and that can be expensive to build. I have seen another solution which is to tie two ships together with cable and spin them around each other, like a dumbbell thrown in the air. Each ship gets its own artificial gravity, and you don't need a huge spacecraft to do it. That may very well be the way we create artificial gravity for long voyages in the decades to come!

For more info, try NASA's Space Settlements: A Design Study website, which is a teachers' and students' aid in station designs.