Author's Note:

When I'm not busy writing, I'm actually an aerospace engineer. I work on a satellite mission that studies the gravity of the Earth. Every now and then, I get asked to do some sort of outreach thing, either with teachers or kids, where I explain what my satellites do, or what gravity is, or just talk about space or science in general.

So I figure, why not post what I do for those outreach events here, where other kids (and adults, if you're interested!) can get to them. Teachers and parents, feel free to take these ideas and use them. Students, feel free to read through them. I'm going to try to design them to make you think, not just to give you information to memorize. That's because science isn't really about memorizing -- it's about learning new ways to look at the world.

If you have any questions on science, please ask. There's a good chance I'll know the answer, or can find someone who does. I love science, and I hope through this "story" I can share some of that love with you guys!

--Ruatha

A Thought Experiment: Designing a Colony Ship to Mars

Congratulations! You’ve just been given the honor of designing the very first colony ship to go to Mars. Your ship needs to hold a hundred people for the eight-month journey from Earth. The doctors at NASA tell you that if you leave a hundred people in zero-gravity for so long, their bones may become too weak to support them once they arrive at Mars. So you’re going to have to – somehow – make some artificial gravity onboard your ship. But this isn’t Star Trek and you don’t have any “anti-gravity plates”. So how are you going to manufacture something enough like gravity to keep those colonists healthy?

Need some hints to get you started? See how quickly you can figure out a way to do it!

1.) Think about what happens when you a bucket of water in circles.

2.) Try this little experiment you can at home. Get a pail with a handle on it. Fill it halfway with water and then bring it outside. Now, if you turn the pail upside-down over your head, all the water will fall out. But try this: hold the bucket out to your side and spin it in a quick vertical circle. If you move it fast enough, the water will stay in the bucket, even when the bucket is upside-down or sideways!

3.) Imagine that instead of water in the bucket, there were tiny people. What way would the bucket-people fall if they jumped? Would they fall towards the bottom of the bucket, or out the top? Whatever way they fall is the direction of the artificial “gravity” that they feel.

Still stuck, or curious what others have come up with? Read on for one possible answer:

The easiest way we know to make artificial gravity is to spin the ship. The spinning will make everything fall to the outside of the ship, which acts enough like gravity to make the NASA doctors happy (and to keep your colonists’ food from floating away from them when they eat!). The faster you spin it, the more artificial gravity there will be.

Here are a few more things to think about when designing your colony ship:

1.) What shape will your ship be? A cube? Sphere? Cylinder? Dumbbell-shaped? Does it matter, in terms of the artificial gravity system?

2.) Where will there be the most artificial gravity in your ship? Where will there be the least? Why?

3.) Assuming you put the same amount of spin into your artificial gravity, would a “fatter” ship or a “thinner” one produce more gravity at the inside edge of the ship? What about a “longer” one versus a “shorter” one?

4.) Imagine if you left your ship hollow and stood on one side of the inside. You’d be held down by the artificial gravity, so everything would seem normal. Then imagine you asked a friend to go stand on the far side of the ship. He’d appear upside-down to you! Why wouldn’t he fall?

There are a lot of other questions to think about when designing an artificial gravity system for a space ship. For example: How fast would you want to spin it up? Where would you put all the things – beds, couches, desks, refrigerators – to keep them from floating away when the ship had no gravity, before it got spun-up? Where do you put your communications antenna, which always needs to point to the Earth, if your ship is spinning? Will the colonists feel any sideways motion from the spinning, or will it all be “down” to the edges of the ship? And finally, what’s the difference between “real” gravity and “artificial” gravity?

How the bucket trick works: The water stays in your bucket, even when it’s upside-down, because whenever you spin something, you give it an acceleration away from the point you’re spinning it at. So when the pail is upside-down, gravity wants to pull the water down, but your spinning makes the water move up. If you spin the bucket at the right speed, the up and down tendencies will cancel out, making the water stay in place, even if there’s nothing under it. (Satellites stay in orbit around the Earth for exactly the same reason. What does that tell you about how fast you need to launch a satellite to keep it from crashing?)

Possible answers to the questions asked above:

1.) Most of the time, when people talk about creating artificial gravity by spinning, they’re thinking about a cylindrical ship. That’s because it’s an easy shape to build that spins around one axis (right up the middle). People could live all along the insides of the cylinder and feel the same amount of gravity. But really, almost any shape with a lot of surface area for people to live on would do. So maybe, depending on how you need to attach your engines and communications equipment, it might be better to have a different shape.

2.) The most artificial gravity will be at the points farthest from the spin axis – which, for a cylindrical ship, means at the outside of the cylinder. The amount of gravity your colonists feel will get smaller and smaller as they go closer to the spin axis, until at the very center of the ship, they’ll feel no gravity at all! This happens because far away things move faster when they are spun than close-up things. (You can test this by spinning a string around your head and watching how fast the end moves.)

3.) Length (distance along the axis of rotation) doesn’t make any difference, but thickness (distance away from the axis of rotation) does. Since you get more artificial gravity far away from the spin axis, this means a “fat” ship will have more gravity at the every edge of the ship than a “thin” one.

4.) He wouldn’t fall because the artificial gravity would be pulling him towards the outside of the ship, too. What is “down” to you is really “up” to him! (It would be very confusing to live on a hollow ship. Then again, if the Earth was transparent, we would see that the people on the other side of the planet were all standing upside-down to us, too!)