Thursday, February 09, 2006

Space Travel for Dummies, Part 2


"East takes you out. Out takes you west. West takes you in. In takes you east."
-- Larry Niven, The Integral Trees

Once the main engines shut down, things get quiet. Not completely quiet. That would be bad. There's a constant hum and rattle of pumps pumping and motors whirring, but if that ever stops, breathing is liable to become just a little bit difficult. But the rumble of the main engines has stopped, which means that you've reached a stable orbit.

Now what?

Last time we took a look at how you get into orbit in the first place. Now, we're going to look at what you can do once you get there. It's not enough just to get into space. You need to know how to get around, so that you can go where you want to go, not just bore holes in the sky.

Let's rejoin Superman back on Mount Everest, where he just caught a baseball that he threw all the way around the world. What do you think would happen if he threw it just a little bit harder? There's only one way to find out ... He pulls another ball back, this one farther still, and lets it rip. Just like the other ball, it never touches the ground, but unlike the other ball, its path takes it not on a level path around the world, but slightly upwards. By the time it reaches Brazil, it's several miles higher than the first ball's path. But, it starts to fall closer to Earth after that. About two hours later, Superman catches it when it comes back.

Now, what's going on, here? In a word, it's all about energy. Neglecting atmospheric friction, the energy of an object in orbit is constant. If we start with an object in low Earth orbit, and give it just a slight kick so that it goes a little faster, its orbit is no longer a circle, it's an ellipse. The low point on the ellipse touches the original circular orbit. The high point on the ellipse sticks out just a little bit.

Consider the opening quote from The Integral Trees. It's a pretty good description of how basic orbital maneuvering works. Going East -- adding speed to your trajectory -- takes you Out -- farther away from Earth. Going Out takes you West -- as you climb up, you trade speed for altitude, and end up going a bit slower. If you left another spaceship back in your original orbit, if you look down, you'll see it pass under you. Going West -- taking speed away from your trajectory -- takes you In, or closer to Earth. This works the opposite way: you take a little bit of energy away from your orbit, and start to lose altitude. But now you're trading altitude for speed, and you'll pass anything you left in your original orbit. Which brings us -- you guessed it -- to the last bit: "In takes you East."

These facts are the foundation of the art of orbital rendezvous. You don't catch up to another spaceship by burning your engines constantly until you match position. No, what you do is enter a higher or lower orbit, and wait. At just the right time, you make an orbital adjustment burn, and match trajectory with your target. The actual math is a lot more complicated than that, but that's the general idea. You spend most of your time in space waiting for the right time to make these maneuvers.

The other thing is, you're not always in the same orbital plane as the spaceship you want to dock with. The way you want to handle this is to make sure you launch into the right orbital plane to begin with. That's why you hear about "launch windows" all the time. It's possible to change your orbital plane once you get into space, but plane change maneuvers drink fuel like nobody's business. You never want to do one unless you have to. It's always easier to make sure you get the plane right the first time around. But if you do have to, the way you change the plane of your orbit is that you fire your rocket at right angles to your current direction of travel. That is, instead of firing ahead or behind, you fire your rocket sideways.

In any case, now that we've gotten ourselves into space, we know how to move around and get where we want. Next time, we'll talk about going a little farther out: to the Moon, and beyond.

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