Tuesday, March 07, 2006

Space Travel for Dummies, Part 3


If you've been following along, by now you know the basics of how to get into orbit in the first place, and how to move around in Earth orbit once you get there. Which will do as far as things like going to the space station or putting a new satellite up go, but sometimes you'd like to step a bit further out. It's not all that much different than what we've already been talking about. You know the old saying, "What goes up, must come down?" Well, it's not always true.

If you throw something hard enough, it won't be coming back.

Let's revisit our good friend Superman up on Mount Everest. He still has a few baseballs left, and intends to put them to good use.

If you remember, last time he threw the ball a little harder than he had to in order to achieve a circular orbit, and put it into an elliptical orbit. If he puts more muscle into the throw, he can get it to sail higher and higher into the sky. The orbits get bigger and bigger, and they take longer and longer to get back to their starting point. Starting from 90 minutes, the orbit times extend to two hours, three hours, six hours, and longer. Then, you start measuring the orbits in days, not hours. They blast up to ten, twenty, a hundred thousand miles high. Eventually, he starts going home after he throws, coming back a couple of days later for the catch.

He's got one ball left.

He winds this one waaaaaaaaay back, and lets it go. It leaves his hand at over 36,000 miles per hour. If he weren't Superman, the shock wave would knock him down. Then, he packs up his stuff and goes home. He's done.

This one, you see, isn't coming back to Earth. Ever.

Here's what's happening: Gravity loses strength at the square of the distance from Earth. But, gravity can only drain your speed away at a linear rate. Most of the time, that's enough. It saps all of your vertical speed and pulls you right back. But if you're going fast enough, gravity can't suck speed away quickly enough, and no matter how far away you get you've always got a little speed left over.

So, that last ball Superman threw will sail across the Solar System forever. It's in a permanent Solar orbit, and probably won't ever come back to Earth.

This is an important principle, because that's what lets us leave Earth and strut our funky stuff across the Solar System. While gravity extends everywhere, it's not infinitely powerful, and can be overcome with enough effort. Sometimes, you even get to cheat a little bit and make gravity work for you.

That's the essence of what they call the gravity assist or "slingshot" maneuver. It's a way that you can get a speed boost and direction change for free, if you line everything up just right.

Ordinarily, changing speed and direction takes energy. And the only practical tool we have right now for doing that is a chemical rocket. Which means you have to bring along fuel to do it. Since you have to bring along fuel, you have to bring along fuel tanks, and that drives up the ship's weight.

That's bad. As the current NASA administrator, Mike Griffin, once wrote: "Spacecraft, like turkeys, are bought by the pound."

But, sometimes, you get to pull one over on Mother Nature. Here's how it works:

Imagine a skater, carrying a grapnel hook on a bungie line. He wants to round the corner, but doesn't want to expend the effort to turn his skates. So, he throws out his line, hooks a pole, and swings around.

By timing your approach and direction just right, you can swing around a planet just like the skater swung around the light pole.

As you fall in, coming from behind and below, you speed up as the planet pulls you in. As you speed away, the planet's gravity isn't able to suck your speed away quickly enough to get it all back. Basically, you've stolen a tiny bit of the planet's orbital momentum, and used it to speed up and change the direction of your orbit.

We've gotten pretty good at this. Voyager, Galileo, Cassini, all of them went through numerous slingshot encounters to achieve their missions.

And that's about it for basic orbital maneuvers. Once you escape Earth orbit, getting to and from another planet in the Solar System works just the same as getting to and from something in Earth orbit. You burn in the direction of travel to gain energy and go farther out, and you burn against the direction of travel to lose energy and go in.

Which brings us to a trick question. Remember this: you can amaze your friends at parties, because almost no one ever gets the right answer the first time.

Here's the problem: you've got a package of dangerous waste that you want to be rid of forever. You've got two options. One, drop it into the Sun, and two, send it into interstellar space.

Which one is easier?

Sending it into interstellar space, of course.

This stunned me the first time I heard it, but the numbers absolutely work out.

Earth's orbital velocity is about 30 kilometers per second. As a rule of thumb, escape velocity is about one and a half times orbital velocity. So, once you've escaped Earth, you have to gain about another 15 km/sec to achieve Solar escape velocity.

But to drop something straight down, you have to shed ALL of your orbital velocity. So, to drop something into the Sun means you have to find a way to shed a full 30 km/sec.

It's a hard fact to swallow, but I assure you, it's true.

Next time: How the heck to we get back home?

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