So you want to get into space? In some ways, that’s actually the easiest part of space travel. Just go up far enough, and voila, you’re in space!

Just how far?  Well, that’s a little fuzzy.

This picture isn’t space just yet. This is only 100,000 feet, about 18 miles up. This picture was in fact taken from a balloon. Some fighter jets can reach this altitude. Meteors burn up before they descend to this altitude — a spacecraft here wouldn’t survive long.

The commonly recognized boundary of space is called the Kármán line, more than three times this altitude, at 100km (62 miles) above sea level. While it is difficult to reach this altitude, this barrier was broken by a rocket built by a group of hobbyists in 2004.

So, you don’t need to be a big government agency or defense contractor to reach space. What exactly, then, is so darn hard about space travel?

Short answer: everything else.

Ok, you want the long answer? Let’s start with what you do once you’re up there. The hobbyist rocket reached an altitude of 72 miles, but it reached it in the same manner that a ball thrown straight up might reach 50 feet. It hits that height, and then it has no momentum left. Then it starts to fall.

In a manner of speaking, you could say that it starts to “fall” the minute its engine cuts out. When the air is so thin that you can effectively neglect its resistance, there’s really nothing special physically about the moment you switch from moving up to moving down. In this kind of world, you’re in one of two states: either there’s a force acting on you, or there isn’t.

If you’re standing on the ground, there’s a force acting on you — the ground pushing up on your feet. It’s something so common that we actually consider this the null case. In reality, the only time there is no force acting on you is when you are in free fall. Only then do you get to see Newton’s physics’ true behavior, unadulterated by the constant acceleration of gravity.

“But wait,” you say. “If you’re in free fall, you are under the constant acceleration of gravity.”

Not so fast there.  Einstein showed that gravity is not a force per se, but rather a curvature in space itself.  Without getting into the mind-bending physics of relativity, we can use this way of thinking:

Right now, you’re probably sitting at a desk, on a couch, at a table, on the floor.  Maybe you’re standing up, maybe you’re laying down.  What forces do you feel acting on your body?  The force of gravity pushing you down?  Really?  Are you sure it isn’t the chair, or the floor, or the couch pushing you up?  Close your eyes for a moment.  Imagine that there is no gravity acting on you.  What would make you feel the force you are feeling?

Well, if your chair was accelerating upwards at a rate of exactly 1 g, would that be any different?  To look at this in a slightly more silly way, imagine you’re in a sealed room with no windows.  You have no idea where you are — you might be in deep space, you might be in Spokane, Washington.  All you know is that everything is pulled down at 1 g.  You hold a tennis ball in front of your face and let it go.  It hits the ground in a little over half a second.  You do the same with a hammer.  It falls at exactly the same speed.  So does everything else you drop.  Without harboring any preconceived notions of gravity (hard to do when you’ve grown up with it), which do you think would be more likely, that everything had some attraction to the floor with a force perfectly tuned so that everything falls at exactly the same speed?  Or would it be more likely that everything you let go of is actually at rest, and you are accelerating upward at 1 g?  Does the tennis ball feel any force between the time you let go of it and the time it hits the ground?

This long digression is really meant to underscore a specific concept — the minute a force is removed from an object, it enters a state of free fall.  If you fall off a roof, you enter free fall because the ground stopped pushing you up.  Likewise, when a rocket’s engine shuts down, the rocket enters free fall, even if it is still moving up.

So, back to the whole getting in space thing.  One of the first things you want to do once you get into space is to figure out a way to stay there.  One way to do this is to have enough of a force maintained on you to keep you at that altitude.  You can climb Mount Everest, and when you reach the top, you stay at 29,000 feet because Mount Everest keeps pushing you up.  The problem is, exerting a force with a rocket requires fuel, and lots of it.  You can’t keep it up for long.  There’s got to be a better way.

Fortunately, the earth isn’t flat, and “down” isn’t absolute.  Rather, the earth is nearly spherical, and “down” just points toward its center.  That means “down” where you are is not the same direction as “down” where I am.  It’s not even the same direction as “down” at your next door neighbor’s house.  To illustrate, if you were to hang a plumb line from the top of each of the towers of the Golden Gate bridge so that the bottom just touched the water, the tops would be approximately two inches farther apart than the bottoms would be.

We can take advantage of this fluidity of “down” to keep our spacecraft up in space.  Take a ball and throw it as hard as you can, level with the ground.  It curves downward, right?  But remember that “down” isn’t the same everywhere.  Likewise, “level with the ground” isn’t the same direction everywhere either.  As the ball moves away from you, the Earth curves away from its path, even as it falls toward the ground.  At the speed you can throw a ball, this curvature is imperceptible, but if you can go faster, the ball will travel farther, and the earth will curve away even more.  We can take this idea to an extreme, then.  There should be some speed at which you can throw a ball so that the Earth curves away from it just as fast as it is being pulled toward the center.  You can think of this as the ball continuously falling toward — but always just missing — the ground.  Once you can do this in a stable manner, you’re in orbit around the Earth.  You’re not staying up, rather, you’re falling, but the earth is falling away from you fast enough that you never hit it.

So…. the only thing you need to stay up is enough sideways velocity to create an orbit.  Just how much is this velocity?  Well, it depends on your height.  I’ll save the math for a later post, but long story short, the higher you are, the less velocity you need to stay in orbit.  To enter a stable low Earth orbit, like where Space Shuttle Atlantis will be going in a couple weeks, you need to reach a velocity of approximately 17,500 miles per hour.  That’s a mind-bogglingly fast speed — about five miles every second.  To put that in perspective, at that speed, you can travel from New York City to Los Angeles in just slightly more than eight minutes.  In fact, a low Earth orbit has an orbital period — the time it takes to completely circumnavigate the earth — of only an hour and a half.

So, to recap, to get into space and stay there, you have to 1) get at least 60 miles up (you probably want higher so the thin, wispy remnants of the atmosphere don’t drag you down sooner than you want), and 2) you have to get up to 17,500 miles per hour once you’re there.  More on just how you do that tomorrow.