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Astronaut Ed Lu's second letter from the ISS

04 June 2003

Flying

Part 1 of 2

I thought I'd write next about what it is like to fly. That of course
is how we get around up here on the space station. The main
difference between life up here and life down there is that things
don't drop to the floor here when you let go of them, and that
includes yourself. Rather than walking around as we do on the ground,
we fly around inside the station. It takes some practice getting used
to it, but you get better rapidly. I thought I was pretty good at
flying after 2 short shuttle flights, but after working with Ken,
Nikolai, and Don (who had been up here for almost 6 months), I
realized I have a lot to learn.

On about our second day onboard ISS, Sox and I had the task to pump
some water into a container for use in the galley, so off we flew
from the Service Module (the main Russian living compartment) to the
node (where all our water is stored). We were each carrying some
items in one hand (pumps, hoses, cables, etc.), so we only had one
hand free. Off he flew, and I couldn't keep up with him over the 70
feet or so to the node. We had to fly through a module called the
FGB, which is a narrow 35-foot long corridor with equipment strapped
and "velcroed" to the walls, ceiling, and floor. The faster I tried
to go, the more I bumped into stuff. Sox flew straight as an arrow
down to the node, while I moved along the handrails down the
corridor, leaving a cloud of debris I had knocked off the walls
behind me. It turns out flying with one hand tied behind your back
isn't so easy! 

Our flying up here takes place under the jurisdiction of Newton's
laws of motion. Over 300 years ago Isaac Newton wrote down his famous
laws of mechanics - which was a great stroke of genius at the time,
but you quickly realize up here how obvious they are if you are
weightless and don't have pesky gravity utterly dominating the
mechanics of moving around. Unfortunately Newton didn't have the
advantage of living on a space station. His first law, which states
that objects in motion will tend to remain in motion, and objects at
rest will tend to remain at rest, is the very first thing you have to
deal with when learning how to fly. When flying across the module,
you will continue in a straight line until you grab onto something or
you hit the far wall. Similarly, if you are floating in the middle of
a module and not moving, you will stay there until you push off some
other object (like a handrail or a wall). I am ignoring the effect of
air resistance and air currents because it doesn't have too much
effect on human flying (it does for much lighter objects).

Flying can be broken down into two tasks: getting from here to there,
and keeping yourself facing the way you want. Engineers call this
translation and orientation, and they are exactly the same tasks that
a spacecraft like the ISS, space shuttle, or Soyuz needs to do when
flying about space. In effect when flying around inside the ISS you
are like a miniature spacecraft. When we talk about translation, we
mean moving your center of mass (also called the center of gravity).
For humans the center of mass is around your belt buckle. And as my
old wrestling coach Mr. Yengo used to say - "Wherever your center of
gravity is going is where you are going." As for orientation, when
you spin an object here, it will rotate around its center of mass, so
that means if you do somersaults here you will see that you will
rotate around a point near your belt buckle. Controlling which way
you are facing means controlling your rotation around your center of
mass.
 
First, getting from place to place. If I want to get from one end of
the laboratory module to the other, all I have to do is push off from
the wall to get my center of mass moving, fly across the lab, and
stop myself on the other side. Easy! But remember that since you fly
in a straight line, you can't make midcourse corrections unless you
grab onto something along the way, which is fine but you lose style
points for that. The next thing to think about is how hard you push
off. If you push too hard, you end up going really fast, and the next
thing you know you are crashing into something on the other wall.
Again, with nothing to slow you down in the middle of the module you
are kind of helpless until you hit the far wall. It turns out that
you don't need to push off from the wall as hard as you might think.
On the ground, it takes a lot of work to move around because you are
constantly fighting the force of gravity trying to make you fall to
the floor. Up here, a push of maybe a few pounds is about right to
fly across the module at a comfortable speed. It is easy to fall to
the temptation of really flying fast, but you have to be careful to
not knock your head on the many hatches and bulkheads here. Every bit
of momentum you put into moving your center of mass (i.e., flying
somewhere) has to be taken out at the other end when you stop, so
flying slowly takes less energy. 

 - Continued -

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