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ED LU LETTERS`



Submitted by Arthur - N1ORC

Lately, it seems like every time I look out the window I see Canada. A
few weeks ago it seemed that it was always the southern Andes Mountains
and Tierra Del Fuego. While I have nothing against Canada or the Andes
Mountains, I got to wondering why that is. It turns out there is a
fairly simple explanation for the "Oh Canada!" effect and the "The Andes
Again!" effect, but you have to understand a little bit about orbits
first. Plus, many of the really neat things I've been describing about
living on the ISS are a result of being in orbit - so it's worth a
mention. 
You may have noticed that I keep mentioning the speed we are traveling
at up here - about 18000 MPH. That is the key to what keeps us from
falling back down to the ground. In fact, we are always falling towards
the Earth, it's just that we manage to keep missing it. I'll explain.
Think of standing on the ground and throwing a baseball. The harder you
throw it, the further it goes before gravity pulls it to the ground.
Obvious. Now imagine you are incredibly strong and can throw the
baseball all the way across the country, or even half way around the
Earth before it lands. Now reach back and throw it even harder - perhaps
it goes three fourths of the way around the Earth. What if you throw it
even faster? Then maybe it will fly almost completely around the Earth
and land right at your feet. Now throw it just a bit harder. What will
happen? If there was no atmosphere and therefore no air resistance to
slow the ball down, the ball would fly all the way around the world,
right past your feet, and keep going. Since it doesn't slow down, it
keeps right on going and continues around the Earth again and again. The
ball would be in orbit. 
For the physicists and engineers out there, you know the story isn't
quite that simple, but the basic idea is correct. The trick to being in
orbit is to get going fast enough that you go all the way around the
Earth in the time it takes gravity to turn your direction around. While
gravity is pulling you downwards all the time and making you curve
around the Earth, the curvature of your trajectory isn't enough to
actually run into the ground. If you think about a bit you'll see that
there are some complications, namely you have to throw the ball at the
proper angle so it doesn't run straight into the ground, and also you
have to show that the trajectory doesn't diverge after repeated laps. Of
course you also have to make sure you don't go too fast, or you will
just fly away since the Earth's gravity won't be strong enough to pull
you back again. 
So that's all it takes, a lot of speed and initially a little bit of
aiming to make sure you don't hit the ground, and as long as you are
high enough so you are out of the atmosphere you will just keep going
round and round the planet. For orbiting the Earth at our altitude, that
required speed is about 18000 MPH. That is the purpose of the big rocket
that our Soyuz spacecraft sat on top of on the launch pad, and is also
the purpose of the Space Shuttles main engines and solid rocket boosters
- they both serve to lift their respective spacecraft high enough to get
out of the atmosphere, and then to reach orbital speed. Once that is
complete, they are no longer needed - the force of gravity will keep the
spacecraft in orbit around the Earth. The Space Station and everything
in it (including Yuri and myself) are just coasting along in orbit, much
like the moon also orbits the Earth. 
The interesting thing about orbits is that the closer you are to the
planet, the faster you need to go. This makes sense since the force of
gravity decreases as you move away from the planet. That's how Newton
first figured out his law of gravitation, by reasoning that the moon was
in orbit and figuring out that the force of Earth's gravity pulling on
the moon had to be much less than it was on the surface of the Earth.
The effect of this is that if you momentarily speed up in orbit, you
will climb to a higher orbit where in fact your speed will decrease. We
make use of this fact during the rendezvous of the Space Shuttle and
Soyuz with the ISS. 
Even though we are above almost all the atmosphere, there are still
traces left at this altitude which do cause some drag on the space
station. This has the effect of slowing us down slightly over time. This
then has the effect of lowering our orbit where in fact our speed will
then increase again, but at a lower altitude. In order to stay out of
the atmosphere, we have to periodically boost our orbit back up again. A
few weeks ago we fired the engines for a few minutes on the Progress
which was already docked to the Station in order to compensate for this
small drag. The engines of the Progress are small compared to the huge
station, so our acceleration was very little, and in fact we only used
it to increase our speed by 1 meter/sec, about walking speed. We tried
to watch out the window to see the engine firing, but we don't have a
window which can see straight backwards, so we couldn't actually see
much. We were able to show that the station was very slowly accelerating
by letting a pen float in the air. It slowly started to move towards the
rear of the station. Actually, it was the station wall which was slowly
accelerating towards the pen. 
A common misconception is that the reason we are weightless because we
are beyond the Earth's gravity. In fact, the reason we are in orbit is
exactly because we are being pulled downwards by gravity - as I said
earlier it is only because we are going fast that we manage to keep from
hitting the Earth. The reason we are weightless here is that the entire
ship around is also being pulled by gravity in exactly the same way, so
we are both falling around the Earth together. It is the same feeling
that you get when in a roller coaster going over the top, you feel light
in your seat for a moment because the seat is falling out from under
you. 
In a sense the entire Space Station has been pulled out from under us.
In fact, when flying around and doing flips inside the Space Station, I
am just doing exactly what divers do when they do flips as they dive off
a diving board. They are "weightless" also while they are in the air, it
is just that they only get a second or so until they hit the water. We
get 6 months. 
As I described in my last letter, our orbit path is like a big hoop
around the Earth that we circle round and round. Meanwhile, the Earth is
rotating on its axis once a day inside the hoop. Since the Earth is
pretty close to a perfect sphere (but not quite), its rotation doesn't
affect our orbit very much (think about how you would notice if a
perfect sphere was rotated around - answer - you wouldn't). As I
mentioned before, the hoop of our orbit doesn't go around the equator,
but rather is tilted by 51.6 degrees. You can figure out why that is
pretty easily; our launch site Baikonur is not on the equator. Since our
orbit has to start from Baikonur, it has to be tilted relative to the
equator in order to pass over that point. 
I've included a picture here of a computer program we use to give us our
current position and show the track of our orbit across the ground. In
the middle of the screen, the two small red rectangles with the little
white circle between them is supposed to represent the Space Station.
The white circle around it is roughly the patch of ground you can see if
you look down. Right now the Space Station is over Western Sahara (you
can see the zoomed insert in the lower left) and moving southeast
towards the bottom right hand corner. The white dotted lines show the
path that the Space Station will follow in this orbit, as well as the
next two orbits. The reason the 2nd and 3rd orbits are displaced to the
left is that during the 90 minutes it takes us to complete a lap around
the hoop, the Earth has rotated by one 16th of a revolution to the
right, so our orbit track is displaced to the left by that much each
time. You can see that we never cross over any point with latitude
greater than 51.6 degrees, so we never get to see the North or South
poles from here. You can also see that if we do go over a point, we go
over it twice a day: once going northeast, and once going southeast. 
The shaded areas of the map are the areas in darkness, and the rest of
the map is in daylight. If you are wondering why the day-night line
curves up and down, it is for the same reason that our orbit curves up
and down - namely the sun isn't over the equator so that while half the
Earth is lit up that half doesn't line up with the equator or one of the
lines of longitude. You can see that now in the summertime, very far
northern points will always be in daylight. 


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