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Ed Lu Letter from Space #6

4 July 2003

Orbits

Part 1 of 2

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.

 - Continued -

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