| Aurora
Australis | |
The
Expedition 7 crew captured this image of the Aurora
Australis, or "southern lights," from the International
Space Station.
Watch
aurora videos taken by the Expedition 6 crew.
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Expedition
7
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Watching
the World Go By
One of
my favorite things to do when I have time off is to just watch
the world go by. Whenever I get a chance, I spend time just
observing the planet below. It turns out you can see a lot
more from up here than you might expect. First off, we aren't
as far away as some people think - our orbit is only about
240 miles above the surface of the Earth. While this is high
enough to see that the Earth is round (believe me, it is),
we are still just barely skimming the surface when you consider
that the diameter of the Earth is over 8000 miles.
So how
much of the Earth can we see at one time? When you are standing
on the ground, the horizon is a few miles away. When in a
tall building, the horizon can be as far as about 40 miles.
From the ISS, the distance to the horizon is over 1000 miles.
So from horizon to horizon, the section of the Earth you can
see at any one time is a patch about 2000 miles across, almost
enough to see the entire United States at once. It isn't exactly
seeing the Earth like a big blue marble, it's more like having
your face up against a big blue beach ball. When I look out
a window that faces straight down, it is actually pretty hard
to see the horizon - you need to get your face very close
to the window. So what you see out a window like that is a
moving patch of ground (or water). From the time a place on
the ground comes into view until when it disappears over the
horizon is only a few minutes since we are traveling 300 miles
per minute.
When
looking out a sideward facing window, you can see the horizon
of the Earth against the black background of space. The horizon
is distinctly curved, so as I said earlier, I can see that
the Earth is not flat. The edge of the Earth isn't distinct
but rather is smeared out due to the atmosphere. Here you
can get a feel for how relatively thin the atmosphere is compared
to the Earth as a whole. I can see that the width of the atmosphere
on the horizon is about 1 degree in angular size, which is
about the width of your index finger held out at arms length.
For those of you who are farsighted, it is also about the
height of a person when seen from about 100 yards away (the
length of a football field). At a distance of 1000 miles,
that translates into a height of about 20 miles. There really
isn't a sharp boundary to the atmosphere, but it gets rapidly
thinner the higher you go. Not many airplanes can fly higher
than about 10 miles, and the highest mountains are only about
6 miles high. Above about 30 miles there is very little air
to speak of, but at night you can see a faint glow from what
little air there is at that height.
Here
is a typical view out the window - blue ocean and clouds.
Note how thin the atmosphere is on the horizon.
Since
we orbit at an altitude about 40 times higher than the tallest
mountain, the surface of the Earth is pretty smooth from our
perspective. A good way to imagine our view is to stand up
and look down at your feet. Imagine that your eyes are where
the ISS is orbiting, and the floor is the surface of the Earth.
The atmosphere would be about 6 inches high, and the height
of the tallest mountain is less than 2 inches, or about the
height of the tops of your feet. Almost all of the people
below you would live in the first one quarter of an inch from
the floor. The horizon of the Earth is a little over 20 feet
away from where you are standing. If you are standing on top
of Denver, then about 15 feet to one side you can see San
Francisco, and about 15 feet to the other side you can see
Chicago. At this same scale, the Earth that you are standing
on would be a sphere with a diameter of about 160 feet. If
you want to complete the effect, you can start walking and
take a step about every 20 seconds.
The
San Francisco Bay Area - Some things you can have fun looking
for are the Golden Gate Bridge and the Bay Bridge, Alcatraz
Island, Golden Gate Park, and Stanford University. Also look
for the red salt ponds on the bay near Fremont.
Now I
thought I'd take you on a guided tour of an orbit around the
Earth. Take a globe and imagine a hoop representing our orbit
around the equator. Now tilt the hoop by 51.6 degrees, and
that is what our orbit track looks like. The Earth rotates
on its axis every 24 hours inside the hoop, while we go much
faster around the hoop, making a lap every 90 minutes. I'll
write more about the mechanics of orbits and ground tracks
in a later installment. For now though you can see that if
we start our tour along the inclined hoop from over the equator,
we will at first be moving in a northeasterly direction, and
by a quarter of an orbit later will be at the northernmost
point of our orbit traveling in an easterly direction. The
orbit then becomes southeast, we cross the equator into the
Southern Hemisphere and by the time we reach the southernmost
point of our orbit we are again traveling due east. The final
quarter of an orbit takes us back to the equator, but not
over the original point since by then the Earth will have
rotated 1/16th of a revolution in the 90 minutes (or 1/16th
of a day) it took us to travel once around. If our orbit hoop
was completely fixed in space (which it is not quite exactly),
then we will see 16 different orbit tracks each day with the
pattern repeating itself every day.
One of
my favorite orbit tracks starts over the equator southwest
of Hawaii. At this point, looking down you will just see water
and clouds. The Pacific Ocean is a deep bright blue color,
and typically over the equator there are scattered bright
white clouds. In about 3minutes, off to the left of our track
you can see the islands of Hawaii. You can easily see the
standing clouds over the mountains as the trade winds blow
up the mountainsides. When you fly right over the top of the
islands you can look down and see the city of Honolulu near
Pearl Harbor. If the weather is good and the air is clear
(and it almost always is in Hawaii) you can see objects as
small as maybe a quarter of a mile in size with your bare
eyes. What matters most for spotting objects is usually the
contrast with surrounding areas. For instance there is a large
runway (8R if you care to know the name) at Honolulu International
Airport that has been built out on the edge of the water that
you can very easily see from space since it is easy to pick
out against the blue color of the water. With binoculars you
can see much smaller objects like ships and individual buildings!
Here
is a closeup shot of Honolulu - you can see the airport on
the left, and if you look carefully you can see my old apartment!
(well, almost). However, you can see Diamondhead Crater, Waikiki
Beach, Punchbowl Cemetery, and a whole lot else.
After
Hawaii passes off to the left, again you see mostly ocean
for a few minutes as we head northeast towards the California
coast. We cross the coastline just north of San Francisco,
and looking down you can see the cities of San Francisco,
Oakland, and San Jose surrounding San Francisco Bay. Cities
have a grayish color, probably because of all the asphalt
and buildings. They are not always easy to spot unless they
are located near an easily recognizable feature (like San
Francisco Bay) or the surrounding areas have a very different
color or brightness (cities surrounded by forests for instance).
In the bay near Fremont (where my parents live) are huge maroon
red ponds which are very easy to spot from space. This color
is from bacteria growing in the ponds where they evaporate
water to collect salt. The Great Salt Lake in Utah has a similar
color. Looking to the left of our track you can see a line
of white snow capped volcanoes running up the Cascade mountain
range and Washington State in the distance. To the right you
can look down the central valley of California to the Baja
Peninsula in Mexico.
We continue
northeast heading over the Rocky Mountains, over Yellowstone
Park, and up into Canada. When I lived in Colorado I remember
the big afternoon thunderstorms that we would get in the summertime.
From space, you see that this area is covered with isolated
thunderhead clouds that pop up like mushrooms in the late
afternoon. Actually, thunderstorm clouds look more like flattened
cauliflower heads when viewed from above.
Southern
Canada is covered with a myriad of small lakes. If the sun
is overhead, you can see the sun glint off the lakes, rivers,
and streams - briefly lighting them up as the reflection point
moves across the surface of the Earth with you. Looking left
we pass Hudson Bay. Even now in the summertime there is ice
on parts of the bay. To the right pass the Great Lakes. One
thing that is surprisingly easy to see from space is airplane
contrails, the white condensation trails left behind highflying
jets. You can see the white lines converging on Chicago from
all directions. Continuing eastward, we pass over the St.
Lawrence River, over Newfoundland, and then out over the Atlantic
Ocean. The total time to cross North America has been about
12 minutes.
Over
the North Atlantic Ocean lately there have been wide expanses
of clouds covering the ocean like a blanket. These large weather
system clouds look like a white 1970s era textured carpet
with bumps and ridges and the occasional thunderhead cloud
popping up through. As we head southeast towards the coast
of Africa, you can see in the distance the red deserts of
the western Sahara. The color here is a chalky red, almost
like the color of red bricks. Lately there have been large
dust storms over the desert blurring out any detail on the
ground. We skim the coast heading southeast over the war-torn
countries of Sierra Leone and Liberia, and continue towards
the Cape of Good Hope at the southern tip of Africa.
If Hawaii
is in daylight, then this side of the Earth will be on the
night side. Since we orbit the Earth 16 times a day, we see
16 sunrises and sunsets each day. Looking backwards towards
the horizon you can see the sunset. Sunsets and sunrises are
beautiful, with a very thin distinct color layers in the atmosphere.
They range from orange and red near the surface to various
distinct shades of blue, purple, and finally black. The sun
rises and sets fairly quickly at the speed we are flying,
taking just a few minutes for us to go from dark to light
or vice versa. Looking down on the ground you can see the
line dividing the day and night sides of the Earth. If there
are high clouds, you can often see the long shadows they cast
when the sun is low in the sky.
At night,
you can easily see city lights. Larger cities are very easy
to pick out, as well as sometimes the lights along major roads
between them. As we round the southern tip of Africa, you
can see the lights of Cape Town. City lights have a yellowish
hue, which I think is due to the fact that most streetlights
are sodium vapor lamps which have a yellowish tinge. We then
head eastward over the southern Indian Ocean.
At night,
the dominant thing you see when you look down is thunderstorms.
Lightning lights up the clouds in sometimes spectacular displays.
At any given time at night, especially over the tropics, you
can see one or more lightning storms going on. The lightning
flashes illuminate the clouds from within, and ripple through
the storm systems. I enjoy turning off all the lights in the
docking compartment, and watching thunderstorm systems at
night through its sideward facing windows. The southern Indian
Ocean is a great place to watch thunderstorms.
As we
near Australia, if you look towards the horizon southward
you can see the aurora. The aurora look like glowing green
curtains which move upwards from the top of the atmosphere.
The curtains intersect the atmosphere in a curved line, which
appears as a bright green line south of Australia. There are
times when we actually fly through the aurora, and you can
look downwards and see the green glow below you. Sometimes
there are traces of red along with the predominant green.
We've taken some time-lapse movies looking towards the horizon
as we fly past the aurora.
The next
part of this orbit takes us northeast across Australia. In
the daytime, you can see the bright red color of the deserts
of central Australia. At night, by the absence of lights you
can see how few people live in this area. To the right is
the coastal city of Sydney. The final part of this orbit crosses
over the Great Barrier Reef and various South Pacific islands
on our way back towards the equator. Most of the small islands
dotting the South Pacific are ringed by coral reefs. The most
striking thing about these reefs and small atolls is the bright
almost iridescent aquamarine green color of the water. And
that takes us to the finish of this orbit - total time around
the world is 90 minutes. In a later installment I'll write
about some of my other favorite orbit tracks. I'm only including
a few pictures here so this e-mail isn't too huge. If you
want to see lots more pictures that we have taken, go to the
Expedition
7 Gallery or to the NASA
Earth Observation Web Site where you can find lots of
interesting stuff.
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