From: Johnson Space Center
Posted: Thursday, June 26, 2003
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.
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.
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!
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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