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The Webfooted Astronomer - July 2000


The Satellites of Summer

By Pat Lewis

AS we look up at the starry sky, probably all of us have noticed a speck of light crossing the sky among the stars. Did you wonder whether it was an airplane at high altitude or, perhaps, an artificial Earth satellite? More likely than not, the object was a man-made satellite, one of several thousands now orbiting the Earth.

These objects don't shine by their own light, but most of them have shiny metallic surfaces that catch sunlight, and many are large enough to be seen by the naked eye. By observing the object's direction and speed, you can estimate its altitude and make a good guess about what kind of satellite it is.

First, look for flashing wing lights. A plane a few miles high may move across the sky at about the same rate as a satellite. If it is very high you may not be able to see the wing lights with just your eyes. Look at it in your binoculars. If there are no strobe lights, then it's a satellite, about 250 miles up.

Notice the direction it goes and how long it takes to cross the sky. Satellites may move north to south, or south to north, or west to east, but never from east to west. When satellites are launched, they always head eastward to take advantage of the Earth's rotation, going more than 1,000 miles per hour near the equator. This saves a lot of fuel. The north-south satellites may be military reconnaissance satellites in polar orbits because that allows them to look over the entire Earth every day. These military satellites often have lower orbits than others; if so, they will be moving faster across the sky.

Of the thousands of objects circling the Earth, only a relative few are actually spacecraft. All the rest are an assortment of debris, space trash. For example, back when the little 20-inch Sputnik was going around the globe and we looked up at it in awe, what we really saw was the 100-foot-long upper stage of its rocket, which went into orbit along with it.

The U.S. Delta rocket's final stage does this too, but the final stage of this rocket still carries some liquid fuel, which may leak. In 1992 a Delta upper stage blew up after 15 years in orbit, becoming hundreds of pieces of orbiting scrap metal, each of which now has to be separately numbered and tracked.

If you see a satellite whose light seems to be blinking off and on, it's probably one of these pieces of space junk, tumbling as it travels in its orbit, sometimes catching the sunlight, sometimes not.

Sometimes you will watch a satellite move steadily across the sky, and it suddenly disappears. Where did it go? It disappeared into the Earth's shadow. Light from the Sun shines through space all around the Earth, illuminating whatever objects are there—the Moon and the planets, for example. But extending out from the Earth is a huge cone of shadow. You can't see it, but it's there. In winter it stands almost straight upward after sunset, making winter a poor time to see satellites.

In summer when the shadow slants off toward the south, you can watch a satellite moving north to south or west to east in bright sunshine. Then suddenly it plunges into the invisible shadow, just as the Moon does in a lunar eclipse.

When you're out observing this summer, watch for the satellites. You might as well-—they're watching you!

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