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When the shuttle burns out of orbit, What is its rate of descent? During daylight hours, at what altitude does it become a naked eye object?

I have watched a shuttle land. I saw the next shuttle that went up after the "Challenger" disaster land at Edwards Air Force Base in California. You *hear* the shuttle before you see it: it produces a characteristic double sonic boom as it crosses the California coast. Then everybody looks for it --- in those days typically thousands of people turned out for each landing --- and it takes about 3 or 4 minutes from when it is spotted until it lands, which is much quicker than if you were watching airplanes at the airport, because the shuttle lands faster than a jet and glides downward at a much steeper angle. The outside limit on seeing far-away objects is the resolution of your eye --- how small a thing can you make out? As a rough estimate, you can see something L feet long at a distance of D = 1720 * L/x feet away, if you can resolve something x seconds of arc across. To estimate how big x is for you, look at craters on the moon or a bird flying across it: the moon is 30 seconds of arc across, what fraction of the moon would an object have to be for you to recognize it? If you can make out things 1/5 the size of the moon, x = 6 and you can see 100 foot objects (the size of the shuttle) 30,000 feet away. But the problem with the shuttle is not the outside limit, but *finding* the thing in the sky. If you have ever watched a child's balloon rise up in a clear sky, taken your eyes off it for a moment and then tried to find it again you know what I mean. It helps that the shuttle is black (from below) but normally it is not spotted until it is well within the outside limit.

christopher grayce

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