Ask A Scientist© Physics Archive

Question:  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?
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Answer:  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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