Re: RCS VALUES + decaying object brightness

From: Daniel Crawford (
Date: Sat Jan 04 2003 - 13:13:29 EST

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    Thanks much to all for the replies to the original question and follow-ups.
    It is clear that size isn't the only thing that matters.  Angle, shape,
    distance, etc., are some of the variables that determine how "bright" an
    object would be WRT an observer.
    I asked the original question about a year late.  I have learned that it is
    difficult to find/track a piece of small orbiting debris. Just because an
    object is cataloged, under 200KM, and making an overhead pass, doesn't
    really mean that you can see it.  I have tried many times, with little
    success.  I didn't know better...  :)  After lots of non-observes, I know a
    little better.
    I have an interest of looking for the bits and pieces of things that are
    really low and coming down soon, regardless of the size.  Perhaps I am
    foolish, but I figure that some day, if I really keep trying to track down
    de-orbiting debris, I will see a decay.  Sure, the odds are low, but much
    better if you know where you should be looking.  It is a free way to pass
    the time; the only cost is my neighbors looking at me funny at night  :)
    This brings up the further question.  How "big" would an object need to be
    to see a decay?
    Again, I know that the odds are small in observing one, but if....
    An object was decaying overhead.  I would expect that the size of the object
    would be proportional to the brightness and length of time the object would
    be visible.  Any thoughts to how bright and how long a decay would be with
    respect to the mass/surface area of the object?
    For an example, how bright and how long will Starshine be when it decays?
    How about the pieces of an object that exploded?  Large rocket bodies and
    Thanks. Clear, dark and warmest regards.
    Daniel Crawford
    ----- Original Message -----
    From: "Michael McCants" <>
    To: <>
    Sent: Saturday, January 04, 2003 10:46 AM
    Subject: Re: RCS VALUES
    > Phillip Clark wrote:
    > >Quite a few years ago Nick Johnson had a paper published
    > Perhaps it might be important to know about how many years ago.
    > >So, unless the system has improved, the RCS values are very nice to look
    > >but useless in reality !
    > Since Jay got the RCS values included in the SSR, we now have nearly
    > ten years experience with those values.
    > Some values are clearly quite wrong.
    > Some special values seem to mean "unknown value" or "default system
    > value" and should be ignored.
    > Values for objects that are usually higher than 10000 miles are
    > often much too small or much too large.
    > Taking the median (not the mean) of the last 50 or 60 values seems to
    > produce a median value which is fairly well correlated with the
    > observed visual magnitude, but of course the optical reflection
    > properties (flashes and large changes in brightnes) cannot be expected
    > to always compare to what amounts to an "average" radar reflection
    > value.  And, of course, some objects are white, some are black, and
    > some are polished, so the RCS only provided a "guideline" as to what
    > optical brightness might be expected.
    > If an object explodes into many pieces, a visual observer would be
    > wise to look for the ones with a larger RCS before looking for the
    > ones with a smaller RCS.  :-)
    > Summary:  I have been including the median RCS value in my printed
    > visual predictions for many years and I compare my observed magnitudes
    > with that median RCS value and I have not found all that many
    > "discrepancies" in over 2500 different LEO objects in the last decade.
    > Mike McCants
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