[amsat-bb] Re: Life Expectancy ARISat-1
wb3jfs at cox.net
Sun Aug 7 00:47:31 PDT 2011
If anything, maybe this calls for another Chicken Little Contest to see who
can come the closest to predicting re-entry.
----- Original Message -----
From: "Phil Karn" <karn at philkarn.net>
To: "James" <kb7tbt at gmail.com>
Cc: <amsat-bb at amsat.org>
Sent: Sunday, August 07, 2011 12:34 AM
Subject: [amsat-bb] Re: Life Expectancy ARISat-1
> On 8/6/11 8:55 PM, James wrote:
>> Thank you for your informative contribution..
>> Anyone with a real answer?
> Well, an educated guess can be made by looking at a plot of ISS altitude
> vs time:
> The ISS is periodically reboosted, accounting for the sudden jumps in
> this sawtooth-like graph. Obviously ARISSat-1 won't be reboosted, so if
> you extrapolate the downward-sloping parts of the graph you can get a
> rough idea of what will happen.
> The ISS orbital decay rate varies with changes in upper atmospheric
> density with solar activity, but also because of changes in its attitude
> and the operation of the solar panels.
> The orbital decay rate also depends on qthe ballistic coefficient of the
> object. This has units of mass divided by area -- the mass of the object
> divided by the cross-sectional area it presents in its direction of
> flight. The larger the ballistic coefficient, the less its deceleration
> from drag as it flies through the thin upper atmosphere.
> The ISS probably has a larger ballistic coefficient than any other
> satellite simply because it's so huge. The volume of most objects
> increases as the cube of the size while the cross-sectional area
> increases with the square. Since mass is usually a function of volume, a
> large object will generally have a higher ballistic coefficient and last
> longer in a given orbit than a small object.
> Obviously there are exceptions to the "large lives longer" rule such as
> the "Echo" balloons. The actual ballistic coefficient for any given
> satellite has to be computed from its actual mass and dimensions and its
> orientation relative to its velocity vector. The ISS is a huge
> satellite, but it also has lightweight solar wings that greatly increase
> its cross-sectional area without increasing its mass very much, so they
> decrease its ballistic coefficient somewhat.
> ARISSat-1 is far smaller than the ISS, but it is fairly heavy for its
> size and it lacks large solar wings that create a lot of drag. This will
> reduce its decay rate, but it will still probably decay more quickly
> than the ISS.
> It was tossed out the back of the ISS against the velocity vector, and
> that immediately put it in a lower energy orbit with a higher mean
> motion. But any further increase in mean motion will be due to orbital
> decay, and from that we should be able to estimate its ballistic
> coefficient and how it will likely behave in the future. Determining an
> exact lifetime would be difficult because of the difficulty of
> predicting solar activity, but a good estimate can probably be made.
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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