[amsat-bb] Re: Life Expectancy ARISat-1
rwmcgwier at gmail.com
Sun Aug 7 04:48:06 PDT 2011
I would prefer having a set of orbital elements that actually correctly predicts AOS/LOS times for a couple of days to start my "SWAG" on when it will reenter.
Sent from my iPad
On Aug 7, 2011, at 3:47 AM, "Jeff Yanko" <wb3jfs at cox.net> wrote:
> If anything, maybe this calls for another Chicken Little Contest to see who
> can come the closest to predicting re-entry.
> Jeff WB3JFS
> ----- 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.
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