If anything, maybe this calls for another Chicken Little Contest to see who can come the closest to predicting re-entry.
73, Jeff WB3JFS ----- Original Message ----- From: "Phil Karn" <[email protected]> To: "James" <[email protected]> Cc: <[email protected]> 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: > > http://www.heavens-above.com/IssHeight.aspx > > 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. > > --Phil > > _______________________________________________ > Sent via [email protected]. Opinions expressed are those of the author. > Not an AMSAT-NA member? Join now to support the amateur satellite program! > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb > _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
