I remember that a few years ago with some other bird Like Ande or something like that?
Joe WB9SBD The Original Rolling Ball Clock Idle Tyme Idle-Tyme.com http://www.idle-tyme.com On 8/7/2011 2:47 AM, Jeff Yanko wrote: > 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 > _______________________________________________ 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
