The concerns was, that it might have a still partly filled fuel tank - potentially creating a risk at re-entry, a concern similar to that for the re-entry of USA 193 in 2008 (which, for that reason, was destroyed on-orbit by a SM-3 missile). This greatly worried me, the more as no news on this was appearing from either the Japanese, or US Space Command (who no doubt must have been aware that IGS 1B was coming down - an object like this will certainly be tracked).
My assessment of a potentially still half-full tank, was, as I indicated, at best an "educated guess". Noted amateur observer Ted Molczan from Toronto has now made an independant assessment of the situation, notably on the absolute amount of fuel left. Below I quote from his analysis, in which he writes (after first noting that he gets similar results to mine as to the probable time of decay, i.e early 2012):
"I agree that IGS 1B could decay from orbit in 2012, with perhaps half of its initial fuelmass; however, I believe that its initial fuel mass probably was far less than that of USA193 - between approximately 28 kg and nearly 100 kg, compared with the 450 kg of USA 193. Ifhalf of IGS 1B's fuel has been expended, then between 14 kg and 50 kg may remain - at most10 percent of USA 193's fuel load. Only the Government of Japan knows for certain thequantity of fuel that remains, but if my estimate is in the ballpark, then the risk of anuncontrolled decay from orbit would be much less than for USA 193."
"USA 193 carried about 450 kg of fuel, none of which had been expended by the time of its impending decay, due to its failure soon after it reached orbit. I believe that IGS 1B may have considerably less fuel for the following reasons:
1. IGS 1B was designed to operate at a considerably higher altitude than USA 193 (485 km vs. 360 km), which means that it was subject to far less atmospheric drag, which would have decreased the quantity of fuel required for orbit maintenance.
2. IGS 1B's total mass is reportedly about half that of USA 193 (1200 kg vs. 2300 kg). For a given velocity change, the fuel expenditure varies in direct proportion to total spacecraft mass.
3. IGS 1B died four years into what was reportedly a five year mission, so might already have expended most of its fuel."
[note from Marco Langbroek: but its sister ship IGS 1A is still maintaining orbit 8 years later, as I indicated in my original post, suggesting that these satellites carry more fuel than for a minimum 5 year mission]
"With respect to points #1 and #2, assuming that IGS 1B's ballistic coefficient (mass divided by cross-sectional area) is similar to that of USA 193, and that its fuel supply was designed to enable operating up to twice the reported 5 year design life, i.e. 10 years, then the total velocity change required to maintain 485 km altitude would have been about 53 m/s (metres per second). Assuming IGS 1B uses the same fuel as USA 193, then for its mass of 1200 kg, the required initial fuel mass would have been just 28 kg - far less than that of USA 193.
Factoring in point #3: assuming provision of fuel for 10 years operation, then IGS 1B might have consumed 40 percent of its fuel by the time it died, four years after launch. Considering that its first couple of years of operation coincided with the tail end of the previous solar maximum, its fuel use could have been somewhat greater; assuming for the sake of argument that half its fuel has been expended, then 14 kg would remain.
I based this rough estimate on data found in the respected textbook/reference Space Mission Analysis and Design III, specifically the annual velocity change required to maintain low Earth orbits against decay, depending on altitude, ballistic coefficient and solar activity. I assumed that fuel for attitude control was negligible, and that IGS 1B was not designed to be de-orbited at the end of its useful life (the latter would have increased the initial fuel mass to nearly 100 kg, with perhaps 50 kg remaining after four years of operation, still far less than USA 193 carried.)"
I have high trust in Ted's assessment: and the result is somewhat of a reassurrance: 14 to 50 kg of fuel is an order of a magnitude less than the 450 kg of fuel of USA 193. While no uncontrolled re-entry is without danger, these figures from Ted's assessment lead me to think that IGS 1B is clearly less of a threath than USA 193 was.
Ted's assessement is exactly the kind of thing I called for in my earlier post, when I wrote:
Instead of watching this one quietly go down, I would prefer to see a good risk assessment done [...] a clear argument presented as to why it would not be a danger in this case, given all the fuzz created around falling fuel tanks with USA 193.Ideally, this should of course have come from the Japanese themselves (which refused to say anything pertinent to one of the reporters that contected me over this, besides the simple statement that there was "no risk"). In absence of that, Ted's assessment is a good thing to have.