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Just how much warning do you need? If a strange satellite came within 10 km of your satellite every month, would you start to worry? Apparently not, not that is, if you are Iridium. I have been puzzling over the probability of one satellite, even if it’s from the larger population of dead satellites, hitting another. I just don’t believe in such long-shot outliers (it must be something like 10,000 times less likely than space junk hitting a satellite if it was simply a matter of random chance) and figured something else was going on, even given the increased density of stuff near the poles. It turns out that there was: these two satellites shared nearly the same altitude range (as defined by their apogees and perigees) for about six years. Every 35 days or so they came within 10 km or each, which is apparently the error associated with determining close approaches. (I have a pretty cool graph that shows this by looking at the debris generated by this collision.)
Something happened to Cosmos 2251 between 1999 and 2003: its orbit unaccountably shifted down to Iridium’s orbit. Perhaps there was a leak of remaining fuel. Perhaps it was hit by another piece of debris. There is no way for me to know what happened. But once it did, the fate of these two satellites seems fixed as the cosmic clockwork of Newtonian physics ground on. (Was Russia irresponsible in leaving unused fuel on board? That might change my feeling that this was clearly an example of Iridium irresponsibly not being aware of these conjunctions.)
What can be done about this? I’m still working on that but to me its starting to make sense, as the desirable orbital altitudes start to fill up, to de-orbit dying satellites. I estimate that this will result in an increase in launch costs (unfortunately, not the only cost associated with putting additional fuel on board) of only about $1 million for satellites launched using Western launch services and half that for satellites launched by non-Western services.
Update: Unfortunately, the interpretation of these graphs (especially the one at the top of the post) is less straight forward than I had hoped. In order to lessening that confusion, I’ve added a new image that should help explain what is going on. If not, I also wrote a comment that might help some more.