While researching for this question I ran across something called Celestrak Socrates. I requested the ten closest predicted approaches, and saw distances of closest approach of the order of 100 meters or even less. I recalled this question but this kind of close approach doesn't apply, since I don't think Iridium and OrbComm got together and planned this.
I figured there must be some mistake, so I clicked "Analysis" and got the TLEs for the two spacecraft, Iridium 55 and "OrbComm FM18. I plugged them into Skyfield and sure enough, at about 17 seconds after
2016-12-15 13:14:00 UTC it shows them passing about 100 meters from each other at 6,674 m/s relative velocity. I had to use 1 millisecond time spacing to resolve the event.
Question: Is 75 meters an exceptionally close distance for two satellites to pass at >6,000 m/s? I get the feeling that since this is a "weekly top-10" list, this kind of close approach must be happening frequently. Are these satellite constellations under control to such precision that a ~100 meter miss is routinely acceptable, or did I stumble upon a fluke somehow?
I suppose this will happen even more often if SpaceX put's their first 1,600 satellites at a single altitude/inclination in 32 intersecting planes.
1 25272U 98019A 16348.53470940 .00000004 00000-0 -55193-5 0 9992
2 25272 86.3948 4.6359 0002006 88.0295 272.1131 14.34219879979454
1 25414U 98046B 16347.49264999 -.00000013 00000-0 46447-4 0 9990
2 25414 44.9965 56.3156 0006146 236.8164 123.2093 14.32449704958255
above: Celestrak Socrates screen shot circa 2016-12-13 18:30 UTC
I went back to the page (about an hour and a half later, I've added estimated times of page loadings before screen shootings) and the predicted distance of closest approach has changed from 0.075 kilometers to 0.113 kilometers, which is precisely the value I get from Skyfield. The TLEs have not changed, gee, what's going on?
update: Figured this out at least. The epoch for the Iridium 55 TLE is
16348.53470940 which is about 12:50 UTC. Clicking the Analysis button around 18:00 UTC seems to have retreived the updated TLE, and not the TLE that was used to generate the display. It could very well be that if I had updated the page at that moment, the new minimum distance 0.113 km would have appeared.
Twice each day, CSSI runs a list of all satellite payloads on orbit against a list of all objects on orbit using the catalog of all unclassified NORAD two-line element sets (TLEs) releasable to the public to look for satellite conjunctions over the next seven days. The reason for limiting the search to conjunctions with payloads is to give satellite operators an opportunity to plan—or obtain planning services—to move their satellites out of harm's way (if their satellite is maneuverable), or to take other appropriate measures. Since there currently is no way of knowing which payloads are still active, all payloads are considered.
above: Celestrak Socrates screen shot circa 2016-12-13 20:12 UTC