The big news in my industry right now is the collision between the satellites Iridium 33 and Cosmos 2251. While the Cosmos spacecraft was reportedly non-operational since 1995 Iridium 33 was operational and in service.
In some of the press the choice of wording (e.g. “out of control” and “slammed into the Iridium craft”) has been such as to unfairly suggest that Cosmos 2251 was at fault. For example, from BBC News:
“A collision occurred between an Iridium 33 satellite and a Russian Cosmos 2251 military satellite,” Major General Alexander Yakushin said.
The satellite was launched in 1993 and ceased to function two years later, he said according to the AFP news agency.
Russia has not commented on claims the satellite was out of control.
The Russian Major General said that Cosmos 2251 ceased to function in (or around) 2005, so by definition it is “out of control,” which is to say it was orbiting the Earth as dictated by fairly well understood laws of physics. However, the term “out of control” is used by BBC (and others) in a way that implies that Cosmos 2251 is flying around all over the place in an unpredictable manner endangering other spacecraft left and right.
The truth is, it doesn’t really matter, this could just as well have happened between any two operational spacecraft. The key here is that in general, no one is looking. And even if you are looking, it is difficult to know what to do.
I first heard of the event about twelve hours after the fact from Dr T.S. Kelso at the Center for Space Standards & Innovation. Dr Kelso operates the venerable CelesTrak website and one of the services his site provides is SOCRATES (Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space). SOCRATES compares Two-Line Elements (parameters describing satellite orbits, hereafter referred to as TLEs) from the US Air Force in order to predict close approaches between objects in orbit around Earth. SOCRATES had predicted a close approach between Iridium 33 and Cosmos 2251 of 584 m at the time of collision. This highlights a significant problem: the data generally available to predict close approaches isn’t very accurate. The actual accuracies of any given TLEs are not well known, perhaps not even to the Air Force. In this case the data was obviously off by about 584 m (the satellites themselves may be up to tens of meters large).
So, should Iridium and other satellite operators look out for, and maneuver their satellites to avoid, anything that comes within a given distance of their own spacecraft? Apparently a 584 m predicted miss distance is not safe so for the sake of argument let’s assume that we use that as a safety threshold. Searching for “IRIDIUM” in SOCRATES right now returns 16 close approaches with distances less than 584 m over seven days:
|Time of Closest Approach (UTC)||Object 1||Object 2||Min Range (km)|
|2009 Feb 13 14:38:21||IRIDIUM 16||COSMOS 1275 DEB||0.137|
|2009 Feb 16 05:47:41||IRIDIUM 97||CBERS 1 (ZY 1A)||0.184|
|2009 Feb 13 03:22:09||IRIDIUM 16||FENGYUN 1C DEB||0.213|
|2009 Feb 15 23:59:47||IRIDIUM 45||SL–8 DEB||0.251|
|2009 Feb 12 19:38:39||IRIDIUM 24||COSMOS 1275 DEB||0.274|
|2009 Feb 12 07:19:35||IRIDIUM 26||NOAA 6||0.315|
|2009 Feb 12 14:18:32||IRIDIUM 36||CZ–4 DEB||0.348|
|2009 Feb 18 18:08:06||IRIDIUM 59||COSMOS 990||0.363|
|2009 Feb 16 00:12:50||IRIDIUM 54||THORAD AGENA D DEB||0.373|
|2009 Feb 17 07:48:07||IRIDIUM 38||DELTA 1 DEB||0.417|
|2009 Feb 17 06:36:02||IRIDIUM 50||ARIANE 40 R/B||0.483|
|2009 Feb 12 17:16:04||IRIDIUM 21||SL–8 DEB||0.490|
|2009 Feb 18 01:35:15||IRIDIUM 54||OPS 5058 (DMSP 5B F2)||0.491|
|2009 Feb 15 03:23:34||IRIDIUM 11||SL–18 DEB||0.495|
|2009 Feb 12 12:41:48||IRIDIUM 81||COSMOS 1190||0.555|
|2009 Feb 16 11:51:54||IRIDIUM 4||TITAN 3C TRANSTAGE DEB||0.578|
Note that none of the above include debris originating from the Iridium 33/Cosmos 2251 collision - most likely because the Air Force hasn’t begun to publish that data yet. And we’re only looking at close approaches involving the 92 Iridium spacecraft, while there are over 10000 known objects out there. Check out the SOCRATES Top 10 listings at any given time for even closer approaches. The point is that services like SOCRATES predict lots of close approaches where the miss distance is significantly below the known to be unsafe 584 m, but actual collisions are still extremely unlikely and (to date) extremely rare.
Now, from the point of the satellite operator it should be understood that maneuvering a satellite to avoid a conjunction generally carries a quantifiable monetary cost with it. If the available information is so inaccurate you don’t even know for sure if you’ll be improving the situation by maneuvering (which is often the case when the information is TLEs) chances are you’ll do nothing and hope for the best. Indeed, there is a good chance you won’t even bother looking.
What is needed to make it operationally feasible to react to predicted close approaches is better data. Operators like Iridium have to know that if the data says the miss distance is, say, 100 m they are safe, and that for the infrequent cases where the miss distance is less the risk is so high that it is worth doing the analysis and spending the fuel necessary for an avoidance maneuver.
I have more to say on this topic but it’s getting late. In the mean time, check out some visualizations courtesy of Analytical Graphics, Inc.