What’s The Chance?

Econo-blogger extraordinaire Felix Salmon writes:

“Low-Probability Disaster of the Day, Exosphere Edition”:http://www.portfolio.com/views/blogs/market-movers/2009/02/12/low-probability-disaster-of-the-day-exosphere-edition?tid=true

From “Andy Pazstor”:http://online.wsj.com/article/SB123438921888374497.html:

Pentagon brass, satellite industry executives and NASA leaders for years have publicly expressed concern about the dangers of orbital debris. But the odds of a direct hit between satellites were considered so small as to be basically unthinkable.

Is there a way of distinguishing, ex post, between (a) the ex ante probabilities having been wrong, and (b) the collision having been genuinely improbable? I’m going with (a).

This is a really meaty question, actually. It’s not, as one might suppose, a deterministic problem, as there is a human element involved in steering active satellites away from anticipated collisions. (Expect some hard questions to be asked about how this collision came about.) It’s just the sort of complex and continuously occurring situation that “nuclear deterrence wonks often think about”:http://press.princeton.edu/titles/5301.html, in fact.

So if one were to assess the probability of a sat-sat collision beyond a first approximation, it would probably involve interrogating experts to get their probability estimates. And no, you really could not know if their probabilities for this or any other specific question were right or wrong. But you can establish, “after the fact”:http://press.princeton.edu/titles/7959.html, which experts provide accurate probabilities, and which don’t. What is more, with creativity and good preparation, you can even get a pretty solid idea of who is accurate and who isn’t “well before the fact”:http://www.us.oup.com/us/catalog/general/subject/Philosophy/History/20thCContemporary/?view=usa&ci=9780195064650. All without resorting to “black magic”:http://www.malleusmaleficarum.org/?p=6.

One thing you probably shouldn’t do is to try to judge the accuracy of past predictions by what happens now, “post-Iridium-Cosmos”:http://www.totalwonkerr.net/1847/iridium-cosmos. This event presumably reduces the odds of the next sat-sat collision, since operators will be more vigilant for a good while. There’s that slippery human element again.

Then again, sat-sat collisions are the least of our worries. The real threat is sat-debris collisions. Not only are these relatively common — David Wright identifies “seven such incidents since 1991”:http://www.ucsusa.org/nuclear_weapons_and_global_security/space_weapons/technical_issues/colliding-satellites.html — but we should expect the fresh debris generated by Tuesday’s debacle to accelerate the trend.

For The Record

A side note. “Andy Pasztor’s WSJ article”:http://online.wsj.com/article/SB123438921888374497.html contains a startling inaccuracy:

Industry officials say Iridium has identified the Russian craft as a Cosmos series satellite launched in 1993, weighing more than a ton and including an onboard nuclear reactor. That couldn’t be independently verified. Experts have said the chance of radioactive debris surviving a fall through the atmosphere and reaching inhabited areas is very small.

Rest assured, there was no nuclear reactor on board. Although “such a thing has been contemplated”:, so far as I’m aware, it’s never been done, and at least in my naive estimate, the practicalities seem daunting. That is presumably why this claim “couldn’t be independently verified” — which maybe means that it shouldn’t have been published, either.

(And let’s not even get into “Project Orion”:http://books.google.com/books?id=r_Gu4f0QxrkC.)

What Pazstor’s confused source probably was thinking of was a “radioisotope thermoelectric generator”:http://www.absoluteastronomy.com/topics/Radioisotope_thermoelectric_generator (or RTG), used to power deep space probes, the occasional satellite, or, in Russia, “various other things that require power, but people don’t routinely visit”:http://www.bellona.no/bellona.org/english_import_area/international/russia/navy/northern_fleet/incidents/37598. (RTGs have also provided Iran’s nuclear research agency with “an embarrassingly lame excuse for experimental Po-210 production”:http://www.iaea.org/NewsCenter/Focus/IaeaIran/iaea_note172004.pdf.) Long story short, a pellet of plutonium (or other radioisotope) gives off heat, which is used to generate electricity.

But, as it happens, this wasn’t the case, either. Pavel Podvig has “flagged”:http://russianforces.org/blog/2009/02/collision_in_space.shtml the dead bird as a “Strela-2M”:http://www.astronautix.com/craft/strela2m.htm comsat, which apparently involved chemical batteries of some type.

Hey, live and learn.

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