Thursday, February 21, 2008
Celestial Events and Meaningless Probabilities
Photo by Atomicshark via Flickr under a creative commons license
There were two fascinating celestial events last night. My wife and I were standing on the front lawn, bundled up from the cold, looking at the total eclipse of the moon, which occurred under clear skies from 10:00 to 10:50 pm Eastern time.
Earlier in the evening I was listening to the radio debate about the US Navy shooting down a disabled spy satellite in a deteriorating orbit. When I got up this morning I heard on the radio news that the Navy hit their target, then later in the story the newscaster said, "the odds of being hit by falling space debris are one in a trillion."
That is a completely meaningless probability.
Does it mean that each individual , such as myself has a one in a trillion probability of being hit or any person, out of over four billion people living today has a one in a trillion probability of being hit? Does the probability apply for today, for this event, or over my lifetime?
I later found an online AP article by SETH BORENSTEIN that went into more detail describing various probabilities related to being hit by space junk that were bandied about in the news this morning.
The AP story was a little more specific than the radio news report about the one in a trillion odds. The article said that Center for Orbital and Reentry Debris Studies, which studies these issues, "puts the odds of anyone being hurt by any piece of re-entering space junk at one in a trillion, saying you are far more likely to get hit by lightning." This statement of probability is better than what the morning newsreader said, but still contains ambiguities. Does anyone refer to one specific person, such as me, or the entire human population? Does any piece of space junk mean any piece from this satellite, or any of the hundreds of pieces that re-enter the atmosphere every year?
The end of the article included a more sensible point by David Ropeik, a Boston risk communications consultant, and author of Risk: A Practical Guide for Deciding What's Really Safe and What's Dangerous "This is the type of risk that shouldn't be reduced to mere numbers It's the nature of the risk, not the number." Of course the morning drive time radio news isn't about making sensible points.