Falling Satellites



In the last blog I discussed a tweet posted by NHS Suffolk about the health risks of smoking. It said:

“If you are thinking of having a cigarette, remember it could kill you ow.ly/7bWFt”

There was another thing that amused me about the tweet, and that was in the replies it received. There were two of them and both suggested that a person could be killed by their next cigarette due to something falling on them while they stood outside smoking it.

One of them was more specific, and said the person smoking could get hit by a falling piece of space junk coming back to Earth. It seems a random retort, but in fact, back on that weekend in October 2011, the newsrooms were full of reports an old NASA satellite was due to fall back into the Earth’s atmosphere and that a small piece the size of a “washing machine” or a “small van” would likely make it through re-entry and come crashing back down onto Earth’s surface somewhere. News studios entertained experts and discussed how big a splash or how big a dent the piece of space junk would make, depending on where it landed, and discussed the risk it posed to human life.

They talked about how fast it would be travelling, what noise it would probably make and whether we would see it in time to get out of its way if we happened to be the unfortunate individual under its path.

1 in 100 billion.

According to the European Space Agency, that’s the approximate annual odds of being severely injured by falling satellites. I suspect it’s similar to the annual odds of being killed by falling satellites (“this huge piece of junk fell out of the sky and hit me on my foot!”). But either way, incredibly small. So small in fact, that we shouldn’t have any column inches or airtime dedicated to a discussion about how we could save ourselves if we happen to be in its path.

It’s a great example of the availability bias where we believing something is more likely to happen because it is more prominent in our minds. The availability bias is a subconscious mental shortcut we take when judging the chance of something happening. We quickly scan our memory and judge something to be more likely if we can recall it more easily, and things that come to mind more easily tend to be more recent or more emotionally vivid.

Satellites falling “out of control” to “smash” back into earth is a vividly and emotionally distinctive idea. Add to that the fact that we spent time listening to experts even referring to the possibility of being struck by one also makes it feel more likely. It's just another example of human’s not getting their heads around risk and chance.

Note: We didn’t get to find out where the satellite landed. Both it, and all the media stories surrounding it, vanished a day or so later.

Note 2: Only 3 days ago my BBC news app flashed me a story about the similar-sized defunct Chinese Tiangong-1 space lab that was also just hours from crashing back to earth. That one landed in the South Pacific somewhere. You might have seen that story too.

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