A Space Oddity

Since the re-entry of NASA’s Upper Atmosphere Research Satellite (UARS) last September, a number of journalists have written articles which read like the plot summary of Deep Impact. But does returning space debris really pose a threat to us?

With around 1,000 operational spacecraft currently in orbit, it’s important to note that re-entry events are a frequent occurrence. On average one piece of space debris returns to Earth each day, though not all are as big as those associated with the re-entry of NASA’s UARS.

Granted, the thought of a 6 tonne satellite plummeting towards the ground with no one knowing where or when it will ‘land’ is unnerving. However, the reality of the re-entry of the UARS was quite different. It is thought that only 532kg of the satellite made it to the Earth’s surface, with the single largest piece of debris estimated at 160kg. Considering that approximately 71% of the Earth’s surface is covered by ocean, and much of the land mass is sparsely populated, the risk to human life was small (NASA calculated that the risk of human injury was 1 in 3,200 and the risk for one human was 1 in 21 trillion, subject to your position on Earth).

NASA’s inability to accurately predict the time or location of the satellite’s re-entry caused some anxiety. However, given the low risk this seemed somewhat unreasonable. Predictions are complicated primarily by the effects of variations in solar activity on the Earth’s upper atmosphere (thermosphere). When solar activity is high, ultraviolet radiation from the Sun heats and expands the thermosphere, increasing atmospheric drag and the orbital decay rate of spacecraft, and thus causing early re-entry.

If you are still concerned about future re-entries you could avoid falling debris altogether by travelling away from under the satellite’s orbit (or ‘ground track’). For the UARS, this would have meant, you would have had to travel further north or south than 57 degrees latitude (the Portland Building is 52.94?N).

Taking everything into account, my advice is the same as that of The Hitchhiker’s Guide to the Galaxy: DON’T PANIC.

James Gordon 

ScienceThis Issue

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