THE residents of Japan and other Pacific Rim countries might feel a twinge of
anxiety next week as Mir passes nearby on its final orbit, but the chances of
anyone being hit by debris are tiny.
There are around 10,000 man-made objects in orbit, and although on average
one falls to Earth every day, nobody has ever been hit, as far as we know.
鈥淭here鈥檚 a lot of world out there, and not a whole lot of people,鈥� says Nicholas
Johnson, NASA鈥檚 programme manager for orbital debris. Averaged over the whole
Earth, the world鈥檚 population density is only 12 people per square
kilometre.
To calculate the odds of human casualties from an uncontrolled re-entry, NASA
scientists first work out the population density of the strip of the Earth鈥檚
surface under the satellite鈥檚 orbit. Then they estimate how many chunks of
satellite will survive re-entry, and how big they鈥檒l be. After that, it鈥檚 easy
to calculate the odds of a person being hit.
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Johnson says NASA鈥檚 official policy is that if the odds of a fragment hitting
someone are higher than 1 in 10,000, the satellite must be brought down in a
controlled re-entry. When NASA decided to bring down the Compton Gamma Ray
Observatory last June, for instance, studies showed there was a 1-in-1000 chance
that part of the 14-tonne satellite would hit someone if it fell to Earth
uncontrolled. So NASA brought the satellite down safely in the Pacific
Ocean.
NASA had a harder time with Skylab in 1979. The odds were 1 in 152 that it
would injure someone, and NASA could only control its tumble to increase
atmospheric drag. When last-minute calculations revealed Skylab might hit North
America, engineers set it spinning a little early. It hit the Australian outback
instead.