Today, let us fill infinity. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
Thermodynamics students have
to learn about an important thought model. It is
the infinite thermal reservoir -- a region
so large you can dump heat into it without
increasing its temperature. Or you can cool it
without reducing its temperature.
You may blink at the notion of cooling something
without reducing its temperature, but that's just
what makes the concept so interesting. Suppose I
lower a sealed tin container of ice cubes into some
tropical ocean. I come back an hour later and pull
up a tin filled with tepid water. I've removed
thousands of Btu's from the ocean without changing
its temperature noticeably.
For a century, thermo teachers would suggest
oceans, rivers, and the atmosphere as examples of
thermal reservoirs. Not until well after WW-II did
we realize that certain power plants, located along
rivers, were rejecting enough low-temperature
energy to warm the rivers and damage their biology.
It's been a hard lesson, but we've gradually caught
on to the fact that the outdoors is not
infinite. Now we're finding that's also true for
what Captain Kirk called "space, the final
In a recent Science magazine article,
space consultant Richard Crowther offers an
alarming picture of what we've done to the region
of space that reaches from just outside Earth's
atmosphere to the level of geosynchronous orbit,
just over twenty-two thousand miles up. Into that
region, we have, by now, dumped over four million
pounds of trash.
Close to ten thousand items have a dimension of
over ten centimeters, and they've been catalogued.
Those are the objects that could destroy a space
vehicle. But hitting anything larger than even a
centimeter (say a lens cap or a bolt) could have a
lethal effect. Over a hundred thousand such items
are now in orbit.
Over ninety-nine percent of the items are smaller
than a centimeter, even though they're less than a
thousandth of the total mass. We often hit those
items, and some have done serious damage.
Trash tends to cluster in one of two shells. One is
a thin shell at the geosynchronous orbit level. The
other lies in the low orbit range used by space
shuttles and space stations. It's around two
hundred miles up. Within a decade or so, low-orbit
junk is slowed by fringe molecules of the
atmosphere. It eventually falls back to Earth. Junk
in the outer shell can last indefinitely.
Already, we're designing space vehicles and flight
patterns, not so as to eliminate impacts,
but simply to reduce the amount of damage
that's done. We keep track of those ten thousand
large items -- part of a broken-up satellite, a
dropped glove, a lost structural member, stuff that
could destroy a vehicle. We track those pieces and
But some thirty-five million tiny bits and pieces
represent a danger that astronauts simply have to
face. And they remind us of the mischief we've done
by letting ourselves regard infinite reservoirs as
anything more than a mathematical abstraction.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
Crowther, R., Space Junk -- Protecting Space for
Future Generations. Science, Vol. 296, 17
May 2002, pp. 1241-1241.
For more on this matter see:
The Problem of Space Debris
NASA image showing large low orbit junk as of 1998
The Engines of Our Ingenuity is
Copyright © 1988-2002 by John H.