Sky and Telescope - July 2018 - 39

Bad Behavior in Orbit

JULES V ER NE: ESA / N ASA; LONE STA R: N ASA; M A P: G REGG DINDER M A N / S&T; SOURCE: ESA

All of these techniques are working to keep space junk at
a lower level, but they can't prevent deliberate explosions
and collisions. These have been another big source of debris
historically, one that is completely avoidable. Soviet satellites
often carried self-destruct packages to prevent them falling into American hands if they reentered. Unfortunately,
sometimes these packages would go off by accident. The
Soviet missile early warning system used one-ton infrared
observatories called Oko (old Russian for "eye") to watch for
American missile launches from orbit. These satellites only
lasted a few years at best, remaining in highly elliptical orbits
as space junk, and their self-destruct systems had a regrettable habit of activating months or years after the satellite's
demise, strewing debris on a path ranging all the way from
LEO to GEO altitudes.
The U.S., USSR, and China have also all played with
weapons designed to destroy an enemy's space systems.
Fortunately, none of these weapons have been used against
an opponent, but they've been tested against the country's
own satellites. The weapons have usually been aimed at a
dedicated target. But a U.S. Air Force test in September 1985
did use an F-15 to take out a still-operational U.S. Navy solar
physics observatory, causing some bitterness from the Navy
scientists whose data suddenly stopped coming in. (Wags at
the time suggested the traditional Air Force/Navy rivalry had
escalated to confusion about who their real enemy was.)
Deliberate collisions are simple to avoid: Let's just not do
that sort of thing. But that still leaves the accidental ones.
At the moment, the debris from accidental collisions is still
a small fraction of all the space junk (only about 3%), so do

we really need to worry about it? The problem is that if you
have ten times the number of satellites, you typically make
ten times the amount of all the other kinds of debris. But you
get a hundred times as many collisions; the probability of an
orbital crash goes as the square of how much trafﬁc there is.
So if we keep increasing our use of space, in a few decades the
collisions are likely to surpass everything else and become the
main space debris problem. Then we'll be in trouble - Kessler's prediction may come true.
So far, our efforts to reduce how much debris we generate
have met with mixed success. Attention is now shifting to
more active measures.

Space Garbage Trucks
With tens of thousands of small debris objects ﬂoating in
near space, some fairly imaginative, and indeed desperate, ideas have been suggested. Powerful Earth-based lasers
could melt small debris pieces. Or, a satellite could deploy a
huge sticky net, perhaps covered in the aerogel used to collect solar wind samples, and act as a sort of space vacuum
cleaner. (Of course, an actual vacuum cleaner wouldn't
work in the vacuum of space!) Such devices go under the
heading of active debris removal, the term of art for space
garbage collection.
Perhaps these removal mechanisms aren't really needed,
though, because it turns out the smallest objects aren't the
worst problem. Most of them have a fairly large area-tomass ratio: They're more like feathers than cannonballs, so
they're strongly affected by atmospheric drag. In orbits below
500 kilometers, objects will usually reenter over the course
of the solar cycle. Every 11 years, as our star reaches solar

p REENTRIES AROUND THE WORLD Debris fragments have been recovered on the ground worldwide. Even so, the hazards posed by reentering
spacecraft and debris are extremely small. Space agencies and nation states typically aim for a casualty risk of less than 1 in 10,000 probability for a
single uncontrolled reentry.
s k y a n d t e l e s c o p e . c o m * J U LY 2 0 1 8

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Sky and Telescope - July 2018

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