Sky and Telescope - July 2018 - 36
The major spacefaring nations are now taking steps to lower
the risk of a Kessler cascade. Although no international law
controls space littering, informal agreements exist between
the world's major space agencies. The Inter-Agency Space
Debris Coordination Committee provides a forum for these
Iridium Collision Debris
Chinese Antisatellite Debris
p A GROWING PROBLEM The amount of space junk has increased
dramatically over a few decades. Most debris comes from accidental
rocket explosions or deliberate military tests. Single collision events,
such as the deliberate Chinese test or the accidental Iridium/Cosmos
crash, can worsen the situation significantly. Littering of inert parts,
rocket stages, and dead satellites makes up most of the rest. Active
satellites (dark purple) are only a small fraction of the orbital traffic.
J U LY 2 0 1 8 * S K Y & T E L E S C O P E
t WHO'S LITTERING? The littering by civilian and
commercial satellites is relatively minor compared to
the debris generated by military satellites (medium
purple) and exploded rocket stages (dark purple).
agencies to set recommendations - for
example, how high above GEO you should
boost your dying satellite so it won't bump
into operational ones. The leading centers
for research on debris are the Orbital Debris
Program Ofﬁce at NASA's Johnson Space
Center in Houston, Texas, and the European
Space Agency's Space Debris Ofﬁce in DarmCommercial
stadt, Germany, which also hosts regular
international conferences on the problem.
These groups have found that there's no
one-size-ﬁts-all solution when it comes to space junk. Different kinds of junk need different approaches.
Active satellites are "passivated" at the end of their missions. This neologism indicates that the owners try to get rid
of all energy sources that might cause the satellite to blow up
at a later date - usually, by venting all rocket propellant and
all the ﬂuid from any batteries. In the past, battery explosions
had been another signiﬁcant contributor to space junk.
If the satellite is in a low orbit, it will be lowered toward
Earth as much as possible before getting rid of its rocket fuel.
The idea is to make it vulnerable to atmospheric burn-up.
Getting the satellite to reenter immediately is best, since you
can then control where it burns up, but there may not be
enough fuel to do this. Even reducing the perigee a bit will
help, since the atmosphere gets much denser the lower you
go, and atmospheric drag will become more effective in bringing the satellite down, perhaps in months instead of decades.
For satellites in GEO, it wouldn't be practical to bring the
satellite down from that high. Instead, they go into a socalled "graveyard orbit," a few hundred kilometers above the
geostationary belt. Satellites placed here will stay out of the
belt for hundreds of years, even with the perturbing gravity of
the Moon and Sun.
Such actions mark a drastic change from 30 years ago,
when most low-orbit satellites didn't have the ability to
change their orbits at all. A rocket put them in space, and
they orbited solely under the inﬂuence of gravity and air drag.
Nowadays, most satellites with a mass of more than a few
hundred kilograms have their own rocket-propulsion systems
to alter the orbit at mission's end.
But recent years have brought us a new problem: Since
2003 nanosatellites (less than 10 kg) have become common,
most using a standard design known as a CubeSat (S&T: Nov.
2013, p. 64). More than 500 CubeSats are now in orbit, and
almost none of them have their own rocket engines, posing a
challenge for other satellites. Even if a CubeSat is still operating, if it can't get out of the way it might as well be space
debris as far as an approaching satellite is concerned. Until
recently many CubeSats were launched to low orbits with
G REGG DINDER M A N / S&T; SOURCE: J. MCDOWELL (2)
All of these orbits are roughly circular -
relatively few satellites operate in highly elliptical orbits (HEO), where the low and high
points (perigee and apogee) are very differ10.5%
ent from each other. But a lot of junk lies in
HEO, mostly from rocket stages, which were
left there while delivering a satellite to GEO.
Still, most of the known junk is in LEO,
and thanks to the 2007 Chinese military
weapons test, the majority of that is in SSO.
That distribution may be an illusion,
though, as it's much harder to detect small
debris in the higher orbits farther from
Earth. The same object in a 10 times higher
orbit is 1/100 as bright to optical telescopes
and appears 1/10,000 as bright to radar
reﬂection. We'll know better what the high-orbit debris
situation is in a few years, when new satellites dedicated to
mapping high-altitude space debris give us the true picture.
It's likely to be depressing news.