Sky and Telescope - July 2018 - 35
Thousands of pieces of debris orbit Earth,
and it's going to take a coordinated effort
to solve the problem.
ixty-some years ago, Sputnik became Earth's ﬁrst artiﬁcial satellite. But it wasn't the only thing launched
that day - the main rocket stage of the launch vehicle
also ended up in orbit. There are now more than 9,000
metric tons in orbit around Earth, and 80% of that is orbital
debris - "space junk."
Space is famously big, so you might think
that even tens of thousands of orbiting
objects would have plenty of room to themselves. Indeed, the average distance between
debris at any moment is hundreds of miles.
But each of those objects is ﬂying around
Earth at swift speeds: 28,000 km/hr (17,400
mph) in the lowest, fastest orbits. They sweep
through so much space in the course of a
short time that the occasional cosmic collision is not only likely but inevitable.
On February 10, 2009, a half-ton communications satellite, Iridium 33, smashed into
a dead Russian satellite at a relative speed of
41,940 km/hr and an energy of 54,000 megajoules. (A single
megajoule is the energy of a one-ton truck hitting you at 100
mph.) In a fraction of a second, both satellites were reduced
to thousands of pieces of shrapnel, many of which remain in
orbit today and pose a threat to other space trafﬁc.
As the amount of space junk increases, we risk what's
called the Kessler syndrome, in which collisions become so frequent that a chain reaction gradually reduces the near-Earth
satellite population to aluminum confetti and makes space
qJUNK NEAR EARTH The vast majority of debris exists in low-Earth
orbit (left), but a significant number of objects are in or near geostationary orbit (center), which, aligned with Earth's equator, allows satellites to match our planet's spin. A polar perspective (right) provides a
different view of the density of objects in LEO and GEO orbits. View
the debris in motion at https://is.gd/spacedebrismovie.
LIT TERBUGS: ESA; JUNK NE AR E A RTH: NASA (3)
We can distinguish two main kinds of junk: Deliberate littering includes dead satellites, expended rocket stages, and
discarded parts such as covers and lens caps. Debris can also
result from destructive events, such as rocket explosions,
satellite collisions, and antisatellite tests. As the number
of satellites has gone up, the number of different types of
space junk has also increased over time. In the classic Space
Race years of the 1960s, only a few dozen satellites were
operating at any one time, but today there are almost 2,000
- and the amount of orbital garbage has ballooned accordingly. The junk increased dramatically in 2007, when China
tested an antisatellite missile and destroyed one of its
weather satellites, and again in 2009, thanks to the Iridium
collision. A handful of incidents have undone decades of
efforts to reduce the amount of space junk.
Any piece of debris orbits Earth in a path
similar to that of the satellite that made it.
Most satellites are either in low-Earth orbit
(LEO), between 200 and 1,700 km above
the surface, or in the 35,800-km-high geostationary orbit (GEO), where satellites take
24 hours to circle the planet in order to
stay in the same location on the sky as seen
from Earth. GEO is mostly used for communications and television-broadcasting
satellites, although low-power communications payloads used for cellphone and email
trafﬁc can be found in LEO, too.
A special set of near-polar orbits within
LEO are known as Sun-synchronous orbits (SSO), where satellites pass over the same part of Earth at roughly the same
local time every day. Here you can ﬁnd the satellites that
image Earth, both for civilian mapping and government
At intermediate heights (medium Earth orbit; MEO)
between LEO and GEO, the intense Van Allen radiation
belts make it harder for satellites to operate. Nevertheless,
GPS navigation satellites are among those that operate here
in 12-hour orbits.
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