As Discussed by Donald J. Kessler
March 8, 2009
The “Kessler Syndrome”
is an orbital debris term that has become popular outside the
professional orbital debris community without ever having a strict
definition. The purpose of this writing is to clarify the
intended definition, to put the implications into perspective after 30
years of research by the international scientific community, and to
discuss what it may mean to future space operations.
As far as I am aware, the term
originated with a colleague, John Gabbard, who worked for NORAD.
NORAD maintained a catalogue of man-made objects in orbit, but
did not maintain a breakup record of events in orbit. John
unofficially kept a record of major satellite breakup events, which
later proved very useful in understanding the sources of smaller
orbital debris. John is known for his description
of these events with a graph we now call a “Gabbard Plot”.
When I met John in 1978, I had just published
the Journal of Geophysical Research (JGR) paper, “Collision
Frequency of Artificial Satellites: The Creation of
a Debris Belt”. This paper predicted that
around the year 2000 the population of catalogued debris in orbit
around the Earth would become so dense that catalogued objects would
begin breaking up as a result of random collisions with other
catalogued objects and become an important source of future debris. These
finding were important for three reasons:
- At the time, it was generally assumed that
there were very few objects in orbit that were too small to catalogue,
although there was no definition as to what limiting size was in the
catalogue. The paper illustrated that even if this
assumption were correct, future collisions between catalogued objects
would produce a large amount of small debris fragments. This
small debris population would be more hazardous to other spacecraft
than the natural meteoroid environment immediately after the first
- Each collision would also produce several
hundred objects large enough to catalogue, increasing the rate that
future collision breakups would occur….resulting in an exponential
growth in the collision rate and debris population.
- The only way to prevent this exponential
growth was to reduce the number of rocket bodies and non-operational
spacecraft left in orbit after their useful lifetime.
It was the second prediction that caught John
Gabbard’s attention. While talking to a reporter
shortly after the publication of the JGR paper, John used the phrase
“Kessler Syndrome” to summarize my prediction of a future cascading of
collisions in orbit. The reporter published the
phrase. Perhaps it was a 1982
Popular Science article that made the term more popular, since the
Aviation and Space Writers Association gave the author, Jim Schefter,
the 1982 National Journalism Award for the article. However,
regardless of the source, the label stuck, becoming part of the
storyline in some science fiction, and a three-word summary describing
orbital debris issues.
However, not all who have used the phrase
have referred to it in the context of its original meaning.
It was never intended to mean that the cascading would occur
over a period of time as short as days or months. Nor
was it a prediction that the current environment was above some
critical threshold…although the concept of a critical threshold was an
important possibility that was studied in detail more than 10 years
later. The “Kessler Syndrome” was meant to describe
the phenomenon that random collisions between objects large enough to
catalogue would produce a hazard to spacecraft from small debris that
is greater than the natural meteoroid environment. In
addition, because the random collision frequency is non-linear with
debris accumulation rates, the phenomenon will eventually become the
most important long-term source of debris, unless the accumulation rate
of larger, non-operational objects (e.g., non-operational payloads and
upper stage rocket bodies) in Earth orbit were significantly reduced.
Based on past accumulation rates, the 1978 publication predicted
that random collision would become an important debris source around
the year 2000, with the rate of random collisions increasing rapidly
after that, if the accumulation rate were not reduced to near zero.
Findings Since 1978
Combined with the discovery that 42%
of the catalogued objects were the
results of only 19 explosions in orbit of U.S. upper stage rockets
that NORAD was not tracking “all man-made objects” as generally
believed, NASA took these findings and predictions seriously.
Beginning in October of 1979, I was given funds to begin
research for data to more accurately define the current and future
debris hazard, and understand techniques to limit the future growth in
the debris population. With these funds, we
accomplished our objectives with a combination of modeling,
measurements that sampled the environment, ground tests to simulate
space collisions, and coordination with the space community to
determine cost-effective techniques to minimize future growth of the
We sampled the small debris environment by developing and using ground
telescopes and powerful, shorter wavelength radars. We
also analyzed recovered spacecraft surfaces for impacts using scanning
electronic microscopes, which allowed us to determine the chemistry of
the objects causing those impacts. Together
with the Air Force, we conducted hypervelocity ground simulation of
collisions and examined ground explosion data to more accurately
predict the amount of small debris generated. We
also developed much more elaborate computer models which we used to
test our assumptions and ground data against the data we obtained by
sampling the environment. We used these computer
models to test the effectiveness of various techniques to minimize
future growth in the debris population. These
efforts were lead by a team of scientists in what is now known as the NASA Orbital Debris
Program Office. Other international governmental agencies
participated in this research, forming an international organization
now known as the Inter-Agency
Space Debris Coordination Committee (IADC). The
following conclusions were reached as a result of this research:
Significance of the “Kessler Syndrome” Today
- The hazard from the debris that was too
small to catalogue had already exceeded the hazard from the natural
meteoroid environment. The
sources of that debris included not only explosions, but paint
flecks from spacecraft surfaces, exhaust from solid rocket upper
stages, and leaks of coolant from nuclear reactors.
- Better data and more accurate modeling by
NASA and the international community support the conclusion that the
long-term threat to the environment is collision cascading, as
predicted in 1978.
results supported by data from USAF tests, as well as by a number
of independent scientists, have concluded that the current debris
environment is “unstable”, or above a critical threshold, such that any
attempt to achieve a growth-free small debris environment by
eliminating sources of past debris will likely fail because fragments
from future collisions will be generated faster than atmospheric drag
will remove them.
- Although the rate of growth in the
catalogued population has been reduced as a result of new operational
procedures that minimize the possibility of explosions in orbits and
leaving non-operational upper stages and payload in orbit for periods
longer than 25 years, the catalogued population continues to increase,
but at a lower rate than it was increasing prior to the 1978 paper.
On February 10, 2009 the Iridium 33 and Cosmos 2251 satellites collided
with a velocity of ll.6 km/sec, at an altitude of 790 km. The
collision was catastrophic, likely producing hundreds of fragments
large enough to catastrophically breakup other satellites, and tens of
thousands of fragments large enough to damage other satellites.
This is the first clear example of what was predicted in 1978.
Although there have been three other random collisions between
catalogued objects since 1991, none of those were catastrophic.
Although all existing data and analysis support the major conclusions
presented in the 1978 JGR paper, there are minor differences.
The most obvious is the difference between the predicted growth
rate in the catalogue population of 510 objects per year compared with
the actual growth rate, which was less. There were
a number of conditions that contributed to the lower rate: 1.
The success of the orbital debris program in establishing international
agreements that reduced the number of accidental explosions in orbit.
These explosions had been a major source of catalogued debris.
2. An abnormally high solar activity increased the upper
atmospheric density and caused more satellites to reenter. 3.
The declining economy and eventual fall of the USSR
significantly reduced the number of Soviet launches. As
a result of these conditions, the actual average growth rate over the
last 50 years was about 300 objects per year. This
rate would have been lower, had it not been for the Chinese
anti-satellite test in 2007, which produced over 2000 fragments large
enough to catalogue. A rate of 300 objects per year
is close to the lower assumed rate in the 1978 JGR paper. This
average growth rate would predict the first collision between
catalogued objects to have occurred around the year 2000, and it was
assumed to be a catastrophic collision.
The lower growth rate of 320 objects per year in the 1978 paper
predicted two collisions by 2009, both catastrophic. Although
the actual number of collisions is too few to be statistically
meaningful, they may indicate that the actual collision rate could be
higher than predicted, but fewer are catastrophic. This
higher collision rate would be consistent with the uncertainty in
spacecraft area subject to collisions, as was noted in 1978. In
publications, the collision area was shown to be about 2.5 times
greater than adopted in 1978. The
2000 publication also concluded that not all cataloged fragments were
massive enough to cause a catastrophic collision…this would be
especially true if the colliding fragment hit an antenna, stabilizer
boom, or solar panel, or if the target were the empty tank of an upper
stage. The presences of antennae, solar wings, and stabilizer booms
were ignored in 1978, and obviously hitting one of these areas will
only transfer a fraction of the impact energy to the entire spacecraft
structure, reducing the likelihood of a catastrophic breakup.
Also an impact into the empty fuel tank of an upper rocket stage
may not transfer all the impact energy to the rocket body
structure….again not causing a catastrophic breakup. We
may have been lucky that only one of the four collisions since 1991 was
catastrophic…or it may be that only one out of four of the collisions
between catalogued objects will be catastrophic. The
1978 prediction of collision frequency becomes more consistent with the
actual collision frequency by simply assuming that the area used in
1978 is the average catastrophic collision area, which was the intent
of the paper. However, a more accurate
understanding of both the non-catastrophic and catastrophic collision
frequency is achieved by using data generated since 1978 in more
accurate models currently used by the Orbital Debris Program Office.
Despite the absence of random catastrophic collisions, the predicted
fluxes of smaller debris in 1990 and beyond in the JGR paper are not
too different from what has been measured as a result of the orbital
debris program. Accidental explosions and a few
intentional collisions almost certainly contributed to the similarity….
and possibly some non-catastrophic collisions involving an
un-catalogued object also contributed. However, the
major contributors were a number of small debris sources that were
discovered since 1978. Even though these sources
have produced a debris environment in the past that is about the same
as predicted from collisions, past
debris sources are fundamentally different from future random
collisions between catalogued objects. The
past sources produce debris at a rate that is proportional to the
number of objects in orbit, while the future frequency of
collisions will produce debris at a rate that is proportional to
the square of the number of objects in orbit. For
example, if one were to double the number of upper stages and payloads
in orbit, each having a probability that they would explode, then the
rate that debris is generated by explosions would also double.
However the rate that debris is generated by collisions between
these objects would increase by a factor of four.
The 1978 prediction of a catastrophic collision between catalogued
objects of 0.013 per year was based on a catalogue containing 3866
objects; today, the catalogue contains about 13,000 objects, or more
than 3 times as many objects. This gives a
collision rate that is more than 10 times what it was just over 30
years ago, or 0.13 per year….which is the same as one catastrophic
collision between cataloged objects every 8 years….with the time
between collisions rapidly becoming shorter as the catalog continues to
grow. The larger fragments from either explosions
or collisions will further accelerate the rate of collisions.
Most of the collisions in the 1978 paper were predicted to take place
between 800 km and 1000 km altitude. That is even
truer today. Not only is this region rapidly
growing, certain altitudes contain a high concentrations of satellites,
and the inclinations of their orbits are near polar, both conditions
increasing the probability
that they will collide, and do so with collision velocities that
average more than 10 km/sec.
We are entering a new era of debris control….an era that will be
dominated by a slowly increasing number of random catastrophic
collisions. These collisions will continue in
the 800 km to 1000 km altitude regions, but will eventually spread to
other regions. The control of future debris
requires, at a minimum, that we not leave future payloads and rocket
bodies in orbit after their useful life and might require that we plan
launches to return some objects already in orbit.
These control measures will significantly
increase the cost of debris control measures; but if we do not do them,
we will increase the cost of future space activities even more.
We might be tempted to put increasing amounts of shielding on
all spacecraft to protect them and increase their life, or we might
just accept shorter lifetimes for all spacecraft. However,
neither option is acceptable: More shielding not
only increases cost, but it also increases both the frequency of
catastrophic collisions and the amount of debris generated when such a
collision occurs. Accepting a shorter lifetime also
increases cost, because it means that satellites must be replaced more
often….with the failed satellites again increasing the catastrophic
collision rate and producing larger amounts of debris.
Aggressive space activities without adequate safeguards could
significantly shorten the time between collisions and produce an
intolerable hazard to future spacecraft. Some of
the most environmentally dangerous activities in space include large
constellations such as those initially proposed by the Strategic
Defense Initiative in the mid-1980s, large structures such as those
considered in the late-1970s for building solar power stations in Earth
orbit, and anti-satellite warfare using systems tested by the USSR, the
U.S., and China over the past 30 years. Such
aggressive activities could set up a situation where a single satellite
failure could lead to cascading failures of many satellites in a period
of time much shorter than years.
As is true for many environmental problems, the control of the orbital
debris environment may initially be expensive, but failure to control
leads to disaster in the long-term. Catastrophic collisions between
catalogued objects in low Earth orbit are now an important
environmental issue that will dominate the debris hazard to future