CCNet DEBATES 4 August 1998


Steven Ostro and Carl Sagan

From Astronomy & Geophysics: The Journal of the Royal Astronomical
Society 39(4) 1998, pp. 22-24*

* The permission by the editor of A&G to circulate this article on  
  the CCNet is greatfully acknowledged.

Can civilizations survive without developing space travel? Steven Ostro
and the late Carl Sagan explore the consequences of cosmic impacts for
the longevity of galactic civilizations.

The absence on Earth and in astronomical data of unambiguous signs of
interstellar spacefaring civilizations - at a time when we ourselves
are, with Pioneers 10 and 11 and Voyagers 1 and 2, capable of
rudimentary interstellar flight - has been called the Fermi Paradox
(e.g. Shklovskii and Sagan 1966, Brin 1983). Among the explanations
offered (se synopses by von Hoerner 1995 and Crawford 1997) are that we
are being visited and either can detect the visitors (UFOS, although
the evidence is wholly non-compelling) or cannot (the “zoo hypothesis”
of Ball 1973); that there are intractable physical or economic
impediments to interstellar spaceflight (Purcell 1963; Drake and Sobel
1992); that diffusion limits on interstellar space flight are such that
we have not yet been visited (Newman and Sagan 1981); that the
evolution of life and technical civilizations is much more improbable
than we might guess from our own presence (Hart 1975, Tipler 1980,
Sagan and Newman 1983, Diamond 1995); or that even if technical
civilizations arise abundantly, they tend to destroy themselves before
achieving extensive interstellar spaceflight (Shklovskii and Sagan

Another possible explanation has been offered (e.g.. Ashkenazi 1995):
there is no general imperative, it is argued, for intelligent species
to invent spaceflight - even among civilizations that are
technologically superior to our own. Only under special conditions, it
is suggested, will spaceflight develop. An analogous argument can be
made regarding radar astronomy and interplanetary radio communications.
It is perfectly possible, advocates of these ideas propose, that the
Galaxy is filled with highly advanced civilizations that never develop
high technology, or that prudently abandon it - or even that develop
high technology but not high-power radar transmitters and not
spaceflight. Then we can understand why we have not received radio
signals or visits from other civilizations in space.

We argue here that there is a common factor that will drive many
technical civilizations into space, that those civilizations which
choose not to become spacefaring will for this very reason be rendered
extinct, and that, by a kind of natural selection, all sufficiently
long-lived civilizations must be spacefaring (and are likely to have
developed radar/radio techniques).


We now recognize that the Earth orbits the Sun amidst a swarm of comets
and asteroids, that impacts of these objects with the Earth are
inevitable, and that civilization-threatening impacts should occur on a
timescale (i.e. average interval) of 100,000 to 1,000,000 years
(Chapman and Morrison 1993, Toon et al. 1997). For the Earth, the
threshold for catastrophic global effects is the impact of an object of
diameter 1-3 km..  The kinetic energy of impact, E =100,000 to 1,000,000
Mt, where 1 Mt= 4.185 x 10 to the power of 15 joules, is so high that
a globe-enshrouding stratospheric cloud of impact debris (dust,
asteroidal sulphates, and soot from wildfires ignited by the impact's
thermal radiation) will lower light levels, drop temperatures, destroy
the ozone shield and have many other deleterious effects, especially
through the global termination of agriculture. The deaths of a
significant fraction of the human population are anticipated. Much
lower-energy impacts (E=10,000 to 100,000 Mt, projectile size -500 m,
average interval of order 60,000 years) into an ocean could raise
tsunamis able to inundate a kilometre of coastal plain over entire
ocean basins, killing up to 1% of our population. Much higher energy
impacts (E>10,000,000 Mt, projectile size of order 10 km, interval >20
million years) can cause mass extinctions; it is widely thought that
the Cretaceous/Tertiary impact of 65 million years ago eliminated at
least 60% of Earth's species (Sharpton and Ward 1990, Sheehan and
Russell 1994).

We do not argue here about what impact energy threshold would be
necessary to wreck global civilization with confidence, what the
average interval is between impacts that could render the human species
extinct etc (Adushkin and Nemehinov 1994). Whatever these numbers are,
if we wait long enough, the relevant event will occur.

To deal with the asteroidal component of the terrestrial impact hazard,
it is probably sufficient to have interception capabilities restricted
to within a few AU of Earth. At their current discovery rate,
Earth-based surveillance will locate the entire population of
kilometre-sized, possibly hazardous asteroids during the next century
or two. Unless we are unlucky enough to suffer a catastrophic collision
in the interim, it will be a simple matter to mitigate a
civilization-disrupting asteroid collision, because there will be
adequate warning rime. Means have been suggested for deflecting
asteroids into harmless trajectories or, if the warning time is too
short, destroying them. In all cases the technical solutions require
implanting or attaching devices to the object or exploding nuclear
weapons near it (Ahrens and Harris 1992, Melosh et al. 1994).  It is
difficult to imagine practicable means of deflecting which do not
require a substantial capability to interplanctar robotic spaceflight.
For the shortest-warning-time scenarios, space missions piloted by
humans might be required for mitigation of an impending collision, if
the advantages of onboard human intelligence outweigh the associated
cost and risk.

The odds of a very-long-period comet (LPC) collision in a millennium
are similar to odds of an equally energetic asteroid collision in a
century, but dealing with the LPC component of the impact hazard is
orders of magnitude more challenging. Ground-based and spaceborne
reconnaissance of physical properties is intrinsically much more
difficult than for asteroids, so deflection or destruction of a
threatening LPC would require more exotic weaponry and very much longer
warning times than would a threatening asteroid. However, we cannot
detect LPCs much more than a few months before their arrival in the
inner solar system (Marsden and Steel 1995), because coma-producing
evaporation of volatiles by insolation typically doesn't turn on until
a comet gets within about Jupiter's distance from the Sun, because even
very large (>10 km) inactive nuclei far beyond those distances
generally are too dim for VIS/IR telescopic detection, and because
LPC motion against the star background is inconspicuous.

Sooner or later human civilization must confront the asteroid/comet
collision hazard or become extinct. Dealing with interplanetary
collision hazards over a period of centuries or millennia will
naturally take our spacefaring society further out into the solar
system - to improve surveillance of incoming comets, if for nothing
else. As technology advances and the life-span of our species (and its
successors) lengthens, a slow outward transition fro, interplanetary
travel towards cometary source regions and interstellar spawflight
seems conceivable (Sagan 1994).


In the long run, the threat of interplanetary impacts must play a role
in the evolution not only of our civilization, but of any others that
may have evolved on the planets of other stars with residual small-body
populations. The nature of the small-body collision hazard that
confronts extra-terrestrial civilizations may be very different from
ours, depending on such factors as the physical and chemical
characteristics of the planet and its biosphere, the biological and
sociological nature of the civilizadon, and of course the collision
flux itself.

Disks of planetesimals are thought to be a ubiquitous stage in the
formation of planetary systems. Our system contains several primordial
subpopulations of small bodies that feed potential impactors into
Earth-crossing orbits: the main asteroid belt, which supplies nearearth
asteroids; the Kuiper Belt, which supplies short-period comets; and the
Oort Cloud, which supplies LPCS. Both the existence of the source
populations and the mechanisms that maintain Earth's collision flux
are, in part, a consequence of the radial distribution of large masses
in the solar system, which in rum are an outcome of the initial
physical conditions (especially the gas/dust ratio) in the primitive
solar nebula (Lunine 1995).

For example, our Oort Cloud was probably populated by gravitational
ejecta from the Uranus and Neptune regions (Femandez 1978; Fernandez
and Ip 1983).  If there are no planets that play the role of Uranus and
Neptune in systems otherwise like our own, their Oort Clouds may be
very thinly populated or nonexistent, although they may retain
densely-populated peripheral Kuiper belts.  Stars in open and globular
clusters, stars closer to the centre of the Galaxy and stars
experiencing more frequent encounters with Giant Molecular Clouds, may
all experience higher impact fluxes at their terrestrial planets, if
any. in our system, the main asteroid belt is the remnant of
planctesimals that were kept from accumulating into a planet by the
disruptive gravitational effect of Jupiter, and Jupiter's mean-motion
resonances are the key mechanism for injection of main-belt asteroids
into Earth-rossing orbits. On the other hand, Jupiter gravitationally
shields the Earth from LPC impacts; Wetherill (1994) has calculated a
2.5 order-of-magnitude increase in the cometary flux at the Earth had
the planet Jupiter never formed. Thus there may be many possibilities
for residual smallbody populations and the evolution of the impact flux
in systems containing Earth-like planets. For example, gas-rich
planetary accretion disks might create giant planets in the “wrong”
place (Ward 1997), or gas-poor disks may generate terrestrial planets
but neither category (Jupiter/Saturn or Uranus/Neprune) of giant planet
and hence a negligible impact flux.


Impacts during the first -100 million years of Earth's history may have
played a role in the origin of life, both in preventing its "permanent”
establishment during the post-accretional heavy bombardment (Maher and
Stevenson 1988) and in delivery of volatiles and prebiotic organic
molecules (Chyba et al. 1994). Our Moon, which is thought to formed by
the impact of a Mars-sized planctesimal on the proto-Earth (Hartmann
and Davis 1975, Cameron and Ward 1976), has prevented flue tuations in
Earth's obliquity that would have destabilized the climate and
short-circuited evolution (Laskar et al. 1993, Laskar 1997). 
High-energy impacts are thought to have caused at least some, if nor
most, of Earth's mass extinctions (Rampino and Haggerty 1996 and
references therein) and may have increased the rate of evolution, in
the sense of Gould and Eldredge's (1993) punctuared equilibrium. More
frequent, lower-energy impacts maY have catalysed the evolution of
biological diversity between mass extinction events (Morris 1998).

We conjecture that Earth's rich and complex history of asteroid/comet
collisions has accelerated the appearance of intelligent life on our
planet, and that the timescale for the evolution of life and the
emergence of extra-terrestrial technological civilizations depends on
the distribution and dynamics of small bodies left over from planet
formation. Large impact fluxes might increase the rate of evolution,
whereas too high a flux clearly would be inimical to the development of
civilization. Conversely, too low a flux might forestall the appearance
of intelligent life. In any event, for our single available sample of a
technological civilization, the same interplanetary collision flux that
may have been instrumental in its creation also constitutes a definite
threat to its long-term existence. One consequence of these arguments
is that the Galaxy does not contain many civilizations that are both
long-lived and have never developed at least a robotic spacefaring


Likewise, the utility of electromagnetic radiation in controlling
spacecraft, in returning data from spacecraft to the home planet, and
in studying the nature and trajectories of asteroids and comets, is so
high that successful avoidance of the collision hazard without
development of radio telemetry and radar remote sensing seems similarly
ifnplausible. Currently, laser radars are orders of magnitude less
sensitive than planetary radar telescopes. Development of this
technology may someday enable laser radars to detect reflections from
Earth-crossing asteroids that permit imaging and astrometric
measurements as precise as is now achievable with radio-wavelength
radars. Similarly, optical telemetry has bandwidth advantages over
radio telemetry and soon will be implemented for some NASA missions.
But centimetre and longer wavelengths can he used around the clock and
in any weather. They also have fundamental advantages in the physical
and dynamical characterization of cornet nuclei, whose comas are
optically opaque but radio-translucent, and in the assessment of the
large-particle component of those comas. Given the paramount importance
of high-precision ranging to the nucleus in predicting a comet's
trajectory, radio-wavelength radars would seem to be essential to
defence against threatening comets (Ostro 1994).

A possible implication for SETI is that transmissions from small-body
radar astronomy, which often are very narrowband, constitute beacons
that might be detectable over galactic distances. Radar astronomy on
Earth already contributes the brightest, albeit very intermittent and
highly directional, electromagnetic signature of our civilization, and
it is likely to become increasingly powerful and less intermittent. 
The several dozen radar-detected Earth-crossing asteroids are a tiny
fraction of the population of objects that are desirable to monitor in
order to maintain the accuracy of orbit predictions. Future generations
of very sensitive radar teiescopes (with very powerful transmitters),
perhaps constructed as part of our planetary defence against LPCS, will
be able to study enormous numbers of asteroids that are both
potentially threatening on long timescales and attractive targets of
robotic and piloted space missions on short timescales. Ironically, the
population of asteroids that can collide with Earth includes the
cheapest destinations for such missions, which might be motivated not
just by concern about the collision hazard, but also by any of the
factors that have driven human exploration in the past. Indeed, given
the accessibility and resource potential of near-Earth asteroids (Lewis
and Hutson 1993), development of an asteroid orbit-manipulation
infrastructure could offer an irresistible return on investment, one of
whose spin-offs would be a head start on systems that ultimately will
be necessary for planetary defence.


However, altering the trajectories of objects in nearby interplanetary
space can introduce perils on timescales much shorter than the average
intervals between natural impact catastrophes (Sagan and Ostro 1994a,b;
Harris et al. 1994). Thus, interplanetary collision hazards may act as
a kind of sieve, simultaneously requiring civilizations to become
spacefaring and to institute stringent controls on the misuse of
orbitengineering technology. These joint constraints may or may not be
so severe as to truncate the longevity of spacefaring civilizations
below the timescales for civilization-ending impacts themselves. One
way or another, interplanetary collisions constitute a unique,
erogenous environmental factor in the natural selection of long-lived

Steven J Ostro, 303-233 Jet Propulsion Laboratory, California Institute
of Technology, Pasadena, California 91109-8099, USA.
"ostro@echo.jpl.nasagov”. Carl Sagan (deceased), Cornell University.
Part of this research was conducted at the Jet Propulsion Laboratory,
California Institute of Technology, under contract with NASA. Work at
Cornell was supported in part by NASA Grant NAGW 1896.


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